An unprecedented polyborate ester anion: X-ray diffraction studies on [1,8-C10H6(NMe2)2H][B 5O6(OMe)4]

Article abstract of DOI:10.1021/ic700433e

The slow hydrolysis of B(OMe)₃ in a CH₂Cl₂ solution in the presence of 1,8-C₁₀H₆(NMe ₂)₂ (5:1 ratio) led to the formation of the novel isolated pentaborate ester anion [B₅O₆(OMe)₄] ^-, which was characterized by a single-crystal X-ray diffraction study as the salt [1,8-C₁₀H₆(NMe₂) ₂H][B₅O₆(OMe)₄].

Full text of DOI:10.1021/ic700433e

Inorg. Chem. 2007, 46, 3801−3803
An Unprecedented Polyborate Ester Anion: X-ray Diffraction Studies on
[1,8-C10H6(NMe2)2H][B5O6(OMe)4]
Michael A. Beckett,*,† Catherine C. Bland,† Peter N. Horton,‡ Michael B. Hursthouse,‡ and
K. Sukumar Varma§
School of Chemistry, UniVersity of Wales, Bangor, Gwynedd LL57 2UW, U.K., School of
Chemistry, UniVersity of Southampton, Southampton SO17 1BJ, U.K., and Pilkington Building
Products R and D, European Technical Centre, Lathom, Lancashire L40 5UF, U.K.
Received March 6, 2007
The slow hydrolysis of B(OMe)3 in a CH2Cl2 solution in the presence
of 1,8-C10H6(NMe2)2 (5:1 ratio) led to the formation of the novel
isolated pentaborate ester anion [B5O6(OMe)4]-, which was
characterized by a single-crystal X-ray diffraction study as the salt
[1,8-C10H6(NMe2)2H][B5O6(OMe)4].
solvent, CH2Cl2.3 Unexpectedly, the novel tetramethoxy-
pentaborate ester salt [C10H6(NMe2)2H][B5O6(OMe)4] (1) was
obtained, and this Communication describes its serindipitous
synthesis and subsequent characterization by a single-crystal
X-ray diffraction study.
Well-characterized homoleptic methoxyborate systems
have hitherto been restricted to the borate esters B(OMe)3,4a
B2(OMe)4,5 and B3O3(OMe)34b and the anion6 in M[B(OMe)4]
Polyborate systems are a class of compounds that are
undergoing a rapid resurgence of academic interest because
of the richness of diversity of their intrinsic structures, their
potential as precursors to new porous materials, and their
practical applications in fluorescence, piezoelectric, and
second-harmonic-generation optical materials.1 We have
recently been exploring solid-state pentaborate chemistry and
are currently investigating structure-directing effects of a
variety of “non-innocent” substituted alkylammonium cat-
ions.2 During the course of these investigations, we examined
the conjugate acid of 1,8-bis(dimethylamino)naphthalene
(proton sponge, PS) as a potential cation. We anticipated
that under ambient conditions the pentaborate salt [C10H6-
(NMe2)2H][B5O6(OH)4] would be formed from controlled
slow hydrolysis of the borate ester B(OMe)3 in a nonaqueous
1
(M ) Li, K, /2Sr). B2(OMe)4 is unique within the series
because it contains a B-B bond. A less-well-characterized
polyborate, B8O9(OMe)6, has also been described;7 it is
believed to contain boroxyl (B3O3) rings but is unlikely to
be a pure compound. To our knowledge, 1 is the first
structurally characterized anionic polyborate ester. It is also
the first compound containing a spirocyclic tetraalkoxy-
pentaborate(1-) anion and as such is a rare example of a
spirocyclic nonhydroxypentaborate(1-) anion; the few previ-
ously characterized examples have been hydrolytically
stabilized by B-C bonds.8
The reaction of the proton sponge with B(OMe)3 in
reagent-grade CH2Cl2 in a 1:5 ratio gave, after 2 weeks and
evaporation of ∼50% of the solvent, a purple solution and
a high yield of a “wet” crystalline solid.9 Crystals of 1
suitable for a single-crystal X-ray diffraction10 study were
obtained directly from this crude material. Elemental analysis
data of the isolated solid were inconsistent with the structural
* To whom correspondence should be addressed. E-mail: m.a.beckett@
bangor.ac.uk. Tel: +44-1248-382374. Fax: +44-1248-370528.
† University of Wales, Bangor.
‡ University of Southampton.
§ European Technical Centre.
(1) (a) Liu, G.-Z.; Zheng, S.-T.; Yang, G.-U. Inorg. Chem. Commun. 2007,
10, 84-87. (b) Liu, Z.-H.; Li, L.-Q.; Zhang, W.-J. Inorg. Chem. 2006,
45, 1430-1432. (c) Schubert, D. M.; Visi, M. Z.; Knobler, C. B. Inorg.
Chem. 2000, 39, 2250-2251. (d) Soler-Illia, G. J. de A. A.; Sanchez,
C.; Lebeau, B.; Patarin, J. Chem. ReV. 2002, 102, 4093-4138. (e)
Liu, Z.-H.; Zhang, W.-J.; Zhang, J.-J. Thermochim. Acta 2005, 439,
151-153. (f) Liu, Z.-H.; Zhang, J.-J.; Zhang, W.-J. Inorg. Chim. Acta
2006, 359-524. (g) Wiebcke, M.; Freyhardt, C. C.; Felsche, J.;
Englehardt, G. Z. Naturforsch. 1993, 48b, 978-985.
(2) (a) Beckett, M. A.; Bland, C. C.; Coles, S. J.; Horton, P. N.;
Hursthouse, M. B.; Varma, K. S. J. Organomet. Chem. 2007, in press.
(b) Beckett, M. A.; Bland, C. C.; Stiefvater-Thomas, B. M.; Varma,
K. S. In 11th International Symposium on Inorganic Ring Systems;
Laitinen, R. S., Oilunkaniemi, R., Eds.; Report Series in Chemistry
ISBN 9524281535; Oulu University Press: Oulu, Finland, 2006; Vol.
70, p 23. (c) Beckett, M. A.; Coles, S. J.; Light, M. E.; Fischer, L.;
Stiefvater-Thomas, B. M.; Varma, K. S. Polyhedron 2006, 25, 1011-
1016.
(3) Heller, G. J. Inorg. Nucl. Chem. 1968, 30, 2743-2754.
(4) (a) Heller, G. #44 Boron oxygen compounds 1. Gemlin Handbuch
der Anorganischen Chemie, BorVerbindungen; Springer: Berlin, 1977;
Vol. 13, section 2.3, pp 86-93. (b) Meller, A. #44 Boron oxygen
compounds 1. Gemlin Handbuch der Anorganischen Chemie, Bor-
Verbindungen; Springer: Berlin, 1977; Vol. 13, section 4.5, pp 156
and 157.
(5) (a) Wiberg, E.; Ruschmann, W. Ber. 1937, 70, 1393. (b) Brotherton,
R. J.; McCloskey, A. L.; Boone, J. L.; Manasevit, H. M. J. Am. Chem.
Soc. 1960, 82, 6245-6248.
(6) Heller, G.; Giebelhausen, A. Z. Anorg. Allg. Chem. 1980, 460, 228-
234.
(7) Steinberg, H. Organoboron Compounds; John Wiley and Sons: New
York, 1964; Vol. 1, pp 462-465.
(8) (a) Nishihara, Y.; Nara, K.; Nishide, Y.; Osakada, K. Dalton Trans.
2004, 1366-1375. (b) Nishihara, Y.; Nara, K.; Osakada, K. Inorg.
Chem. 2002, 41, 4090-4092.
10.1021/ic700433e CCC: $37.00
Published on Web 04/10/2007
© 2007 American Chemical Society
Inorganic Chemistry, Vol. 46, No. 10, 2007 3801

COMMUNICATION
data, and subsequent NMR analysis in saturated CD3OD and
D2O solutions revealed that the solid was a mixture of
(several) compounds.11 Consistent with the formation of 1,
a broad peak at δ 15 ppm (three-coordinate B) was observed
in the 11B{1H} NMR spectrum of a saturated CD3OD solution
of the isolated solid, and a singlet at δ 3.37 ppm (MeOB)
was observed in its 1H NMR spectrum. Attempts to
recrystallize or further purify the solid in order to isolate 1
were unsuccessful.
The structure of 1 is shown in Figure 1. The unit cell
comprises four repeating [(MeO)4B5O6]- anions and four
repeating [C10H6(NMe2)2H]+ cations with no hydrogen-
bonding interactions between the cations and anions. There
is a strong unsymmetrical hydrogen-bonding intermolecular
interaction within the cation. The N11‚‚‚N12 distance in the
cation of 1 is at 2.588(3) Å, with the N11-H11‚‚‚N12 angle
at 156° and N11-H11 and H11‚‚‚N12 distances at 1.05(3)
and 1.59(3) Å, respectively, consistent with literature data12
on structures containing this cation. The tetramethoxypenta-
Figure 1. ORTEP drawing of 1 showing the atomic numbering scheme.
Selected bond lengths (Å) and angles (deg) of the [B5O6(OMe)4]- anion in
1: B1-O4, 1.472(4); B1-O1, 1.473(4); B1-O6, 1.473(4); B1-O3,
1.483(4); B2-O1, 1.346(4); B2-O7, 1.373(3); B2-O2, 1.374(3); B3-
O3, 1.335(4); B3-O8, 1.365(4); B3-O2, 1.385(3); B4-O4, 1.344(4); B4-
O9, 1.361(4); B4-O5, 1.384(4); B5-O6, 1.338(4); B5-O10, 1.371(4); B5-
O5, 1.379(4); O1-B1-O3, 110.7(2); O6-B1-O3, 108.5(2); O4-B1-
O1, 109.4(2); O4-B1-O6, 111.1(2); O4-B1-O3, 108.0(2); O1-B1-
O6, 109.2(2); O1-B2-O2, 122.8(2); O1-B2-O7, 119.3(2); O7-B2-
O2, 117.9(3); O3-B3-O8, 124.3(3); O3-B3-O2, 122.1(3); O8-B3-
O2, 113.6(2); O4-B4-O9, 123.0(3); O4-B4-O5, 122.2(3); O9-B4-
O5, 114.7(3); O6-B5-O10, 122.7(3); O6-B5-O5, 122.7(3); O10-B5-
O5, 114.5(3); B2-O1-B1, 121.5(2); B2-O2-B3, 117.8(2); B3-O3-B1,
122.3(2); B4-O4-B1, 123.1(2); B5-O5-B4, 117.7(2); B5-O6-B1,
123.0(2).
(9) To a solution of C10H6(NMe2)2 (1.15 g, 5.35 mmol) in undried reagent-
grade CH2Cl2 (10 mL) was added B(OMe)3 (3.00 mL, 26.76 mmol)
via a syringe. The solution was stirred, sealed, and left at room
temperature for 2 weeks. This resulted in the formation of a pale-
purple solution and few colorless crystals of 1. The flask was left
open to air to facilitate slow evaporation of the solvent, and the product
was obtained as a crop of “wet” colorless crystals (1.75 g, 65%). A
crystal suitable for X-ray diffraction was obtained directly from this
crude product. Mp: 170-173 °C.
Table 1. Average B-O Distances (Å) in 1 and Related Compounds
compound
B(sp2)-OR B-Oâ B-Oγ B(sp3)-OR
(10) Crystal data for 1. A suitable crystal was selected, and data were
collected on a Bruker Nonius Kappa CCD area detector at the window
of a Bruker Nonius FR591 rotating anode (λMo KR ) 0.710 73 Å) driven
by Collect (Hooft, R. Collect: Data collection software; Nonius BV:
Delft, The Netherlands, 1998) and DENZO (Otwinowski, Z.; Minor,
W. Methods Enzymol. 1997, 276; Macromolecular Crystallography;
Carter, C. W., Jr., Sweet, R. M., Eds.; Academic Press: New York,
1997; part A, pp 307-326) software at 120 K. The structure was
determined in SHELXS-97 (Sheldrick, G. M. Acta Crystallogr. 1990,
A46, 467-473) and refined using SHELXL-97 (Sheldrick, G. M.
SHELXL-97; University of Go¨ttingen: Go¨ttingen, Germany, 1997).
Data: C18H31B5N2O10, M ) 489.50, monoclinic, space group ) P21/
c, a ) 9.8513(5) Å, b ) 16.2689(6) Å, c ) 16.0216(8) Å, â )
1
1.341
1.356
1.359
1.34
1.368 1.381
1.367 1.383
1.355 1.384
1.475
1.468
1.468
1.47
Na[B5O6(OH)4]‚3H2O16
[H2NMe2][B5O6(OH)4]17
[Rh(cod)(PPh3)2][B5O6(OH)4]8a
1.36
1.39
borate(1-) anion has the gross spirobicyclic ring structure
commonly observed in [B5O6(OH)4]- compounds but with
methyl groups replacing H atoms. However, in contrast to
[B5O6(OH)4]- salts, which generally contain “isolated”
pentaborate anions as hydrogen-bonded supramolecular poly-
(pentaborate) structures, the anions in 1 are truly isolated.
Selected anion structural data for 1 are given in the caption
of Figure 1.
Average B-O bond lengths for 1 are compared with those
for some related but supramolecular poly(pentaborate)
systems in Table 1, where data involving the trigonal B atoms
are grouped as B-OR, B-Oâ, and B-Oγ, with Oâ being exo
to the ring and OR bridging to the four-coordinate B atom.
It is evident from the data presented that isolation of the
pentaborate anion, albeit with methyl groups replacing H
atoms, has an insignificant impact on the B-O bond lengths
of the exo B-O and spirobicyclic pentaborate ring system.
It has long been believed that hydrolysis of orthoborate
esters is a facile stepwise process, proceeding via four-
coordinate boron intermediates.13 In a few exceptional cases,
(RO)B(OH)2 derivatives have been isolated, whereas (RO)2B-
(OH) derivatives have yet to be reported.13,14 In nonaqueous
107.749(2)°, U ) 2445.6(2) Å3, Z ) 4, µ(Mo KR) ) 0.102 mm-1
31 132 reflections measured, 5608 unique (Rint ) 0.1451), which were
,
used in all calculations. Final R1 ) 0.0676, wR2 ) 0.1155 [F 2
>
2σ(F 2)]; R1 ) 0.1493, wR2 ) 0.1404 (all data). Crystallographic data
(excluding structure factors) for the structure in this paper have been
deposited with the Cambridge Crystallographic Data Centre as
supplementary publication number CCDC 638456. Copies of the data
can be obtained, free of charge, upon application to CCDC, 12 Union
Road, Cambridge CB2 1EZ, U.K. (fax +44(0)-1223-336033 or e-mail
deposit@ccdc.cam.ac.uk).
(11) Saturated solutions of the “wet” solid in CD3OD and D2O were used
for 1H (500 MHz), 13C (125 MHz), and 11B (160 MHz) NMR spectra.
1H NMR (CD3OD): δ 2.78 (s, 12H, MeN, PS), 3.19 (s, 8H, MeN,
PSH+), 3.37 (s, 6H, MeO), 6.95 (d, 2H, Ar-CH, J ) 7.6 Hz, PS),
7.26 (t, 2H, Ar-CH, J ) 7.6 Hz, PS), 7.33 (d, 2H, Ar-CH, J ) 7.9
Hz, PS), 7.72 (t, 1.3H, Ar-CH, J ) 7.9 Hz, PSH+), 8.00 (d, 1.3H,
Ar-CH, J ) 7.6 Hz, PSH+), 8.06 (d, 1.3H, Ar-CH, J ) 8.2 Hz,
PSH+). 11B NMR (CD3OD): δ 15.6 (96%), 18.7 (4%). 11B NMR
(D2O): δ 12.9 (61%), 17.36 (39%). 13C NMR (CD3OD): δ 44.91
(MeN, PS), 46.68 (MeN, PSH+), 113.90 (Ar-CH, PS), 120.37 (Ar-
CH, PSH+), 122.05 (Ar-CH, PS), 122.71 (Ar-CH, PSH+), 122.94
(Ar-CH, PS), 126.45 (Ar-CH, PS), 128.24 (Ar-CH, PSH+), 130.64
(Ar-CH, PSH+), 137.09 (Ar-CH, PSH+), 139.32 (Ar-C, PS), 145.59
1
(Ar-CH, PSH+), 151.99 (Ar-C, PS). Integration of the CD3OD H
NMR spectra suggests a PS/PSH+ ratio of 3:2, corresponding to an
overall sample composition of ∼3:15:2 PS/B(OH)3/1. Elem anal. Calcd
for 1 (C18H31B5N2O10): C, 44.2; H, 6.4; N, 5.7. Calcd for 3:15:2 PS/
B(OH)3/1 (C78H161B25N10O65): C, 36.8; H, 6.4; N, 5.5. Found: C,
33.9; H, 6.0; N, 5.5.
(13) Steinberg, H. Organoboron compounds; John Wiley and Sons: New
York, 1964; Vol. 1, pp 842 and 843.
(12) (a) Adler, R. W. Chem. ReV. 1989, 89, 1215-1223. (b) Ozeryanskii,
V. A.; Pozharskii, A. F.; Glowiak, T.; Majerz, I.; Sobczyk, L.; Grech,
E.; Nowicka-Schiebe, J. J. Mol. Struct. 2003, 607, 1-8.
(14) Green, D. B. Boron (Part A, boron-oxygen compounds). Supplement
to Mellor’s ComprehensiVe Treatese on Inorganic and Theoretical
Chemistry; Longman: London, 1980; Vol. V, pp 703-720.
3802 Inorganic Chemistry, Vol. 46, No. 10, 2007

COMMUNICATION
solvents, limited hydrolysis of orthoborate esters leads to
condensation and the formation of B3O3(OR)3 or poly-
borates.3,13 A possible mechanism of this reaction has also
been proposed and the reaction’s outcome attributed, in the
absence of excess H2O, to limited stability associated with
B centers possessing one alkoxy substituent.6 The isolation
of 1 from our reaction mixture further supports this proposal.
Dissolution of our isolated solid in D2O (rather than CD3OD)
results in a different 11B NMR spectrum. Although 11B NMR
spectra of aqueous polyborate systems are difficult to
interpret because of the many complex equilibria present,15
the observed spectrum is consistent with full hydrolysis of
1 to the expected hydroxyborate products [B3O3(OH)4]- and
B(OH)3.
The existence of 1, and related compounds with B-C
bonds, clearly demonstrates that the synthesis of hybrid
inorganic/organic polyborate systems is possible, and further
research in this area is ongoing. The physical and chemical
properties of these new materials needs further research
because in time they may well rival the industrial importance
and applications associated with inorganic polyborate com-
pounds.
Acknowledgment. We thank the EPSRC for funding of
the X-ray crystallographic service at the University of
Southampton. We also thank Pilkington Plc for financial
support.
Supporting Information Available: Crystallographic files in
CIF format. This material is available free of charge via the Internet
at http://pubs.acs.org.
(15) Salentine, C. G. Inorg. Chem. 1983, 22, 3920-3924.
(16) Merlino, S.; Sartori, F. Acta Crystallogr. 1972, B28, 3559-3567.
(17) Barber, R. A. Charmant, P. H.; Norman, N. C.; Orpen, A. G.; Rossi,
J. Acta Crystallogr., Sect. E 2004, E60, o1086-o1088.
IC700433E
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