Guanidinium and imidazolium borates containing the first examples of an isolated nonaborate oxoanion: [B9O12(OH)6]3-

Full text of DOI:10.1021/ic000217u

2250
Inorg. Chem. 2000, 39, 2250-2251
Guanidinium and Imidazolium Borates Containing the First Examples of an Isolated Nonaborate
Oxoanion: [B9O12(OH)6]3-
David M. Schubert,*,1a Mandana Z. Visi,1a and Carolyn B. Knobler1b
U.S. Borax Inc., 26877 Tourney Road, Valencia, California, 91355-1847
ReceiVed February 28, 2000
Borate compounds in which boron is bound only to oxygen
have considerable mineralogical and industrial importance. These
contain anionic components composed of BO3 and BO4 groups
that may link together by sharing oxygen atoms to form isolated
rings and cages or polymerize into infinite chains, sheets, and
networks. Many examples of isolated boron oxoanions containing
one to six borons exist in mineral and synthetic borates. However,
isolated boron oxoanions having more than six borons are rare
and no previous examples have nine borons.2 This communication
describes the synthesis and structural characterization of crystalline
guanidinium and imidazolium borates, [C(NH2)3]3[B9O12(OH)6]
(I) and [C3H5N2]3[B9O12(OH)6] (II), containing the first examples
of the isolated nonaborate anion, [B9O12(OH)6]3-. These borates
have resolved oxide formulas [C(NH2)3]2O‚3B2O3‚2H2O and
[C3H5N2]2O‚3B2O3‚2H2O.
Isolated boron oxoanions are found in the commercially
significant metaborates ([B(OH)4]-), tetraborates ([B4O5(OH)4]2-),
and pentaborates ([B5O6(OH)4]-), as well as in triborates
([B3O3(OH)4]- and [B3O3(OH)5]2-).3 Also, some mineral and
synthetic borates contain the isolated hexaborate anion,
[B6O7(OH)6]2-.2 The mineral ammonioborite contains an unusual
example of a large isolated anion, [B15O20(OH)8]3-.2 Polyborate
anions can be regarded as products of varying degrees of
neutralization of orthoboric acid, B(OH)3, with a strong base in
relatively concentrated aqueous solution. The [B3O3(OH)4]- and
[B6O7(OH)6]2- anions, as well as the [B9O12(OH)6]3- anion
described herein, can all be viewed as products of a 1/3 neutraliza-
tion with strong base of boric acid to [B(OH)4]-.
At least three crystalline binary guanidinium borates exist.
Guanidinium tetraborate, [C(NH2)3]2[B4O5(OH)4]‚2H2O (III), was
reported as early as 1921,4 and brief references are made to
guanidinium pentaborates, including [C(NH2)3][B5O6(OH)4]‚2H2O
(IV).5 We have recently found that the novel guanidinium
nonaborate I crystallizes in the [C(NH2)3]2O-B2O3 aqueous
system at temperatures above ca. 45 °C at B2O3/[C(NH2)3]2O mole
ratios (defined as q) greater than ca. 2.5 over a range of
concentrations. Tetraborate III crystallizes at lower temperatures
or q values. At q > 5 either boric acid or guanidinium pentaborate
IV crystallizes, depending on concentrations. Nonaborate I can
be prepared by aqueous stoichiometric or near-stoichiometric
reaction of guanidinium carbonate with boric acid:6
18B(OH)3 + 3[C(NH2)3]2CO3 >45 °C8
H2O
2[C(NH2)3]3[B9O12(OH)6] + 3CO2 + 21H2O (1)
Alternatively, I can be prepared by reaction of guanidinium salts
with borax and boric acid under appropriate conditions.7 Non-
aborate I exhibits temperature-dependent stability in water contact.
At 20 °C, an aqueous slurry of I converts to tetraborate III and
boric acid in about 1 day. At elevated temperatures, I is stable in
aqueous slurry and can be recrystallized from water. Nonaborate
I can also be prepared by aqueous reaction of tetraborate III with
boric acid in hot water.8
We recently found that imidazolium nonaborate II crystallizes
from aqueous mixtures of boric acid and imidazole between room
temperature and 100 °C:9
9B(OH)3 + 3C3H4N2 H O8
2
[C3H5N2]3[B9O12(OH)6] + 9H2O (2)
In contrast to I, nonaborate II is stable in contact with water at
room temperature. The guanidine-borate and imidazole-borate
systems contrast with the extensively studied ammonia-borate
system, where ammonium tetraborate and pentaborate occur under
comparable conditions under which I and II are formed.5
Single-crystal X-ray structures of I and II are shown in Figures
1 and 2.10 The [B9O12(OH)6]3- anion found in each consists of
four B3O3 rings sharing three tetrahedral boron centers in a linear
arrangement. The remaining six boron centers are trigonal with
one attached hydroxyl group. The two inner B3O3 rings each
contain one trigonal and two tetrahedral borons, and the two outer
(6) To a solution of 360.3 g (2.0 mol) guanidinium carbonate in 1.00 L of
water at 45 °C was added 742.0 g (12.0 mol) of boric acid. Mild
effervescence occurred. The mixture was heated to 90 °C, maintained
with stirring for 1 h, then cooled to 45 °C and filtered (pH 8.1). Product
was washed with water and dried at 105 °C, giving 668.4 g (88% yield)
white powder (I). Anal. Calcd: C, 6.30%; H, 4.27%; N, 22.06%; B,
17.02%. Found: C, 6.25%; H, 4.33%; N, 21.46%; B, 17.32%. Crystals
precipitated from the filtrate upon standing (38 g), identified as
guanidinium tetraborate, II, by XRD analysis.
(7) To a solution of 47.8 g (0.50 mol) guanidinium chloride in 225 mL of
water at 40 °C was added 30.9 g (0.50 mol) of boric acid and 72.8 g
(0.25 mol) of Na2B4O7‚5H2O. The mixture was heated to 90 °C,
maintained with stirring for 1 h, then cooled to 45 °C and filtered (pH
8.2). The resulting powder was dried at 105 °C and identified as I by
XRD and titration analysis.
(8) To 86.9 g (0.25 mol) of guanidinium tetraborate, III, in 225 mL of water
at 40 °C was added 30.9 g (0.50 mol) of boric acid. The mixture was
heated to 90 °C and maintained with stirring for 1 h, then cooled to 45
°C and filtered. The resulting product was dried at 105 °C and identified
as I by XRD and titration analysis.
(9) To a solution of 70.0 g (1.0 mol) imidazole in 225 mL of water at 40 °C
was added 185.5 g (3.0 mol) of boric acid. The mixture was heated to
90 °C and maintained with stirring for 1 h, cooled to 45 °C, and filtered
(pH 7.8). The resulting white powder (II) was washed with water and
dried at 105 °C. Anal. Calcd: C, 18.06%; H, 3.53%; N, 14.04%; B,
16.25%. Found: C, 18.13%; H, 3.49%; N, 14.12%; B, 16.67%.
* To whom correspondence should be addressed. Phone: (661)287-6074.
Fax: (661)287-6014. E-mail: dave.schubert@borax.com.
(1) (a) U.S. Borax Inc. (b) McCullough Crystallographic Laboratory,
Department of Chemistry and Biochemistry, University of California,
Los Angeles.
(2) Grice, J. D.; Burns, P. C.; Hawthorne, F. C. Can. Mineral. 1999, 37,
731-762.
(3) The sodium salt of the tetraborate anion, known as borax pentahydrate
(Na2[B4O5(OH)4]‚3H2O ) Na2O‚2B2O3‚5H2O), has by far the greatest
industrial significance with annual world production in excess of one
million metric tons.
(4) (a) Rosenheim, A.; Leyser, F. Z. Anorg. Allg. Chem. 1921, 119, 1-38.
(b) Weakley, T. J. R. Acta Crystallogr. 1985, C41, 377-379.
(5) Bowden, G. H. In Supplement to Mellor’s ComprehensiVe Treatise on
Inorganic and Theoretical Chemistry, Volume V, Boron, Part A: Boron-
Oxygen Compounds; Longman Group Ltd.: London, 1980.
10.1021/ic000217u CCC: $19.00 © 2000 American Chemical Society
Published on Web 04/29/2000

Communications
Inorganic Chemistry, Vol. 39, No. 11, 2000 2251
average B-O distances around tetrahedral and trigonal borons
are 1.480 and 1.370 Å, respectively. The interatomic distances
between the tetrahedral boron centers B3 and B8 and B8 and
B23 are 2.548(4) and 2.604(4) Å, respectively. Compound II also
exhibits extensive hydrogen bonding linking adjacent anions as
well as anions and cations. There is no significant difference
between intracyclic and exocyclic B-O bond lengths in either I
or II.
Polymeric borate structures can be described in terms of
compact, insular groups, referred to as fundamental building
blocks (FBB’s), which form the basis of classification schemes
for crystalline borates.11,12 These schemes define FBB’s by the
number of boron atoms, the number of BO3 and BO4 groups,
and the mode of polymerization between FBB’s to give isolated,
modified isolated, chain, modified chain, sheet, modified sheet,
and network structures. By use of the classification scheme of
Christ and Clark, the nonaborate anion in I and II is described
as “9:6∆ + 3T, isolated”, indicating an isolated B9-FBB having
six BO3 and three BO4 polyhedra.11 However, this scheme cannot
describe connectivity. The recent refinement of borate classifica-
tion schemes proposed by Burns describes this anion as “6∆30:
2∆0 - ∆20 - ∆20 - 2∆0 ”, where ∆ and 0 refer to BO3
Figure 1. Structure of guanidinium nonaborate, I, drawn at 50%
probability level. One of the three guanidinium cations, which has a 2-fold
axis passing through the carbon atom causing a disorder, has been omitted.
and BO4 polyhedra, respectively, and the
-
symbols indicate
rings of polyhedra connected by sharing one boron center.12
Referring to these schemes, one finds only one borate possessing
a FBB similar to the anion in I and II. This is the mineral
preobrazhenskite, Mg3B11O15(OH)9, which contains infinite sheets
based on a modified B9-FBB (9:4∆ + 5T) linked by bridging
-O2B(OH)2- groups.2,13
It has been shown that B3O3 rings having one trigonal and two
tetrahedral borons (∆20) occur more frequently and are more
energetically stable than other configurations.14 Although reaction
of 1 mol of base with 3 mol of boric acid could produce the
[B3O3(OH)4]- (2∆0) anion, formation of the nonaborate anion
allows a larger proportion of boron to participate in ∆20-type
B3O3 rings. The larger nonaborate anion may also be favored by
the presence of larger cations.
Figure 2. Structure of imidazolium nonaborate, II, drawn at 50%
probability level.
rings each contain one tetrahedral and two trigonal borons. In I,
there is a crystallographic 2-fold axis passing through the
centermost boron atom (B8) relating halves of the anion. This
2-fold axis also passes through the carbon atom (CD) of one
guanidinium cation, causing a disorder in this cation. The anion
in II has no crystallographically required symmetry. The non-
aborate anions I and II are stereochemically equivalent.
In the anion of I, the average B-O interatomic distances around
tetrahedral and trigonal borons are 1.471 and 1.363 Å, respec-
tively. The interatomic distance between tetrahedral boron centers,
B3 and B8, is 2.548 Å. Extensive O-H‚‚‚O and N-H‚‚‚O
hydrogen bonding involving all hydroxyl and guanidine hydrogens
links anions and cations as well as adjacent anions. In II, the
Its B/N mole ratio of 1 suggests that I might serve as a boron
nitride precursor. It does not melt below 1000 °C and pyrolyzes
to a white refractory powder. TGA-MS analysis under inert
atmosphere shows that I begins weight loss at ca. 220 °C, initially
involving elimination of H2O by condensation of B-OH groups
followed by loss of some NH3 and CO2 upon further heating,
resulting in a ceramic material having a B/N mole ratio greater
than 1. Compound II exhibits similar thermal decomposition
behavior, with elimination of water commencing at ca. 180 °C.
Supporting Information Available: Crystallographic files in CIF
format for compounds I and II. This material is available free of charge
via the Internet at http://pubs.acs.org.
IC000217U
(10) Crystal data (297 K) for I: C3H24B9O18N9, monoclinic, C2/c, a ) 16.452-
(7) Å, b ) 9.100(4) Å, and c ) 15.266(7) Å, â ) 95.07(1)°, V ) 2276.7-
(17) Å3, Z ) 4, λ ) 0.7107 (Mo KR), R ) 0.066, Rw ) 0.1822 for 2994
unique reflections, 1257 with I > 2σ(I), GOF ) 0.941. Crystal data (297
K) for II: C9H21B9O18N6, monoclinic, P21/c, a ) 16.622(3) Å, b )
9.3324(15) Å, and c ) 15.570(3) Å, â ) 91.170(3)°, V ) 2414.8(7) Å3,
Z ) 4, λ ) 0.7107 (Mo KR), R ) 0.050, Rw ) 0.074 for 5756 unique
reflections, 2863 with I > 2σ(I), GOF ) 0.825.
(11) Christ, C. L.; Clark, J. R. Phys. Chem. Miner. 1977, 2, 59-87.
(12) Burns, P. C. Can. Mineral. 1995, 33, 1167-1176.
(13) Rumanova, I. M.; Razmanova, Z. P.; Belov, N. V. SoV. Phys. Dokl. 1972,
16, 518-521.
(14) Burns, P. C.; Grice, J. D.; Hawthorne, F. C. Can. Mineral. 1995, 33,
1131-1151.