An alkoxide cluster with 18 Li⁺ ions encapsulating two borate anions, [(^tBuO)₁₂Li₁₈(BO₃)₂]

The title compound, bis­(borato)­dodeca(tert-butoxo)­octa­deca­lithium, [Li₁₈(BO₃)₂(C₄H₉O)₁₂], is formulated conveniently as [{(^tBuOLi)₃(Li₃BO₃)}₂(^tBuOLi)₆]. A central 12-membered ring and two outer six-membered rings are formed by alternating Li⁺ cations and alkoxide O atoms. Sandwiched between the central ring and each of the outer rings is a planar array of three further Li⁺ cations surrounding a [BO₃]^3- anion. Thus, the mol­ecule consists of a cationic [Li₁₈(O^tBu)₁₂]⁶⁺ cage encapsulating two borate anions. This compound is the first example of a structurally characterized polynuclear lithium borate, and a rare case of a lithium alkoxide cage with nuclearity greater than eight. All the alkoxide ligands are triply bridging, and the lithium ions have trigonal-planar, trigonal-pyramidal and fourfold coordination, all with major distortions from regular coordination geometry.

S1. Comment

There has been growing interest in the borates of alkali and alkaline earth metals, owing to their potential applications as optical materials. In particular, β-BaB₂O₄ (beta-bariumborate, BBO), discovered by Chen et al. (1985), is an excellent non-linear optical material, possessing many advantageous characteristics, such as a high damage threshold, good optical homogeneity, mechanical strength and chemical stability (Nikogosyan, 1991; Eimerl et al., 1987). Lithium borates are also important materials for non-linear optical applications, for example, LiB₃O₅ (lithium triborate) (Chen et al., 1989). This compound contains a continuous network of B₃O₇ groups with Li atoms. Two B₃O₇ groups share one O atom and are connected to form endless chains running parallel to one axis. The compound is stable and not hygroscopic, with good chemical properties. Recent investigations of lithium triborate as a non-linear medium for optical parametric oscillators showed that it is relevant for low-pump energy sources (<3 mJ), such as in miniature solid-state systems (Withers et al., 1993).

Much of the current interest in metal alkoxides arises from their use as precursors to metal oxide materials, and in recent years, significant effort has been devoted to the synthesis of heterometallic alkoxides as potential single-source precursors to mixed-metal oxide ceramics, catalysts and glasses, since the well defined stoichiometries and solubilities of these alkoxides offer significant advantages for the production of single-phase materials by methods such as sol-gel, CVD and spray pyrolysis (Caulton & Hubert-Pfalzgraf, 1990). We have been investigating the synthesis of single-source precursors for the production of metal borates (Errington et al., 1999) and report here the structure of a unique molecular species containing trigonal BO₃³⁻ units.

The reaction between Li metal and ^tBuOH, with subsequent addition of B(OBu^t)₃ in toluene, yielded the pale-yellow crystalline title compound, (I), after removal of the solvent and crystallization of the residue, although the product contained crystals of various types and was not analytically pure. Mass spectral analysis was hampered by the facile decomposition of the butoxide groups. The compound is volatile and is soluble in methanol and toluene.

The molecular structure of (I) is shown in Fig. 1, with a view along the crystallographic threefold rotation axis, and the central core, without alkyl substituents, is in Fig. 2. The molecule has crystallographic 3 symmetry. The formula may be conveniently rewritten as [{(^tBuOLi)₃(Li₃BO₃)}₂(^tBuOLi)₆], to illustrate the way in which the molecule is constructed from simple units, and these units are emphasized by the different colours employed in the scheme representing the structural formula in the on-line version of the journal. A total of 18 lithium cations and 12 alkoxide anions encapsulate two borate anions, BO₃³⁻. In the centre of the molecule, six Li⁺ and six alkoxide ions form a 12-membered ring (red in the Scheme); this structure resembles that of [Li₆(C₄₃H₆₁O₃)₂], a lithium complex of a tridentate ligand that effectively enforces such a geometrical arrangement (Dinger & Scott, 2000) but seems to be otherwise unknown in lithium alkoxide chemistry. Each end of the molecule is formed by a chair-shaped Li₃O₃ ring of three cations and three alkoxide anions (blue in the Scheme), a common motif in lithium alkoxide and amide structural chemistry. Sandwiched between the central ring and each end ring is a planar borate anion, BO₃³⁻ (green in the Scheme), with three charge-balancing Li⁺ cations (purple) bridging pairs of borate O atoms. As a result, 12 of the cations have threefold coordination (pyramidal for Li1 in the end rings, and essentially planar for Li3 in the centre ring), and the other six, linking the end and centre rings, have a distorted fourfold coordination (Li2), all bonds to lithium being from the O atoms of alkoxide and borate anions. All 12 alkoxide ligands are triply bridging, while the borate O atoms coordinate to four lithium ions each, as well as being bonded to B atoms, a high coordination number for oxygen. Nevertheless, the B—O bonds are not unusually long. Li—O bond lengths range from 1.862 (5) to 2.194 (5) Å, both the shortest and the longest being for Li2, which may be regarded as having a primary twofold coordination, with its shortest bonds to the end and centre lithium–alkoxide rings, and two weaker interactions with a borate anion. This structure reinforces the view of the molecule as a cationic [Li₁₈(O^tBu)₁₂]⁶⁺ cage encapsulating two intact and recognizable BO₃³⁻ borate anions.

The molecular structure of (I) is unusual in a number of ways. High-nuclearity lithium alkoxides are rare; heteroleptic lithium complexes of simple alkoxides and aryloxides are usually small cage or ring oligomers with up to eight lithium centres, and incorporation of neutral auxiliary ligands generally reduces the nuclearity, with four-membered ring dimers and pseudo-cubanes being common [a search of the Cambridge Structural Database (Version 5.24, updated in February 2003; Allen, 2002) gives about 30 relevant structures]. An exception is the polymeric chain structure of lithium phenolate (Dinnebier et al., 1997). Lithium alkoxides of higher nuclearity are found only with additional bridging ligands, such as in Li₃₃H₁₇(O^tBu)₁₆ (Hoffmann et al., 1998) and Li₁₆(OH)₆(O^tBu)₁₀ (Lambert et al., 1995); the homoleptic peroxide complex Li₁₂(OO^tBu)₁₂ is dodecanuclear (Boche et al., 1996).

As far as we are aware, there are no previously reported polynuclear lithium borate complexes. Only one example is known of a BO₃³⁻ anion coordinated to three metal atoms, viz. BO₃(SnPh₃)₃ (Ferguson et al., 1995), analogous to the corresponding silicon compound, BO₃(SiPh₃)₃, which is known in two solvate forms (Murphy et al., 1993; Beckett et al., 1998).

S2. Experimental

A solution of B(OBut)₃ (1.91 ml, 6.75 mmol) in toluene (15 ml) was added to LiO^tBu (0.54 g, 6.75 mmol). After heating to dissolve the reactants, the resulting solution was filtered and the solvent was removed in vacuo. Recrystallization from toluene (10 ml), with cooling to 243 K, gave a large crop of crystals over two months.

S3. Refinement

Cooling the crystals below 200 K led to fragmentation, presumably as a result of a phase transition. At 200 K, high-angle data are relatively weak because of the high displacement of atoms of the alkyl groups, leading to a structure of only moderate precision.

After confirming their location in a difference map, H atoms were positioned geometrically and refined with a riding model, including free methyl rotation about C—C bonds, with U_iso(H) values equal to 1.5U_eq(C).

Computing details

Data collection: SMART (Bruker, 2001); cell refinement: local programs; data reduction: SAINT (Bruker, 2001); program(s) used to solve structure: SHELXTL (Sheldrick, 2001); program(s) used to refine structure: SHELXTL; molecular graphics: SHELXTL; software used to prepare material for publication: SHELXTL and local programs.

Figures

	Fig. 1. A view of the molecule of (I) along the c axis, showing the 3 symmetry. Bonds to C are hollow and other bonds are filled. Displacement ellipsoids are shown at the 30% probability level, and H atoms have been omitted.
	Fig. 2. The core of the molecule of (I), with ^t-butyl substituents omitted. Atoms of the asymmetric unit are labelled.

Bis(borato)dodeca(tert-butoxo)octadecalithium

Crystal data

[Li18(BO3)2(C4H9O)12]	Dx = 1.040 Mg m^−3
Mr = 1119.88	Mo Kα radiation, λ = 0.71073 Å
Trigonal, R3	Cell parameters from 1806 reflections
a = 19.917 (3) Å	θ = 2.7–25.6°
c = 15.608 (3) Å	µ = 0.07 mm^−1
V = 5362.0 (16) Å^3	T = 200 K
Z = 3	Block, colourless
F(000) = 1812	0.28 × 0.26 × 0.20 mm

Data collection

Bruker SMART 1K CCD diffractometer	1502 reflections with I > 2σ(I)
Radiation source: sealed tube	Rint = 0.070
Graphite monochromator	θmax = 28.5°, θmin = 1.8°
Detector resolution: 8.192 pixels mm^-1	h = −12→26
narrow–frame ω scans	k = −16→24
6484 measured reflections	l = −19→17
2682 independent reflections

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)] = 0.078	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.153	H-atom parameters constrained
S = 1.13	w = 1/[σ^2(Fo^2) + (0.0283P)^2 + 9.227P] where P = (Fo^2 + 2Fc^2)/3
2682 reflections	(Δ/σ)max < 0.001
136 parameters	Δρmax = 0.17 e Å^−3
0 restraints	Δρmin = −0.18 e Å^−3

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

	x	y	z	Uiso*/Ueq
Li1	0.2373 (3)	0.6135 (3)	0.4309 (3)	0.0405 (11)
Li2	0.2591 (3)	0.7279 (3)	0.5276 (3)	0.0393 (11)
Li3	0.1911 (3)	0.5523 (3)	0.6406 (3)	0.0453 (12)
O1	0.26943 (10)	0.71919 (10)	0.40686 (10)	0.0364 (4)
O2	0.25341 (9)	0.62817 (9)	0.55788 (11)	0.0339 (4)
O3	0.21670 (10)	0.47140 (9)	0.62143 (11)	0.0367 (5)
B	0.3333	0.6667	0.5611 (3)	0.0308 (11)
C1	0.23241 (15)	0.73629 (15)	0.33940 (17)	0.0390 (6)
C11	0.1508 (2)	0.7138 (2)	0.3664 (2)	0.0736 (11)
H11A	0.1226	0.6590	0.3826	0.110*
H11B	0.1241	0.7224	0.3186	0.110*
H11C	0.1529	0.7456	0.4154	0.110*
C12	0.22818 (18)	0.68994 (18)	0.26008 (17)	0.0519 (8)
H12A	0.2807	0.7032	0.2428	0.078*
H12B	0.2033	0.7024	0.2134	0.078*
H12C	0.1978	0.6344	0.2730	0.078*
C13	0.2789 (2)	0.82177 (18)	0.3188 (2)	0.0688 (10)
H13A	0.2811	0.8519	0.3694	0.103*
H13B	0.2542	0.8336	0.2714	0.103*
H13C	0.3316	0.8352	0.3021	0.103*
C3	0.15478 (16)	0.39429 (15)	0.62941 (17)	0.0432 (7)
C31	0.08583 (19)	0.38415 (19)	0.5769 (2)	0.0701 (10)
H31A	0.0708	0.4216	0.5961	0.105*
H31B	0.0422	0.3314	0.5846	0.105*
H31C	0.1002	0.3929	0.5162	0.105*
C32	0.1788 (2)	0.33762 (17)	0.5962 (2)	0.0715 (11)
H32A	0.1937	0.3487	0.5358	0.107*
H32B	0.1353	0.2845	0.6018	0.107*
H32C	0.2229	0.3431	0.6296	0.107*
C33	0.1315 (2)	0.37713 (19)	0.7234 (2)	0.0701 (10)
H33A	0.1760	0.3841	0.7572	0.105*
H33B	0.0889	0.3236	0.7292	0.105*
H33C	0.1145	0.4127	0.7443	0.105*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Li1	0.045 (3)	0.042 (3)	0.039 (3)	0.025 (2)	−0.006 (2)	−0.004 (2)
Li2	0.045 (3)	0.042 (3)	0.038 (2)	0.027 (2)	−0.001 (2)	−0.001 (2)
Li3	0.043 (3)	0.034 (3)	0.056 (3)	0.017 (2)	0.015 (2)	0.004 (2)
O1	0.0397 (10)	0.0403 (11)	0.0340 (9)	0.0236 (9)	−0.0040 (8)	0.0019 (8)
O2	0.0285 (9)	0.0328 (10)	0.0398 (10)	0.0149 (8)	0.0030 (8)	0.0037 (8)
O3	0.0378 (10)	0.0240 (9)	0.0399 (10)	0.0092 (8)	0.0015 (8)	0.0002 (7)
B	0.0334 (17)	0.0334 (17)	0.026 (3)	0.0167 (9)	0.000	0.000
C1	0.0416 (16)	0.0403 (16)	0.0399 (15)	0.0241 (14)	−0.0069 (13)	−0.0002 (12)
C11	0.061 (2)	0.117 (3)	0.065 (2)	0.061 (2)	−0.0115 (18)	−0.016 (2)
C12	0.060 (2)	0.057 (2)	0.0410 (16)	0.0310 (17)	−0.0057 (15)	−0.0034 (14)
C13	0.098 (3)	0.045 (2)	0.067 (2)	0.038 (2)	−0.019 (2)	0.0058 (16)
C3	0.0432 (17)	0.0257 (14)	0.0472 (16)	0.0070 (13)	0.0034 (14)	0.0052 (12)
C31	0.052 (2)	0.051 (2)	0.080 (3)	0.0053 (17)	−0.0167 (18)	−0.0019 (18)
C32	0.080 (3)	0.0284 (17)	0.097 (3)	0.0205 (18)	0.009 (2)	−0.0023 (17)
C33	0.069 (2)	0.056 (2)	0.058 (2)	0.0111 (19)	0.0156 (18)	0.0159 (17)

Geometric parameters (Å, º)

Li1—O1	1.906 (5)	C11—H11A	0.980
Li1—O1^i	1.890 (5)	C11—H11B	0.980
Li1—O2	2.006 (5)	C11—H11C	0.980
Li2—O1	1.913 (4)	C12—H12A	0.980
Li2—O2	1.989 (5)	C12—H12B	0.980
Li2—O2^ii	2.194 (5)	C12—H12C	0.980
Li2—O3^ii	1.862 (5)	C13—H13A	0.980
Li3—O2	1.901 (5)	C13—H13B	0.980
Li3—O3	1.942 (5)	C13—H13C	0.980
Li3—O3^iii	1.900 (5)	C3—C31	1.524 (4)
O1—Li1^ii	1.890 (5)	C3—C32	1.521 (4)
O1—C1	1.421 (3)	C3—C33	1.526 (4)
O2—Li2^i	2.194 (5)	C31—H31A	0.980
O2—B	1.3799 (16)	C31—H31B	0.980
O3—Li2^i	1.862 (5)	C31—H31C	0.980
O3—Li3^iv	1.900 (5)	C32—H32A	0.980
O3—C3	1.414 (3)	C32—H32B	0.980
B—O2^ii	1.3799 (16)	C32—H32C	0.980
B—O2^i	1.3799 (16)	C33—H33A	0.980
C1—C11	1.514 (4)	C33—H33B	0.980
C1—C12	1.521 (4)	C33—H33C	0.980
C1—C13	1.511 (4)
O1—Li1—O1^i	133.2 (3)	C1—C11—H11A	109.5
O1—Li1—O2	95.3 (2)	C1—C11—H11B	109.5
O1^i—Li1—O2	101.5 (2)	C1—C11—H11C	109.5
O1—Li2—O2	95.7 (2)	H11A—C11—H11B	109.5
O1—Li2—O2^ii	94.38 (19)	H11A—C11—H11C	109.5
O1—Li2—O3^ii	150.2 (3)	H11B—C11—H11C	109.5
O2—Li2—O2^ii	69.44 (16)	C1—C12—H12A	109.5
O2—Li2—O3^ii	114.1 (2)	C1—C12—H12B	109.5
O2^ii—Li2—O3^ii	94.96 (19)	C1—C12—H12C	109.5
O2—Li3—O3	102.5 (2)	H12A—C12—H12B	109.5
O2—Li3—O3^iii	111.4 (2)	H12A—C12—H12C	109.5
O3—Li3—O3^iii	145.8 (3)	H12B—C12—H12C	109.5
Li1—O1—Li1^ii	98.4 (3)	C1—C13—H13A	109.5
Li1—O1—Li2	85.1 (2)	C1—C13—H13B	109.5
Li1^ii—O1—Li2	84.9 (2)	C1—C13—H13C	109.5
Li1—O1—C1	118.9 (2)	H13A—C13—H13B	109.5
Li1^ii—O1—C1	129.6 (2)	H13A—C13—H13C	109.5
Li2—O1—C1	128.0 (2)	H13B—C13—H13C	109.5
Li1—O2—Li2	80.52 (18)	O3—C3—C31	109.6 (2)
Li1—O2—Li2^i	75.17 (17)	O3—C3—C32	110.4 (2)
Li1—O2—Li3	124.2 (2)	O3—C3—C33	109.3 (2)
Li1—O2—B	100.0 (2)	C31—C3—C32	108.4 (3)
Li2—O2—Li2^i	151.4 (2)	C31—C3—C33	109.5 (3)
Li2—O2—Li3	130.1 (2)	C32—C3—C33	109.6 (3)
Li2^i—O2—Li3	77.15 (18)	C3—C31—H31A	109.5
Li2—O2—B	88.85 (16)	C3—C31—H31B	109.5
Li2^i—O2—B	80.84 (15)	C3—C31—H31C	109.5
Li3—O2—B	122.0 (2)	H31A—C31—H31B	109.5
Li2^i—O3—Li3^iv	90.9 (2)	H31A—C31—H31C	109.5
Li2^i—O3—Li3	84.7 (2)	H31B—C31—H31C	109.5
Li2^i—O3—C3	132.8 (2)	C3—C32—H32A	109.5
Li3—O3—Li3^iv	90.3 (3)	C3—C32—H32B	109.5
Li3—O3—C3	116.1 (2)	C3—C32—H32C	109.5
Li3^iv—O3—C3	128.0 (2)	H32A—C32—H32B	109.5
O2—B—O2^ii	119.86 (3)	H32A—C32—H32C	109.5
O2^ii—B—O2^i	119.86 (3)	H32B—C32—H32C	109.5
O2—B—O2^i	119.86 (3)	C3—C33—H33A	109.5
O1—C1—C11	109.4 (2)	C3—C33—H33B	109.5
O1—C1—C12	109.6 (2)	C3—C33—H33C	109.5
O1—C1—C13	109.2 (2)	H33A—C33—H33B	109.5
C11—C1—C12	108.7 (2)	H33A—C33—H33C	109.5
C11—C1—C13	110.8 (3)	H33B—C33—H33C	109.5
C12—C1—C13	109.1 (2)

Symmetry codes: (i) −y+1, x−y+1, z; (ii) −x+y, −x+1, z; (iii) y−1/3, −x+y+1/3, −z+4/3; (iv) x−y+2/3, x+1/3, −z+4/3.