Synthesis, characterization and crystal structures of the boratranes: 2,10,11-trioxa-6-aza-1-boratricyclo[4.4.4.01,6] tetradecane (tri-n-propanolamine borate), and 3-(4-methoxy)phenoxymethyl-7,10-dimethyl-2,8, 9-trioxa-5-aza-1-boratricyclo[3.3.3.01,5]-undecane

Article abstract of DOI:10.1016/j.poly.2011.08.020

The crystal structures of boratranes 2,10,11-trioxa-6-aza-1-boratricyclo[4. 4.4.0^1,6] tetradecane (tri-n-propanolamine borate) 1 as the tri-hydrate, and 3-(4-methoxy)phenoxymethyl-7,10-dimethyl-2,8,9-trioxa-5-aza-1- boratricyclo[3.3.3.0^1,5]-undecane 2 as the partial (0.2) hydrate have been determined. Compound 1 has a near-tetrahedral coordination of both the N (108.8°) and B atoms (111.2°), N → B bond length 1.656 and all-chair tricyclic conformation, whereas 2 has a slightly-longer N → B dative bond length (1.667 ) and the O-B-O angle, 114.8°, was slightly distorted from near-tetrahedral to adopt a flatter conformation. Theoretical calculations on 1 and 2 showed that the B-N distance in each shortened markedly between isolated gas phase molecules and 'solvated' models. Neither structural results, nor calculated parameters, were able to explain the propensity towards slow hydrolysis of boratranes with five-membered rings compared with the relative hydrolytic stability of boratranes with six-membered rings.

Full text of DOI:10.1016/j.poly.2011.08.020

Polyhedron 30 (2011) 2884–2889
Contents lists available at SciVerse ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Synthesis, characterization and crystal structures of the boratranes: 2,10,11-
trioxa-6-aza-1-boratricyclo[4.4.4.01,6] tetradecane (tri-n-propanolamine
borate), and 3-(4-methoxy)phenoxymethyl-7,10-dimethyl-2,8,9-trioxa-5-aza-1-
boratricyclo[3.3.3.01,5]-undecane
b
a
a
c
Robert A. Franich a, , Brian K. Nicholson , Hank W. Kroese , Suzanne S. Gallagher , Roger Meder ,
⇑
Joseph R. Lane b, Brian D. Kelly d
a Scion, Te Papa Tipu Innovation Park, 49 Sala Street, Rotorua 3020, New Zealand
b Chemistry Department, University of Waikato, Hamilton, New Zealand
c CSIRO Plant Industry, QBP, 306 Carmody Road, St. Lucia, Queensland 4067, Australia
d Starpharma Holdings Pty Ltd., 75 Commercial Road, Melbourne, Victoria 3004, Australia
a r t i c l e i n f o
a b s t r a c t
The crystal structures of boratranes 2,10,11-trioxa-6-aza-1-boratricyclo[4.4.4.01,6] tetradecane (tri-n-
propanolamine borate) 1 as the tri-hydrate, and 3-(4-methoxy)phenoxymethyl-7,10-dimethyl-2,8,9-tri-
oxa-5-aza-1-boratricyclo[3.3.3.01,5]-undecane 2 as the partial (0.2) hydrate have been determined. Com-
pound 1 has a near-tetrahedral coordination of both the N (108.8°) and B atoms (111.2°), N ? B bond
length 1.656 Å and all-chair tricyclic conformation, whereas 2 has a slightly-longer N ? B dative bond
length (1.667 Å) and the O–B–O angle, 114.8°, was slightly distorted from near-tetrahedral to adopt a flat-
ter conformation. Theoretical calculations on 1 and 2 showed that the B–N distance in each shortened
markedly between isolated gas phase molecules and ‘solvated’ models. Neither structural results, nor cal-
culated parameters, were able to explain the propensity towards slow hydrolysis of boratranes with five-
membered rings compared with the relative hydrolytic stability of boratranes with six-membered rings.
Ó 2011 Elsevier Ltd. All rights reserved.
Article history:
Available online 5 September 2011
Keywords:
Boratranes
X-ray crystal structure
N ? B transannular bond length
Quantum chemical calculations
1. Introduction
leaching of boron element from treated wood [7]. One approach to
solving this problem has exploited the valence-shell electron-pair
Boratranes are a class of tricyclic boric acid esters which have
been known since the description of the simplest example pre-
pared using triethanolamine in 1951 [1]. The particular structural
feature of boratranes, which explains their stability towards hydro-
lysis [2,3], is the trans-annular nitrogen-to-boron dative bond,
imparting tetrahedral geometry to the N and B atoms, described
as a ‘tryptych’ structure. This structural motif is common to
numerous synthetic compounds collectively referred to as ‘atr-
anes’, the chemistry of which has been extensively reviewed [4].
Boric acid is well-known as a broad-spectrum biocide which has
found economic application for the protection of wood from fungal
decay and wood-boring insects [5]. This benefit of wood protection
has historically been limited to products used in the interior of
buildings, or protected from rain because boric acid and borate
salts are soluble in water, and therefore leach from wood when
wet over a period of time [6], eventually leaving the wood suscep-
tible to decay. Many attempts have been made to limit the rate of
deficiency of boron (relative to the neon octet), and involved the
synthesis and biocidal effectiveness testing of molecules where
the boron atom was coordinated to nitrogen by an electron-pair
dative bond, such as in biguanide complexes [8,9] and in boratr-
anes [10]. The in vitro fungicidal performance of the variety of
boratranes synthesised [10] was found to be dependent on atrane
ring-size, where those with six-membered rings, e.g., tri-n-propa-
nolamine borate 1 and its C-substituted derivatives [10], had lower
fungicidal effectiveness compared with those boratranes having
five-membered rings, e.g., triethanolamine borate and its C-substi-
tuted alkyl and aryl derivatives, such as 2. This may be attributed
to the observation that while five-membered ring boratranes
hydrolyzed slowly to release boric acid, the six-membered species
were indefinitely stable towards water at neutral pH [10].
Hydrolysis of boratranes has been shown to be dependent on
the ability of a water molecule to attack the boron atom to form
an SN2-type transition state leading to a partially-hydrolyzed inter-
mediate structure, as part of a step-wise formation of boric acid
and the corresponding alkanolamine [2,3]. Closeness of approach
of the water nucleophile to the boron atom would depend on both
⇑
Corresponding author.
E-mail address: Robert.Franich@scionresearch.com (R.A. Franich).
0277-5387/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2011.08.020

R.A. Franich et al. / Polyhedron 30 (2011) 2884–2889
2885
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.

2886
R.A. Franich et al. / Polyhedron 30 (2011) 2884–2889
Table 1
of crystalline 2 showed three peaks with retention time 44.53, 44.83
and 45.31 min, and in the area% ratio of 0.40:0.13:0.47, respectively.
The mass spectra for all three GC–MS peaks were nearly identical.
M.p. 152–153° IR (cmꢀ1) 3440 (OH, water of crystallization),
3080 (Ar–H str.), 2972, 2910, 2865 (CH2 str.), 1513 (Ar C@C str.),
1H NMR (ppm) 1.18 (d, J = 7 Hz, 6H, CH3), 3.26 (dd, J = 7 Hz, 3H,
NCH2), 3.51 (dd, J = 7 Hz, 3H, NCH2), 3.83 (s, 3H, ArOCH3), 3.95
(m, 3H, ArOCH2, CH-CH3), 4.3 (m, 1H, CH2–CH–O), 6.9 (s, 4H,
ArH). 13C NMR (ppm) 24.0 (CH3), 45.0, 51.8 (N–CH2), 54.1, 57.1
(O–CH), 55.9 (OCH3), 72.7 (O–CH2), 114.8 115.3, 151.6, 152.2
(ArC). 11B NMR (ppm) 12.0 GC–MS (m/z) 321 (M+ꢂ11B), 320
(M+ꢂ10B), 198 (100%), 197 (20%), 184 (82%), 183 (16%), 154 (44%),
140 (35%), 123 (49%).
Crystal data and refinement details for the boratranes 1 and 2.
1
2
Formula
Mr
T (K)
Crystal system
Space group
a (Å)
C9H24BNO6ꢂ3H2O
253.10
89(2)
trigonal
R3
13.6873(3)
13.6873(3)
5.9896(3)
C16H24BNO5ꢂ0.2H2O
324.78
118(2)
monoclinic
P21/c
16.9480(5)
5.5575(2)
17.9064(5)
97.528(2)
1672.04(9)
4
ꢀ
b (Å)
c (Å)
b (°)
V (Å3)
Z
971.77(6)
3
1.297
Dcalc (g cmꢀ3
)
1.290
0.094
l
(mmꢀ1
)
0.105
Crystal size (mm)
F(000)
0.51 ꢁ 0.45 ꢁ 0.44
0.41 ꢁ 0.30 ꢁ 0.09
2.3. X-ray crystallography
414
696
hmax (°)
Reflections collected
27.5
7352
0.95, 0.89
816 (Rint = 0.037)
0.0291
30.2
42513
1.00, 0.90
4937 (Rint = 0.043)
0.0517
Crystals of
1
were determined as the trihydrate,
N{(CH2)3O}3Bꢂ3H2O, while those of 2, were determined as the par-
Tmax,
min
tial hydrate. X-ray intensity data were obtained from a Bruker
Unique reflections
R1 [I > 2r(I)]
APEX II CCD diffractometer with MoK
a radiation (0.71073 Å),
wR2 (all data)
0.0741
1.154
0.1610
1.014
and were processed using standard software. Corrections for
absorption were carried out using SADABS [24]. The structures were
solved and refined (on Fo2) using the SHELX programs [25,26], oper-
ating under WinGX [27,28]. For 1, Freidel equivalents were
merged, and H atoms were included in calculated positions except
for those of the water molecules which were located and refined
with fixed temperature factors. For 2, a final peak of residual elec-
tron density was assigned as a water molecule in the lattice, disor-
dered over two symmetry equivalent sites with occupancy about
0.2. This is weakly H-bonded to O(2) and was included in the
refinement without the associated H atoms.
Goodness-of-fit (GOF) on F2
2.4. Theoretical methods
The geometries of 1 and 2 were optimized using the B3LYP and
M06-2x hybrid density functional methods, the B2PLYP and
mPW2PLYP double hybrid density functional methods, the B2PLYPd
and mPW2PLYPd dispersion corrected double hybrid density func-
tional methods and the MP2 ab initio method. The 6-31G(d) and 6-
311+G(2d,2p) Pople style basis sets, the cc-pVDZ, aug-cc-pVDZ
and cc-pVTZ correlation consistent basis sets were used. The geom-
etry of 1 and 2 with the MP2/6-31G(d) method in an aqueous envi-
ronment was also optimized using the Integral Equation Formalism
Polarisable Continuum Model (IEF-PCM) and calculated atomic
charges using Natural Bond Orbital (NBO) analysis. The initial geom-
etries were taken from the corresponding X-ray crystal structure.
Harmonic vibrational frequencies were also calculated to verify that
the optimized structures were true minima. All calculations were
completed using GAUSSIAN 09, Revision A.01 [29].
Fig. 2. Molecular structure of 12,10,11-trioxa-6-aza-1-boratricyclo[4.4.4.01,6
tetradecane 1.
]
form an infinite H-bonded helical chain parallel to the c axis with
the second hydrogen atom H-bonded to O of the boratrane.
Other bond parameters for 1 are listed in Table 2.
The structure of 3-(4-methoxy)phenoxymethyl-7,10-dimethyl-
2,8,9-trioxa-5-aza-1-boratricyclo[3,3,3,01,5]-undecane 2 is shown
in Fig. 3, and is the first example of an X-ray crystal structure of
an alkyl-substituted boratrane derived from di-iso-propanolamine.
There are three five-membered rings which adopt an envelope
conformation with the phenoxymethyl-substituted C atom being
the out-of-plane component. For 2, there was partial disorder in
the conformation of the two methyl substituted rings involving
C(4) and C(7), and this was modeled successfully. That the two
methyl-substituted rings are partially disordered with minor com-
ponents adopting the reverse conformation, as shown in Fig. 4, can
be traced back to the original di-iso-propanolamine used in the
synthesis consisting of a 1:1 mixture with methyl groups syn or
anti to each other. The fact that the disorder is not 1:1 in the bora-
trane molecule analyzed showed there was some selectivity in the
formation and crystallization of the anti stereoisomer, an interpre-
tation supported from the GC–MS data acquired for the crystals
and for the solid remaining in the mother liquor.
3. Results and discussion
3.1. X-ray crystal structure determinations
Crystal data and refinement details for 1 and 2 are summarized
in Table 1.
The structure of 1, as the trihydrate, N[(CH2)3O]3Bꢂ3H2O, showed
ꢀ
the boratrane to lie on a three-fold axis of the chiral space group R3,
coincident with the N ? B vector. The six-membered rings have
adopted a chair conformation, as shown in Fig. 2. There is the ex-
pected N non-bonding electron pair interaction between the N
and B atoms, with a distance of 1.656(2) Å, which compares with
the sum of the covalent radii for the three-coordinate atoms of
1.61 Å, so can be regarded as a strong interaction. This also allows
for near-ideal tetrahedral bonding at each of the atoms (O–B–O0
111.19(6)°, C–N–C0 108.76(6)°). The H2O molecules in the lattice

R.A. Franich et al. / Polyhedron 30 (2011) 2884–2889
2887
Table 2
Bond parameters for the six-membered ring (1) and the five-membered ringa (2)
boratranes.
(1)
(2)
Bond lengths (Å)
N(1) ? B(1)
N(1)–C(1)
C(1)–C(2)
C(2)–C(3)
165.6(2)
150.5(10)
151.6(15)
151.2(16)
142.4(13)
144.0(8)
166.7(2)
148.6(2)
C(3)–O(1)
O(1)–B(1)
143.7(2)
Angles (°)
C(1)–N(1)–C(1)0
O(1)–B(1)–O(1)0
B(1)–O(1)–C(3)
N(1)–C(1)–C(2)
C(1)–C(2)–C(3)
108.76(6)
111.19(6)
116.38(7)
112.74(8)
110.83(9)
114.8(2)
114.8(2)
109.2(2)
105.5(2)
a
Average values.
The N ? B bond length of 1.667(2) Å is essentially the same
as in the unsubstituted boratranes with five-membered rings
[18,19]. This N ? B bond length for 2 is slightly longer than that
[1.656(2) Å] found for 1, with six-membered rings, presumably
because of less flexibility for the five-membered ‘tryptych’ rings.
The extra rigidity (compared with 1) is also manifested in distor-
tion from the ideal tetrahedral bond angles, resulting in flattened
geometry about the B and N atoms with bond angles (O–B–O
(av.) 114.8°, C–N–C (av) 114.8° (Table 2)). The presence of a
large substituent on one of the rings has little effect on the over-
all boratrane structure, though small changes would not be
apparent because of the partial disorder in the methyl-substi-
tuted rings. The H2O molecules in the lattice are disordered over
two symmetry-equivalent sites which lie in channels parallel to
the b axis, and are loosely H-bonded to the O(2) atom of the
boratrane.
Fig. 4. Partial disorder of the two methyl-substituted rings in 2.
tal structure value [1.6764(7) Å] and the gas phase electron
diffraction result [1.84(4) Å] [18,19]. This suggested the molecular
environment was having a significant effect on the B–N distance.
To probe this further calculations on 1 were performed which in-
cluded three water molecules (H-bonded to the O atoms as found
in the crystal structure) and also for a dimer of 1 to partially account
for other crystal packing interactions, with and without the water
molecules. As shown in Table 4, by including the various HOHꢂ ꢂ ꢂO
and C–Hꢂ ꢂ ꢂO intermolecular interactions, better, though still not
perfect, agreement is achieved. These intermolecular interactions
withdraw electron density from the O atoms, which in turn make
the B atom more electron deficient and hence attractive to the elec-
tron rich N atom. The structures were also optimized with the MP2/
6-31G(d) method for an aqueous environment using implicit solva-
tion, which gave calculated N ? B distances of 1.709 and 1.691 Å
for 1 and 2, respectively. Both the X-ray and theoretical methods
show that for these boratrane molecules interaction with polar
neighbors has an unusually dramatic effect by shortening the
N ? B distance compared with that found by electron diffraction,
or theory, for isolated gas phase molecules.
3.2. Quantum mechanical calculations
Data for the boratranes 1 and 2 modeled as isolated species
using various levels of theory are summarized in Table 3. There is
mostly reasonable agreement between calculated parameters and
the ones determined from crystal structures, with the notable
exception of the B–N distance where the calculated values are much
longer than the experimental ones. Interestingly, the B3LYP result is
very poor, especially for compound 1 (calc. 2.347 Å versus experi-
mental 1.657 Å), despite this being the most widely-used density
functional method. Better agreement is seen with the MP2/6-
31G(d) method, but even here, the calculated value is ca. 0.1 Å long-
er. Earlier calculations for the unsubstituted five-membered ring
boratrane reported the same lack of agreement for the B–N dis-
tance, and noted that the experimental values differed for the crys-
Table 3 also lists vibrational frequencies nominally assigned to
the B–N stretching mode, though there is strong coupling with
other ring motions. What the values do reflect is that the mode
is at lower energy for the six-membered ring than for the five-
membered one, arising no doubt from the greater flexibility. These
results can be compared with a very recent theoretical study on
five-membered ring group 13 atranes in general [30].
Fig. 3. Molecular structure of 3-(4-methoxy)phenoxymethyl-7,10-dimethyl-2,8,9-trioxa-5-aza-1-boratricyclo[3,3,3,01,5]-undecane 2.

2888
R.A. Franich et al. / Polyhedron 30 (2011) 2884–2889
Table 3
Selected calculated and experimental data for 1 and 2.
B3LYP/6-31G(d)
M06-2x/6-31G(d)
MP2/6-31G(d)
Experimental
1
2
1
2
1
2
1
2
N–B bond length (Å)
O–B bond length (Å)
C–N–B angle (°)
N–B–O angle (°)
O–B–O angle (°)
2.347
1.382
106.8
94.7
119.3
3.457
220
1.828
1.424
101.0
99.4
117.4
7.086
343
1.816
1.417
108.6
102.7
115.3
5.026
298
1.805
1.425
142.3
101.3
117.4
7.239
459
1.774
1.429
109.1
103.5
114.7
5.680
325
1.758
1.436
102.1
100.3
116.9
8.335
488
1.656
1.440
110.2
107.7
111.2
1.667
1.437
103.3
103.4
114.8
Dipole moment (Debye)
N–B stretch (cmꢀ1
)
Table 4
Optimized N–B distance of 1 as both a monomer and dimer, and the corresponding
hydrated monomer and dimer.
Method
N–B distance (Å)
Monomer
Monomer + 3H2O
Dimer
Dimer + 3H2O
B3LYP/6-31G(d)
B3LYP/cc-pVTZ
M062x/6-31G(d)
M062x/cc-pVTZ
MP2/6-31G(d)
2.347
2.383
1.816
1.802
1.774
1.810
1.779
1.718
1.712
1.704
1.883
2.337
1.747
1.746
1.736
1.781
1.770
1.703
1.709
1.703
3.3. Possible reasons for differing hydrolysis reactivities
The structure determinations were carried out partly with the
expectation that the results would help understand the observa-
tion that the five-membered ring boratranes such as 2 hydrolyzed
faster than the six-membered ones, 1 [2,3,10]. However, the new
data have not provided clarity. A proposed mechanism for hydroly-
sis would involve nucleophilic attack by the O of a water molecule
at the B atom, possibly aided by simultaneous H-bonding to one of
the O atoms adjacent to the B atom to give a four-membered tran-
sition state, such as that shown in Fig. 5.
Fig. 6. Space-filling models of the six-membered ring (left) and five-membered ring
boratranes showing very similar access to the B atom (orange), and different
exposure of the N atom (blue). (For interpretation of the references to colour in this
figure legend, the reader is referred to the web version of this article.)
The X-ray crystal structure determinations for the boratranes 1
and 2 have shown only small differences in the N ? B bond lengths
between 1 and 2, which means the earlier proposal that the rates of
hydrolysis depend on the N ? B interaction [2,3] does not appear
to apply here. Another factor is the accessibility of the B atom to
attack but space-filling models show that the boron atoms of both
1 and 2 are similarly exposed from within the ‘tryptych’ structure
(Fig. 6). The B in the less readily-hydrolyzed 1 is slightly less ex-
posed based on the O–B–O angle of 111.19(6)° compared with
partly flattened 114.8(2)° for 2, but the small difference is unlikely
to be a major influence.
Electronic factors will also be involved, so partial atom charges
were calculated. The MP2/6-31G(d) NBO charges for B are +1.48 for
1 and +1.49 for 2, modeled in the aqueous phase. These showed
that the residual +ve charge on the B atoms are not significantly
different for 1 and 2, so the different susceptibility to nucleophilic
attack cannot be explained on that basis.
A possible activating mechanism might involve protonation at
the N atom, thus weakening the N ? B interaction and increasing
the reactivity at B. Protonation of the N atom of atranes by strong
acids is documented [20]. However, examination of the molecular
surface using space-filling models showed that the N atoms of both
1 and 2 are almost completely buried within the hydrophobic atra-
ne cage (Fig. 6). For 2, the N is a little more accessible [C–N–C0 an-
gle 114.8(2)°] compared with 1 [C–N–C0 108.8(1)°], which would
allow 2 to be the more hydrolytically-reactive molecule, in agree-
ment with the experimental results, but given the low energy of
the N ? B stretching vibration, this small difference in the ground
state is again unlikely to have a major effect. The MP2/6-31G(d)
NBO charges for the N atoms in 1 and 2 are ꢀ0.58 and ꢀ0.60
respectively which would indicate that 2 is slightly more basic
than 1, but the difference is barely significant.
This study of the structure and properties of five-membered ring
boratranes represented by 2 and contrasting these with those of 1
attempts to understand at the molecular level the performance of
such compounds as biocides for the protection of biomaterials, such
as wood, against fungal decay. The pH of wood material when wet is
in the range 4–6, and therefore a supply of protons would be avail-
able for bonding to N of a designed, water-insoluble C-substituted
five-membered ring boratrane as a wood decay protectant, result-
ing in a controlled degree of hydrolysis and a slow-release of boric
acid, on demand, as material moisture content varied. However,
neither the X-ray results, nor the theoretical calculations, provide
any compelling explanation for the different hydrolysis behavior
of the boratranes. In the five-membered ring structure 2 there is a
small but significant flattening away from the ideal tetrahedral
coordination around both the B and N atoms, compared with the
N
O
B
O
O
H
O
H
Fig. 5. A schematic for the attack of a H2O molecule as the first step towards
hydrolysis.

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