metal-organic compounds
Acta Crystallographica Section C
Crystal Structure
Communications
(Chen et al., 2003). Oxidation of the methyl groups with
KMnO4 under basic conditions (Tao et al., 2002), followed by
acidification to pH 2, resulted in the formation of a white solid
material, which was anticipated to be the target structure, (2).
Instead, X-ray crystallographic analysis demonstrated that it
was in fact sodium bis(6-carboxy-1-hydroxy-3-oxo-1,3-di-
hydro-2,1-benzoxaborol-1-yl)oxidanium, (3), a novel type of
structure, containing two units of cyclic boronic carboxylic
anhydride connected via a formally positively charged oxygen
bridge.
ISSN 0108-2701
A sodium salt of the dimer of borono-
terephthalic acid anhydride
Scott Simmons, Albert Fratini and Vladimir Benin*
Department of Chemistry, University of Dayton, 300 College Park, Dayton,
OH 45469-2357, USA
Correspondence e-mail: vladimir.benin@notes.udayton.edu
Received 7 January 2011
Accepted 3 March 2011
Online 12 April 2011
The title compound, sodium bis(6-carboxy-1-hydroxy-3-oxo-
1,3-dihydro-2,1-benzoxaborol-1-yl)oxidanium, Na+ꢀC16H15B2-
O13ꢁ, was prepared in two steps from 2-bromo-p-xylene. Its
crystal structure was determined at 140 K and has triclinic
(P1) symmetry. The compound presents a unique structural
motif, including two units of the cyclic anhydride of
boronoterephthalic acid, joined by a protonated, and thereby
trivalent, oxonium center. Association in the crystal is realized
by complementary hydrogen bonding of the carboxyl groups,
as well as by coordination of the sodium cations to the oxygen
centers on the five-membered rings.
The asymmetric unit of (3) exhibits an almost perfect
stacking of the two aromatic rings (Fig. 1), as evidenced by the
torsion angles B2—O1—B1—O3 and B1—O1—B2—O8, both
close to 180ꢂ (Table 1). The dimer is highly symmetric with
Comment
Flame retardants are widely used in polymeric and composite
materials, and their share of the overall polymer additive
market has grown to nearly 30%. While almost half of the
materials in use are halogenated structures, increased
government scrutiny and environmental and toxicology
problems have played an important role in the recent surge of
research work and commercial activity related to the
discovery and implementation of new nonhalogenated flame
retardants. The latter are frequently phosphorus- or nitrogen-
based, but there has also been increasing interest in boron-
containing compounds (Pitts, 1973; Troitzsch, 1998; Morgan et
al., 2000). Borates and boric acid have been shown to act as
synergists and are often used in combination with other flame-
retardant substances, such as halogenated materials (Wu et al.,
2007). Recent years have also seen developments in the area
of boron-containing reactive flame retardants (Armitage et al.,
1996).
In our ongoing effort to design and prepare novel boron-
based reactive flame retardants, we recently endeavored to
prepare boronoterephthalic acid, (2), which led to an unex-
pected synthetic outcome, illustrated in the Scheme. The
starting material, 2-bromo-p-xylene, was converted to a
Grignard reagent under standard conditions, followed by
reaction of the latter with trimethyl borate and subsequent
acidification, to generate 2,5-dimethylphenylboronic acid, (1)
˚
respect to the central O1 atom, with B1—O1 = 1.559 (2) A and
˚
B2—O1 = 1.549 (3) A. The B centers are each part of a five-
membered ring, properly described as a cyclic mixed carb-
oxylic–boronic acid anhydride ring, with a hydroxy group
attached to the B atom. The B—OH distances [1.428 (3) and
˚
1.435 (3) A] are virtually identical, and considerably elon-
gated compared with typical distances for B—O(H) bonds in
related arylboronic acids, such as 4-carboxyphenylboronic
˚
acid [1.275 (3) A; SeethaLekshmi & Pedireddi, 2007] or 4-car-
˚
boxy-2-nitrobenzeneboronic acid [1.346 (3) and 1.365 (4) A;
Soundararajan et al., 1993]. The B—O distances in the five-
membered rings of (3) are even longer, at 1.509 (3) and
˚
1.524 (3) A. Two plausible reasons may be advanced to
account for the differences. First, the degree of B—O
ꢀ-conjugation, which is greater when boron is in a trigonal-
planar environment, results in shorter B—O distances. Thus,
˚
an increase of about 0.1 A is observed for the B—O distance
upon transition from phenylboronic acid [tricoordinated
˚
boron, B—O = 1.371 (7) A] to its diethanolamine adduct
˚
[tetracoordinated boron, B—O = 1.469 (3) and 1.457 (3) A;
Rettig & Trotter, 1975, 1977] or the 2,6-dimethanolpyridine
˚
adduct [B—O = 1.479 (6) and 1.459 (6) A; Vargas et al., 2005].
A second factor could be the steric crowding, which is greater
Acta Cryst. (2011). C67, m123–m125
doi:10.1107/S0108270111008195
# 2011 International Union of Crystallography m123