R.A. Franich et al. / Polyhedron 30 (2011) 2884–2889
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steric factors (e.g., accessibility of the boron atom within the ‘tryp-
tych’ structure, related to the O–B–O bond angles), and electronic
factors, (molecular orbital and dipole moment). With simple al-
kyl-substituted boratranes, such as tri-iso-propanolamine borate,
the hydrolysis rate of the boratrane was considered to be related
to the length of the N ? B dative bond and distortions from the
ideal tetrahedral bond angles around the N and B atoms as a result
of steric interactions between the alkyl substituents [2,3].
Fundamental knowledge of the structural features of the boratr-
anes described in this paper, and relating these to the compound
hydrolysis rates has enabled a rational approach to design, synthe-
sis and formulation for practical application of wood protection
chemicals based on substituted five-membered ring boratranes.
2. Experimental
In the classical, acyclic donor-acceptor complexes of boron tri-
fluoride and borane with ammonia and alkylamines, the N ? B
bond lengths and bond angles have been determined from both
experimental (X-ray crystallography and gas-phase microwave
spectroscopy) and theoretical quantum mechanical calculations
[11]. The N ? B bond length was found to be shorter in the crystal
(e.g., for H3N ? BH3, 156.4 Å) than in the gas phase, 165.7 Å [11],
and bond angles close to those of tetrahedral coordinate B and N
atoms. Large-ring boron-amine complexes, such as with porphine
and porphyrin ligands [12,13] also have N ? B bond lengths in
the range 1.54–1.60 Å, and bond angles close to tetrahedral N
and B coordination, determined by X-ray crystallography. Small-
ring, highly-strained boron–amine complexes have much-short-
ened (e.g., 1.375 Å) N ? B bond lengths [14].
2.1. Materials and methods
All chemicals and reagents were from Sigma–Aldrich and were
used as received. Infrared (IR) spectroscopy was carried out using a
Bruker Vector 33 spectrometer using samples prepared as KBr
discs. Nuclear magnetic resonance (NMR) spectroscopy was carried
out in CDCl3 solution using a Bruker Avance DPX400 instrument
operating at 100.13 MHz for 13C, 400.13 MHz for 1H and
128.38 MHz for 11B nuclei, relative to external standard BH4 at
ꢀ
0 ppm chemical shift. Gas chromatography–mass spectrometry
(GC–MS) was carried out using an Agilent 5790 instrument fitted
with a 30 m ꢁ 0.2 mm HP5 fused-silica capillary column and he-
lium as the carrier gas at 1 mL minꢀ1 flow rate using a temperature
program from 40–280 °C at 4 °C/min. Spectra were acquired in
Whereas N ? B bond lengths determined by X-ray crystallogra-
phy represent the molecular structure in its solid state, in solution
the N ? B bond is dynamic, lengthening in the presence of nucle-
ophiles which approach the B atom in an SN2-type mechanism.
An example displaying this in a crystal structure is a 2,6-bis(N,N-
dimethylaminomethyl)phenylboronate ester, where the N ? B
bond length is stretched to 1.762 Å and the bond angles (116–
121°) around the B atom are flattened close to trigonal coordina-
tion, as would be expected in an SN2-type transition state [15].
There are few published X-ray crystal structures of boratranes.
The simplest, triethanolamine borate (referred to as ‘boratrane’ in
the literature) was shown to have an N ? B bond length of
1.655 Å [16,17]. This contrasts with the N ? B bond length,
1.846 Å, determined in the gas phase by electron diffraction
[18,19]. A crystalline derivative of triethanolamine borate with
two phenyl rings on the C atoms of one ring has a N ? B bond length
of 1.685 Å [20]. Crystal structures of methyl-substituted five-mem-
bered ring boratranes have been recently reported, with N ? B
bond-lengths of 1.663 Å for the hexamethyl-, and 1.684 Å for the
tetramethyl-derivative [21]. The structure of the six-membered
ring boratrane, tri-n-propanolamine borate 1 has been reported
[22], but was derived from visually-estimated photographic data
(R1 = 0.12), and with no coordinates published. A more precise
structure determination was therefore undertaken, and reported
here, as the only example of a six-membered ring system boratrane
for which X-ray crystallographic data have been obtained. Among
the various water-insoluble C-substituted five-membered ring sys-
tem boratrane compounds synthesised as candidate wood protec-
tion chemicals [10], few formed crystals suitable for X-ray study.
One compound, 2 (Fig. 1) did give suitable crystals and the structure
of this boratrane is also reported.
electron ionization mode at 70 eV, 300 lA electron energy, and
are reported as m/z mass-to-charge ratio and intensity relative to
the spectrum base peak.
2.2. Preparation and characterization of the boratranes 1 and 2
2.2.1. Synthesis of 2,10,11-trioxa 6-aza-1-boratricyclo[4.4.4.01,6
]
tetradecane (1)
Tris(propan-3-ol)amine was prepared by LiAlH4 reduction of
tris(2-carbethoxyethyl)amine, itself prepared by Michael addition
of liquid anhydrous ammonia to ethyl acrylate [23]. To tris(pro-
pan-3-ol)amine (19 g, 0.1 mol) placed in a round-bottomed flask
was added toluene (100 mL) and boric acid (6.18 g, 0.1 mol) and
the mixture was stirred and heated under a Dean-Stark water sep-
arator until the theoretical volume of water had been collected.
The solution was cooled, filtered from a small quantity of residue,
and concentrated to give a white solid. A concentrated solution of
this solid was made in boiling 95% ethanol, which was allowed to
gradually cool. Crystals of 1 deposited as needles which were iso-
lated by filtration and dried at ambient temperature in vacuo.
GC–MS analysis of 1 showed a single peak at 42.85 min retention
time.
M.p. 248 °C IR (cmꢀ1) 3420 (OH, water of crystallization), 2956,
2927, 2883 (CH2 str.), 1257 (B–O str.), 592 (N–B str.).
1H NMR (ppm) 2.03 (m, 6H, –CH2–), 4.0 (t, 6H, N–CH2–), 5.09
(m, 6H, O–CH2–). 13C NMR (ppm) 23.6 (–CH2–), 54.1 (N–CH2–),
61.4 (O–CH2–). 11B NMR (ppm) 0.2 GC–MS (m/z) 199 (M+ꢂ11B),
198 (M+ꢂ10B), 140 (100%), 82 (61%), 57 (63%), 42 (64%).
2.2.2. Synthesis of 3-(4-methoxy)phenoxymethyl-7,10-dimethyl-
2,8,9-trioxa-5-aza-1-boratricyclo[3.3.3.01,5]-undecane (2)
To racemic 3-(4-methoxy)-phenoxypropan-1,2-oxirane (1.8 g,
0.01 mol) was added di-iso-propanolamine (1.33 g, 0.01 mol) and a
few small crystals of toluene-p-sulfonic acid. The mixture was stir-
red and heated without solvent at 60 °C for 5 h. The crude alkanol-
amine product was added to toluene (50 mL) in a round-bottomed
flask, and boric acid (0.62 g, 0.01 mol) was added. The mixture was
stirred and heated under Dean-Stark water separation conditions
until the theoretical volume of water had been obtained. The cooled
solution was filtered, and then concentrated to give a white solid. A
portion of the crude boratrane product was dissolved in hot 95% eth-
anol, allowed to cool and to crystallise. The needle-shaped crystals
were filtered and dried at ambient temperature in vacuo. GC–MS
O
OMe
N
B
N
B
O
O
O
O
O
O
(1)
(2)
2,10,11-trioxa-6-aza-boratricyclo
[4.4.4.01,6]-tetradecane
3-(4-methoxy)phenoxymethyl-7,10-dimethyl-2,8,9-
trioxa-5-aza-1-boratricyclo[3.3.3.01,5]-undecane
Fig. 1. Molecular structures and systematic names for compounds 1 and 2.