A borazaaromatic analogue of isophthalic acid

Article abstract of DOI:10.1039/b104794a

A diboradiazaaromatic-2,7-di-tert-butyl-5,9-dihydroxy-5,9-dibora-4,10-diazapyrene- 4,10-diium-5,9-diuide- which is a structural analogue of isophthalic acid has been designed and synthesised. This compound is capable of spontaneous dehydration in solution to form linear oligoanhydrides. These oligoanhydrides can be readily hydrolysed to the starting diboradiazaaromatic under appropriate conditions. This unusual reactivity is mirrored in the solid-state behaviour of 2,7-di-tert-butyl-5,9-dihydroxy-5,9-dibora-4,10-diazapyrene-4,10-diium-5,9-diuid e. A complex network of hydrogen bonds present in the solid-state structure of the borazaaromatic serve to facilitate a facile solid-state dehydration reaction, once again forming oligoanhydrides of molecular weight greater than 3000 Da.

Full text of DOI:10.1039/b104794a

View Article Online / Journal Homepage / Table of Contents for this issue
A borazaaromatic analogue of isophthalic acid
Peter R. Ashton, Kenneth D. M. Harris, Benson M. Kariuki, Douglas Philp,*†
James M. A. Robinson and Neil Spencer
2
School of Chemistry, University of Birmingham, Edgbaston, Birmingham, UK B15 2TT
Received (in Cambridge, UK) 31st May 2001, Accepted 1st August 2001
First published as an Advance Article on the web 16th October 2001
A diboradiazaaromatic—2,7-di-tert-butyl-5,9-dihydroxy-5,9-dibora-4,10-diazapyrene-4,10-diium-5,9-diuide—
which is a structural analogue of isophthalic acid has been designed and synthesised. This compound is capable of
spontaneous dehydration in solution to form linear oligoanhydrides. These oligoanhydrides can be readily hydrolysed
to the starting diboradiazaaromatic under appropriate conditions. This unusual reactivity is mirrored in the solid-
state behaviour of 2,7-di-tert-butyl-5,9-dihydroxy-5,9-dibora-4,10-diazapyrene-4,10-diium-5,9-diuide. A complex
network of hydrogen bonds present in the solid-state structure of the borazaaromatic serve to facilitate a facile
solid-state dehydration reaction, once again forming oligoanhydrides of molecular weight greater than 3000 Da.
Introduction
Solid-state organic reactivity continues to attract significant
interest since its exploitation can provide enhanced rates of
reactions as well as differing degrees of regio- and stereo-
selectivity when compared to reactions performed in solution.1
Solid-state organic reactions can occur between two solids,
between a liquid and a solid or between a gas and a solid.
However, it is transformations within crystals that are of par-
ticular interest to the supramolecular chemist, since molecular
recognition may be exploited to control the structural and
geometric arrangement of potentially reactive molecules within
the solid-state, and hence, control their reactivity. In the past,
investigations of organic solid-state transformations have
mostly concentrated upon [2 ϩ 2] photodimerisations, and
examples utilising intermolecular interactions to align photo-
reactive alkene2,3 and bisalkene4 compounds have been
reported. Somewhat surprisingly, other types of organic solid-
state transformations, especially bond forming reactions which
proceed under thermal control, appear to have been studied
much less extensively,5 and examples of the use of molecular
recognition to facilitate and control such reactions are scarce.
Borazaaromatics are a class of compounds analogous to
aromatic hydrocarbons, in which a C᎐C fragment has been
Fig. 1 The borazaaromatic supramolecular synthon 2 is structurally
analogous to the carboxylic acid synthon 1. Thus, carboxylic acids 3
and 6 share the same recognition properties as borazaaromatics 4 and 7
respectively.
᎐
replaced by an isoelectronic ϪB᎐Nϩ fragment. Although several
᎐
borazaaromatics were synthesised in the 1950s and 1960s, their
chemical properties were never investigated fully.6 Recently, we
identified two important chemical characteristics of these
compounds. Firstly, we recognised a structural similarity
between the recognition properties of carboxylic acids and
hydroxyborazaaromatics. Thus, the hydroxyborazaaromatic
self-complementary hydrogen bonded dimer 2 is analogous
to the carboxylic acid supramolecular synthon7,8 1 (Fig. 1).
Secondly, unlike carboxylic acids, which are relatively inert
towards self-condensation to give anhydrides, hydroxyboraza-
aromatics readily undergo dehydration, in solution or the solid-
state, to form the corresponding anhydride (Scheme 1). The first
system studied9 in depth was 10-hydroxy-10-bora-9-aza-
phenanthren-9-ium-10-uide 4, which is a structural analogue of
phenanthrene and a recognition analogue of benzoic acid 3.
Compound 4 undergoes a solid-state transformation to form
Scheme 1 Hydroxyborazaaromatics are converted into the corre-
sponding anhydride either in the solid-state (by heating) or in solution
(by control of the solvent polarity). Non-polar solvents (e.g. CHCl3,
CH2Cl2, benzene) favour the formation of the anhydride whereas polar
solvents (e.g. acetone, CH3CN, DMSO) favour the formation of the
hydroxy compound.
the anhydride 5 irreversibly when a sample is heated at relativ-
ely low temperatures (100 ЊC, 1 h). In solution, the inter-
conversion between 4 and 5 is under thermodynamic control.
The mechanism for the conversion of 4 to 5 is presumed to
involve nucleophilic attack of the hydroxy group of one mole-
cule at the electrophilic boron centre of a second molecule.
† Present address: Centre for Biomolecular Sciences, School of
Chemistry, University of St Andrews, North Haugh, St Andrews, Fife,
UK KY16 9ST.
2166
J. Chem. Soc., Perkin Trans. 2, 2001, 2166–2173
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b104794a

View Article Online
Although the charge representation of the ϪB᎐Nϩ bond sug-
᎐
gests that the boron has a partial negative charge, the calculated
electrostatic potential surface around the boron centre in 4 is
actually positive, inferring that the N B charge transfer
π-bonding framework is outweighed by the B N charge trans-
fer σ-bonding framework. The enhanced reactivity of these
hydroxyborazaaromatics, in comparison with carboxylic acids,
reflects the modified reactivity of the electrophilic centre—the
carbonyl carbon atom vs. the borazaaromatic boron atom.
Given that the R22(8)10 cyclic homodimer 1 formed by carb-
oxylic acids has been used extensively in the design of organic
solids, we recognised that the similarity between the recog-
nition characteristics of motifs 1 and 2, coupled with the
modified electrophilic chemistry of motif 2, could be exploited
in the creation of chemical systems which are able to (a) par-
ticipate in intermolecular recognition similar to that seen in
molecular solids bearing carboxylic acid functionality and
(b) possess the latent capability of a covalent bond formation
reaction in the solid-state via self-condensation. The tendency
for molecules bearing two or more carboxylic acid groups
to form hydrogen bonded tapes and sheets in the solid-
state, such as those seen in isophthalic acid11,12 and trimesic13,14
acid (benzene-1,3,5-tricarboxylic acid) derivatives, has been
exploited in crystal engineering and solution-phase supra-
molecular chemistry. Therefore, in principle, replacement of the
carboxylic acid recognition motif by a motif that shares the
same recognition characteristics and has enhanced reactivity
towards self-condensation, should lead to chemical systems
capable of forming extended covalently bonded structures
(i.e. polymers).
Scheme 2 Reagents and conditions: (a) Fuming HNO3, 0 ЊC, 30 min
In this paper, we report the use of the electrophilic reac-
tivity of a borazaaromatic analogue of isophthalic acid in the
design of novel polymers which can be formed under equi-
librium control in solution or non-reversibly via a solid-state
transformation.
then 50 ЊC, 20 min, 53%; (b) Mg, Et2O; (c) B(OMe)3, Et2O, Ϫ76 ЊC,
30 min then warm to 25 ЊC, 48% (over two steps); (d) Pd(PPh3)4, 1 M
Na2CO3, DME, EtOH, H2O, 85 ЊC, 5 h, 57%; (e) Pd/C, H2, EtOAc, RT,
24 h, 84%; (f ) PhBCl2, xylene, 138 ЊC, 90 min then AlCl3, 138 ЊC, 19 h,
22%.
reaction of diamine 12 with a BHCl2ؒMe2S complex afforded 14
(isolated as a mixture of oligomers 15) in an estimated 72%
yield.
Results and discussion
The design of a borazaaromatic system which could mimic the
recognition properties of a carboxylic acid such as isophthalic
acid requires an aromatic skeleton in which two boron atoms
are bonded to the same phenyl ring. Therefore, we selected a
pyrene skeleton bearing two boron–nitrogen functionalities,
representing a borazaaromatic that is practical both in terms of
synthesis and aromatic stability. The dihydroxydiazadibora-
pyrene 7 is a structural analogue of isophthalic acid 6 and a
B-phenyl substituted borazapyrene has been reported15 to
possess a similar UV spectrum to pyrene itself.
However, the intended synthesis of 7 required the prepar-
ation of 2,6-dinitrobiphenyl as an intermediate—a compound
which is known16 to have poor solubility in organic solvents.
Assuming that 7 would have similar solubility properties to
2,6-dinitrobiphenyl, we decided to append tert-butyl groups to
the aromatic skeleton in order to improve solubility. We antici-
pated that the addition of these groups at the 4 and 4Ј positions
on the biphenyl scaffold should simplify the 1H NMR spectrum
such that all the aromatic resonances appear as singlets.
4-tert-Butylbromobenzene 8 was nitrated affording the dini-
tro compound 9 and was also converted to the corresponding
boronic acid 10 (Scheme 2). Cross-coupling of 9 and 10 using
standard Suzuki17 methodology followed by catalytic reduction
yielded the 2,6-diaminobiphenyl derivative 12. In order to
ensure that the diamine was suitable for the synthesis of
borazaaromatics, we reacted 12 with PhBCl2 to give the B,BЈ-
diphenyl substituted borazapyrene 13 which, by virtue of its
non-polar character and relative chemical stability, can be
separated and characterised with ease. Our initial attempt to
synthesise the corresponding dihydroxydiboradiazapyrene 14
using the methodology18 applied to the synthesis of 4 (using
BCl3 as the source of boron) was unsuccessful. However, the
Our initial attempts to grow crystals of 14 were unsuccessful,
although, fortuitously, when a solution of 14–15 in CH3OH was
left to stand overnight, crystals of the dimethoxy ester 16
suitable for single crystal X-ray diffraction were formed; more
concentrated methanolic solutions were found to produce a
precipitate almost immediately after the preparation of the
solution. The reaction of 14 with methanol provided further
evidence that the boron centres in this diboradiazaaromatic
would be susceptible to nucleophilic attack in a similar manner
to the electrophilic boron centre in 4.
Single crystal X-ray diffraction studies were performed on
these crystals of 16 for two purposes. Firstly, we wanted to
confirm the molecular structure of 16 and thus, by implication,
to establish the molecular structure of 14. Secondly, in prin-
ciple, it is possible for 16 to participate in hydrogen bonded
ribbons (analogous to those observed in the solid-state struc-
tures of isophthalic acid and its derivatives) using motif 17,
provided the O–Me bond lies anti to the N–H bond. However,
in the crystal structure19 of 16 (Fig. 2), all the molecules of 16
have the O–Me bond lying syn with respect to the N–H bond.
The preference for this syn conformation, however, was con-
firmed by ab initio calculations on the model system, 18. The
(Ar)C–B–O bond angle of the optimised HF/6-31G(d,p) geom-
etry of the anti conformer is somewhat strained (130.8Њ) by the
proximity of the methyl hydrogens to the aromatic hydrogens at
the 6- and 8-positions, ortho to the BOMe group (dH ؒ ؒ ؒ H
=
2.26 Å). The syn conformer has the methyl group in close prox-
imity to the NH hydrogen, but the steric hindrance is reduced
(dH ؒ ؒ ؒ H = 2.47 Å) and thus the (Ar)C–B–O bond angle is less
strained (123.6Њ). Accordingly, the energy of the syn conformer
is estimated to be over 24 kJ molϪ1 less than that for the
J. Chem. Soc., Perkin Trans. 2, 2001, 2166–2173
2167

View Article Online
Fig. 3 (a) 300 MHz 1H NMR spectrum of 14–15 in d6-acetone
1
solution recorded 1 hour after the solution was prepared. (b) The H
NMR spectrum of the same sample 24 hours later showing resonances
for 14 only. The double-headed arrows represent NOEs which are
observed in the 500 MHz 1H NMR spectrum recorded of 14 in
d6-acetone solution at 30 ЊC.
Fig. 2 ORTEP diagram of one of the disordered molecules of 16 in its
crystal structure. Thermal ellipsoids are shown at the 30% probability
level.
hexane). Obviously, the concentration of water is critical in
mediating the rate of hydrolysis. For example, the addition of a
large excess of D2O to a solution of 15 in d6-acetone resulted in
complete hydrolysis to 14 within a matter of minutes.
In order to investigate the effect of solvent (i.e. polar vs. non-
polar) and the presence of water upon the length of the oligo-
mer chain of 15 (represented by the value of n, Scheme 3)
MALDI-TOF spectra were recorded for solutions of 14 and 15
in acetone and CHCl3 in the presence and absence of 4 Å
molecular sieves.
In the MALDI-TOF spectrum of a dilute solution of 14–15
in CHCl3 (ca. 3 mM), the molecular ion [14ϩ] is by far the
largest peak (Table 1, entry A). However, peaks which could
be assigned as oligoanhydrides 15 (n = 0, 1 and 2) could also
be identified. We attribute the lack of larger oligomers to the
presence of water in the CHCl3 shifting the equilibrium to the
hydrated compound 14. Therefore, activated 4 Å molecular
sieves were added to the CHCl3 solution and the spectrum was
re-recorded after two weeks. Oligomers as large as n = 7 were
then observed (Fig. 4 and Table 1, entry B), suggesting that the
removal of water by the molecular sieves shifts the equilibrium
towards anhydride formation (i.e. 15). It must be noted that the
MALDI-TOF spectra do not provide a quantitative measure of
the proportions of the oligomers present in the solution phase
since the intensity of larger mass peaks is extremely sensitive to
the power of the desorption laser; in the spectrum of the same
sample recorded using a more powerful laser setting, the peaks
representing the larger oligomers became more intense at the
expense of a noisier background. However, peaks that could be
assigned to oligomers with more than 10 repeat units (i.e. n > 8)
were not obtained in any spectrum. Interestingly, only linear
oligomers are observed (i.e. all the mass peaks correspond to
anti conformer. The crystal structure of 16 contains different
types of disorder (see Fig. 2). Firstly, the methyl groups of
the two tert-butyl substituents are disordered between two
orientations (related by rotation around the C–C(CH3)3 bond).
Secondly, the whole molecule is disordered between two
molecular orientations (which can be regarded as being related
by rotation about a 2-fold axis passing through the midpoints
of the two B–N bonds). The two molecular orientations have
equal populations, with the B atoms of one molecular orien-
tation and the N atoms of the other molecular orientation
occupying the same sites. From the crystal structure, we cannot
assess whether the two molecular orientations represent two
different ordered domains, or whether there is a uniform distri-
bution of the two disordered molecular orientations through-
out the crystal.
The dihydroxydiboradiazaaromatic was initially isolated in a
pure form by precipitation of the oligomer 15 by addition of
1
hexane to a solution of a mixture of 14 and 15 in xylene. H
NMR analysis20 in d6-acetone (Fig. 3) shows that the oligo-
anhydride 15 hydrolyses to 14 over a period of one day, whereas
the 1H NMR spectrum of the same sample in a relatively non-
polar solvent, CDCl3, shows an equilibrium mixture of 14
and 15. These observations confirm that the reversible electro-
philic solution-phase reactivity seen in other borazaaromatics
(Scheme 1) is also exhibited by the borazapyrene analogue;
thus, the oligomer formation is under thermodynamic control.
The equilibrium between 14 and 15 can be driven to give 14 by
hydrolysis in polar solvents (such as acetone or DMSO) or the
equilibrium can be shifted towards the side of 15 by spon-
taneous dehydration in a non-polar solvent (such as CHCl3 or
2168
J. Chem. Soc., Perkin Trans. 2, 2001, 2166–2173

View Article Online
Table 1 Analysis of MALDI-TOF data
% Area with respect to 14
Aa
B
C
D
E
14
100
100
100
100
100
15,
where n =
0
1
2
3
4
5
6
7
16
7
trace
—
—
—
28
11
6
4
3
2
1
<1
—
—
—
—
—
—
—
—
7
trace
—
—
—
—
—
—
94
54
28
15
8
6
3
—
—
1
Fig.
4
Partial MALDI-TOF mass spectrum of borazapyrene
oligomers 15 (where n = 1 to 7) obtained from a solution of 14–15 in dry
a A Solution in chloroform. B Solution in chloroform dried over 4 Å
molecular sieves. C Solution of 14 in acetone. D Solution of 14 in
acetone dried over 4 Å molecular sieves. E Crystal of [142ؒCH3COCH3]
that has been heated at 160 ЊC for 5 hours.
CHCl3.
These observations demonstrate unambiguously that in
non-polar solvents such as CHCl3, the solution equilibrium
between 14 and 15 can be shifted in favour of 15 by the removal
of water using very mild dehydrating agents (i.e. molecular
sieves). However, it appears that in polar solvents, such as
acetone and DMSO, the equilibrium lies far on the side of 14
and, thus, the reaction proceeds in one direction (hydrolysis of
15 to 14).
Single crystals of a 2 : 1 complex of 14 and acetone suitable
for single crystal X-ray diffraction studies were grown (follow-
ing many attempts using different crystallisation techniques)
by slow diffusion of hexane into a solution of 14 in an acetone–
hexane mixture. Crystallisation from a variety of other solvent
mixtures yielded only fine powders, emphasising the role
of acetone in the formation of large single crystals. In the
crystal structure21 of [142ؒacetone] (Fig. 5) molecules of 14
are arranged in a complex hydrogen bonding network of
O–H ؒ ؒ ؒ O and N–H ؒ ؒ ؒ O interactions. Although this hydro-
gen bonding arrangement does not include the cyclic dimer
motif 2, the (B)O–H ؒ ؒ ؒ O(B) and N–H ؒ ؒ ؒ O(B) interactions
bring the recognition sites, and thus the boron reaction centres,
into close proximity. Fig. 5a demonstrates how the recognition
sites 1 and 2 are linked together by O–H ؒ ؒ ؒ O hydrogen bonds
to the hydroxy group of recognition site 3 to form a chain of
molecules running along the crystallographic c axis. Recog-
nition site 4 is not part of the chain, although it does partici-
pate in N–H ؒ ؒ ؒ O and O–H ؒ ؒ ؒ O hydrogen bonds with the
constituent hydroxy groups of the chain.
This hydrogen bonding network contains a number of
cooperative and polarising interactions22 (Fig. 5b). The
hydroxy oxygen of recognition site 3 accepts two hydrogen
bonds (N–H ؒ ؒ ؒ O from site 4 and O–H ؒ ؒ ؒ O from site 1)
thus making the hydroxy group an enhanced hydrogen bond
donor. The hydroxy group of recognition site 2 participates
Scheme 3 Reagents and conditions: (a) [BHCl2ؒMe2S], xylene, 138 ЊC,
2 h (Me2S removed in vacuo) then AlCl3, 20 h, 72%; (b) MeOH; (c) 4 Å
molecular sieves, CHCl3 or similar non-polar solvent; (d) H2O and
acetone or similar polar solvent.
in an O–H ؒ ؒ ؒ O᎐C hydrogen bond with an acetone molecule,
᎐
oligomers with two terminal hydroxy groups), whereas cyclic
oligomers would have 18 mass units less due to the loss of an
extra water molecule in the cyclisation step.
thus making it an enhanced hydrogen bond acceptor. The
O–H ؒ ؒ ؒ O hydrogen bond between the enhanced donor and
acceptor is somewhat shorter (dH ؒ ؒ ؒ O = 1.76 Å, O–H ؒ ؒ ؒ O
angle 170.5Њ) than the other O–H ؒ ؒ ؒ O interactions in the
structure. Indeed, all the O–H ؒ ؒ ؒ O(H) distances lie within
a range (1.76–1.90 Å) which is slightly shorter than the
O–H ؒ ؒ ؒ O(H) distance (1.92 Å) observed in the structure of
4, emphasising the cooperative nature of the interactions in
[142ؒacetone].
In hindsight, the absence of the predicted recognition motif 2
is readily rationalised. The presence of the tert-butyl groups on
the aromatic skeleton disrupts close packing arising from π–π
stacking, thus disfavouring formation of the planar ribbons
that would be required for 14 to participate in the cyclic dimer
motif 2. We note that carboxylic acids are also known to par-
ticipate in interaction motifs other than cyclic dimers, such as
In order to confirm that the larger oligomer peaks are not
artifacts (for example arising from the aggregation of smaller
oligomers upon ionisation), the MALDI-TOF spectrum was
also obtained for a solution of 14 in acetone. The resulting
spectrum had a peak corresponding to 14, with no oligomers
(15, n ≥ 0) detected (Table 1, entry C). In contrast to the
behaviour of the CHCl3 solution, the presence of molecular
sieves had little effect on the formation of oligomers. Two weeks
after the addition of molecular sieves, the MALDI-TOF
spectrum again showed that the peak corresponding to 14 was
predominant (Table 1, entry D) and showed only a small
increase (to 7% relative intensity) of the peak corresponding to
the anhydride dimer (15, n = 0).
J. Chem. Soc., Perkin Trans. 2, 2001, 2166–2173
2169

View Article Online
Fig. 6 The mass loss, recorded by thermogravimetric analysis (TGA),
on heating a powdered sample of [142ؒacetone]. The sample was heated
from 25 to 300 ЊC at a rate of 100 ЊC per minute, after which time (2.75
min) the temperature was held at 300 ЊC for a further 1.75 minutes.
sample to room temperature, the diffractogram indicated that
the sample remained amorphous.
The solid-state chemical reactivity of [142ؒacetone] was
demonstrated using thermogravimetric analysis (TGA) to
measure the loss of mass (water and acetone) from the solid as
a consequence of the reaction to give the oligoanhydride 15.
Upon heating from 25 ЊC to 300 ЊC (at 100 ЊC per minute) the
total mass loss for powder samples of [142ؒacetone] was about
13% (Fig. 6), of which some amount may be attributed to
decomposition and sublimation (the theoretical mass loss due
to acetone and water is 9.8%). Isothermal TGA measurements
of the mass loss suggest that, in keeping with the powder X-ray
diffraction studies, the reaction rate is fast at 160 ЊC and slow
at 100 ЊC (the loss of acetone and water is complete within
30 minutes at 160 ЊC and within several hours at 100 ЊC).
In order to confirm that the oligoanhydride 15 had been
formed by a solid-state transformation, single crystals of
[142ؒacetone] were heated at 160 ЊC for several hours and were
then analysed by MALDI-TOF spectrometry. Since the heated
crystal had to be dissolved in an appropriate solvent before
being applied to a MALDI sample plate, it was crucial to pre-
pare the MALDI samples such that no oligomer condensation
occurs in the solution phase. Therefore, MALDI-TOF analysis
of a crystal of [142ؒacetone] that had not been heated was per-
formed as a control experiment. As a further safeguard, all
samples were dissolved in acetone since we have demonstrated
that the equilibrium between 14 and 15 in acetone lies on the
side of 14. This solvent ensures that no significant amounts of
oligomerisation occur in solution. The MALDI-TOF spectrum
of a dilute solution of a crystal of [142ؒacetone] in acetone
(ca. 3 mM) shows the molecular ion 14ϩ is the predominant
peak (Table 1, entry D), but contains no peaks corresponding
to the anhydride dimer (15, n = 0) or larger oligoanhydrides.
The spectrum of a heated crystal of [142ؒacetone], prepared
in an identical manner, shows peaks corresponding to oligo-
anhydrides up to n = 7 (Table 1, entry E) confirming that the
solid-state transformation yields 15. Unfortunately, it is impos-
sible to determine the distribution of oligomer lengths in a
quantitative manner from the MALDI data, and it is likely that
higher oligomer chains, if present, are either not ionised or
fragment to form lower oligomers. Interestingly, at a higher
laser power setting, peaks corresponding to much larger oligo-
mers are seen (n = 15ϩ), although at the same power setting the
control sample shows peaks for 15 (n = 0, 1), probably arising
from aggregation.
Fig. 5 (a) The crystal structure of [142ؒacetone] viewed along the
b axis. The tert-butyl groups are disordered and have been omitted,
together with the acetone molecules, for clarity. The numbers 1 to
4
represent the four crystallographically independent N–B–O
functionalities and the bold line represents a chain of molecules linked
together by O–H ؒ ؒ ؒ O(B) hydrogen bonds. (b) A schematic represen-
tation of the hydrogen bonding interactions in the crystal structure of
[142ؒacetone].
chains and expanded rings. Indeed, a recent analysis23 of the
Cambridge Structural Database (CSD, October 1996) suggests
that the probability that a molecule bearing a carboxylic acid
functionality forms cyclic hydrogen bonded dimers is only
about 33%.
It is important to note that the absence of motif 2 from the
crystal structure of [142ؒacetone] does not necessarily preclude
the capability of polymerisation. The recognition sites bring the
reaction centres (boron and hydroxy) into close proximity, sug-
gesting that a solid-state transformation may be able to occur at
relatively low temperatures.
The first evidence for the solid-state reactivity of the
[142ؒacetone] co-crystal was the fact that crystalline samples did
not melt at temperatures below 400 ЊC. When a (transparent)
single crystal of [142ؒacetone] was used for a melting point
experiment, the crystal became opaque at 150–160 ЊC and
displayed no further observable change at temperatures up to
400 ЊC suggesting that some degree of polymerisation had
occurred.
The change in the solid-state structure of [142ؒacetone]
effected by heating was confirmed by variable temperature
powder X-ray diffraction studies. When a sample of [142ؒ
acetone] was heated to 100 ЊC, the powder diffraction peaks
became less intense over a period of hours. When the temper-
ature was raised to 160 ЊC, the diffraction peaks disappeared
completely within a matter of minutes. At these temperatures,
the sample became amorphous. On subsequently cooling this
Conclusions
We have demonstrated that the unique recognition and
reactivity of borazaaromatics can be incorporated success-
2170
J. Chem. Soc., Perkin Trans. 2, 2001, 2166–2173

View Article Online
fully into bifunctional systems. In solution, the bifunctional
borazaaromatic 14 can participate in self-assembly24 mediated
by covalent bond formation forming linear oligomers of
molecular weight greater than 2000 Da. In the solid-state, the
recognition properties of 14 serve to locate the reactive centres
in an appropriate orientation for the transformation to form
oligomeric species to occur at relatively low temperatures. The
positions of the reactive centres within 14 predispose25 the sys-
tem to form linear oligomers. Studies26 are currently underway
in our laboratory to exploit the reactivity of borazaaromatics in
systems which are predisposed to form other, more complex,
structures.
analyte (typically 1 mg dissolved in 1 mL) was layered upon the
matrix and allowed to air-dry.
Computational methods
Ab initio quantum mechanical calculations were performed
using SPARTAN (Wavefunction, Inc., 18401 Von Karman
Suite 370, Irvine, CA 92612, USA, 1998, Version 5.1.1). All
atomic coordinates were optimised fully to an RMS gradient of
less than 1 × 10Ϫ4 au.
Synthetic procedures
4-tert-Butyl-2,6-dinitrobromobenzene 9. 4-tert-Butylbromo-
benzene (20.0 g, 93.8 mmol) was added dropwise to vigorously
stirred fuming nitric acid (d 1.5, 50 mL) at 0 5 ЊC during
30 minutes. The mixture was stirred at 25 ЊC for one hour
and then at 50 ЊC for 20 minutes after which time the red
coloured mixture was poured on to vigorously stirring ice–
water (200 mL). The yellow–green precipitate was collected by
filtration and washed with water (2 × 50 mL) and saturated
aqueous NaHCO3 solution (3 × 50 mL). The solid was dis-
solved in a mixture of EtOH (200 mL) and acetone (20 mL),
with the resulting concentration of the solution under reduced
pressure affording the product as pale yellow–green coloured
needles (15.18 g, 53%). Mp 129–133 ЊC (lit.27 136 ЊC); 1H NMR
(300 MHz, CDCl3, 25 ЊC): δ 7.91 (s, 2H, ArH), 1.37 (s, 9H,
CH3); 13C NMR {1H} (75 MHz, CD3COCD3, 25 ЊC): δ 154.55,
151.56, 124.97, 103.73, 35.65, 30.62; EIMS: m/z (relative %):
302 (21) [M]ϩ, 304 (20) [M ϩ 2]ϩ, 287 (100) [M Ϫ CH3]ϩ,
289 (99) [M ϩ 2 Ϫ CH3]ϩ.
Experimental
General
Xylene was dried and 1,2-dimethoxyethane was purified by
refluxing with sodium and collected by distillation. Et2O was
dried by refluxing with Na–benzophenone and collected by dis-
tillation under N2. All other solvents and reagents were used as
received. Dichloroborane–methyl sulfide complex was supplied
by Aldrich. Aluminium chloride (powder, ≥99%) and phenyl-
dichloroborane (≥98%) were supplied by Fluka. Thin layer
chromatography (TLC) was performed on aluminium plates
coated with Merck Kieselgel 60 F254 and scrutinised under
a UV lamp. Column chromatography was performed using
Kieselgel 60 (0.040–0.063 mm mesh, Merck 9385) silica. Melt-
ing points were determined using an Electrothermal 9200 melt-
ing point apparatus and are uncorrected. 1H NMR spectra were
recorded on a Bruker AC300 (300.1 MHz) spectrometer or a
Bruker DRX500 (500.1 MHz) spectrometer using the deuter-
ated solvent as the lock and the residual solvent as the internal
reference. 13C NMR spectra were recorded on a Bruker AC300
(75.5 MHz) spectrometer using the PENDANT sequence. Elec-
tron impact mass spectrometry (EIMS) was carried out on a
VG PROSPEC mass spectrometer. Liquid secondary ion mass
spectrometry (LSIMS) was carried out on a VG ZabSpec mass
spectrometer equipped with a caesium ion gun. The sample was
dissolved in a small volume of nitrophenyl octyl ether, which
had previously been coated onto a stainless steel probe tip.
Spectra were recorded in the positive ion mode at a scan speed
of 10 s decϪ1. Infra-red spectra were recorded (KBr disks) on a
Perkin-Elmer Paragon 1000 FTIR spectrometer at ambient
temperature. All spectra were recorded at a resolution of
p-tert-Butylbenzeneboronic acid 10. A solution of 4-tert-
butylbromobenzene 21.32 g (100 mmol) in dry Et2O (100 mL)
was added to magnesium turnings and a crystal of iodine. The
resulting exotherm maintained the reaction mixture at reflux
temperature for several minutes. The resulting ethereal solution
of 4-tert-butylphenylmagnesium bromide (100 mmol) was can-
nulated onto a stirred solution of B(OMe)3 (10.4 g, 10.0 mmol)
in Et2O (50 mL) during 20 minutes at Ϫ76
5 ЊC under a
positive pressure of N2. The stirred mixture was warmed to
25 ЊC with stirring overnight and the resulting thick-grey
slurry poured onto a 1.84 M aqueous solution of sulfuric acid
(60 mL). The aqueous layer was extracted with Et2O (2 ×
20 mL), the combined extracts dried (MgSO4), and concen-
trated under reduced pressure to afford a yellow oil. Addition
of Et2O (ca. 5 mL) followed by hexane (ca. 30 mL) precipitated
the product as white needles (4.7 g). Further crops were
obtained by concentration of the mother liquor under reduced
pressure followed by the addition of more Et2O–hexane in
the presence of a small quantity of water (total yield 8.30 g,
47%). Mp 200–201 ЊC (lit.28 (water) 168–170 ЊC); IR (KBr) ν:
2 cmϪ1
.
Powder X-ray diffractograms were recorded on a Siemens
D5005 diffractometer equipped with an Auton Paar HTK 1200
heating stage (operating in reflection mode using Cu-Kα
radiation). The data were recorded for 2θ in the range 8–40Њ in
steps of 2θ of 0.014Њ with a counting time of 0.5 seconds per
step. Data were recorded at 25 and at 10 ЊC intervals within the
range 100–190 ЊC. Thermogravimetric analysis (TGA) experi-
ments were performed on a Perkin-Elmer TGA6 instrument
using nitrogen as the sample purge gas. Temperature scan
experiments were performed using a range of heating rates
(20 to 100 ЊC). Isothermal experiments were performed at
temperatures between 100 and 160 ЊC.
1
3332 (O–H) cmϪ1; H NMR (300 MHz, CD3COCD3, 25 ЊC):
δ 7.83 (AAЈ portion of AAЈXXЈ spin system, 3J(H,H) = 8.2 Hz,
2H, ArH), 7.41 (XXЈ portion of AAЈXXЈ spin system, 3J(H,H)
= 8.2 Hz, 2H, ArH), 7.12 (s, 2H, OH), 1.33 (s, 9H, CH3);
13C NMR {1H} (75 MHz, CD3COCD3, 25 ЊC): δ 153.98,
135.01, 125.22, 35.28, 21.62; EIMS: m/z (relative %): 480 (5)
[M3 Ϫ 3H2O]ϩ, 465 (68) [M3 Ϫ 3H2O Ϫ CH3]ϩ, 178 (68) [M]ϩ,
163 (100) [Mϩ Ϫ CH3].
MALDI-TOF mass spectrometry
MALDI-TOF mass spectra were recorded on a Kratos Kom-
pact Maldi III mass spectrometer. A nitrogen laser (337 nm, 85
kW peak laser power, 3 ns pulse width) was used to desorb the
sample ions, and the instrument was operated in linear time-of-
flight mode with an accelerating potential of 20 kV. A laser
power setting of 94 was used for all experiments. Results from
50 laser shots were signal averaged to give one spectrum. An
aliquot of matrix (a suspension of cobalt powder in a
methanol–glycerol mixture) was deposited on the sample plate
surface. After the matrix had dried, a small volume of the
2,6-Dinitro-4,4Ј-di-tert-butylbiphenyl 11. 4-tert-Butyl-2,6-
dinitrobromobenzene (3.20 g, 10.56 mmol) was added to a
stirred suspension of Pd(PPh3)4 (0.416 g, 0.36 mmol, 0.03 mol
equiv.) in freshly distilled 1,2-dimethoxyethane (70 mL). After
10 minutes, a solution of p-tert-butylbenzeneboronic acid (2.92
g, 16.42 mmol) in EtOH (8 mL) and a solution of Na2CO3 (2.55
g, 24 mmol) in water (24 mL) was added and the mixture heated
at reflux temperature for 5 hours. Upon cooling, the mixture
was filtered through Celite and the filtrate concentrated under
reduced pressure to afford an oil dispersed in water which was
J. Chem. Soc., Perkin Trans. 2, 2001, 2166–2173
2171

View Article Online
added to brine (50 mL) and extracted with Et2O (2 × 50 mL).
The non-aqueous layer was dried (MgSO4) and the solvent
removed under reduced pressure to afford a brown oil. Upon
standing, needle-like crystals formed in the oil; these crystals
were collected by suction filtration and washed with ethanol to
afford the product as grey needles (2.14 g, 57%). Mp 175–177
2H, C(6)H and C(8)H), 7.68 (s, 2H, NH), 7.26 (s, 2H, OH), 6.96
(s, 2H, C(3)H and C(11)H), 1.45 (s, 9H, C(7)(CH3)3), 1.36 (s,
9H, C(2)(CH3)3); 13C NMR {1H} (75 MHz, CD3COCD3, 25
ЊC): δ 151.4, 146.5, 143.0, 141.8, 141.7, 131.4, 106.1, 35.4, 35.1,
32.0, 31.8; LSIMS: m/z (relative %): 348 (37) [M]ϩ, 252 (100).
1
2,7-Di-tert-butyl-5,9-dimethoxy-5,9-dibora-4,10-diazapyrene-
4,10-diium-5,9-diuide 16. Solutions of 14 and 15 in methanol
gave 16 in situ, which precipitated as colourless rhombic crystals
either upon standing for hours or immediately if the solution
is sufficiently concentrated. Mp 330.5–333.5 ЊC; EIMS: m/z
ЊC; H NMR (300 MHz, CDCl3): δ 7.93 (s, 2H, ArH), 7.42
(d, 3J(H,H) = 8.4 Hz, 2H, ArH), 7.17 (d, 3J(H,H) = 8.4 Hz, 2H,
ArH), 1.42 (s, 9H, CH3), 1.33 (s, 9H, CH3); 13C NMR {1H}
(75 MHz, CDCl3): δ 153.96, 152.38, 151.04, 127.90, 127.61,
127.22, 125.86, 123.66, 35.67, 34.87, 31.38, 30.95; EIMS: m/z
(relative %): 356 [M]ϩ (12), 341 (100) [M Ϫ CH3]ϩ. Anal. Calcd
for C20H24N2O4: C, 67.58; H, 6.77; N, 7.92. Found: C, 67.40; H,
6.79; N, 7.86%.
1
(relative %): 376 (100) [M]ϩ, 361 (75) [M Ϫ 15]ϩ. Neither H
NMR nor 13C NMR spectra could be obtained since 16
is almost insoluble in CD3OD. NMR spectra could not be
recorded in other solvents since 16 is hydrolysed readily to 14.
2,6-Diamino-4,4Ј-di-tert-butylbiphenyl 12. 2,6-Dinitro-4,4Ј-
di-tert-butylbiphenyl (1.90 g, 5.33 mmol) was added to a mix-
ture of palladium on carbon (5%, 563 mg, 5 mol%) and EtOAc
(50 mL) and the mixture degassed under reduced pressure.
Hydrogen gas was introduced via a balloon and the mixture was
stirred for 24 hours after which time it was filtered through a
pad of Celite. The red coloured filtrate was evaporated to dry-
ness under reduced pressure and the solid residue dissolved in
CH2Cl2 (ca. 5 mL). Trituration with hexane (ca. 20 mL)
afforded 12 as a light-orange powder (1.32 g, 84%). Mp 212–
214 ЊC; IR (KBr) ν: 3434 (N–H), 3420 (N–H), 3343 (N–H)
Acknowledgements
This research was supported by the University of Birmingham
and the Engineering and Physical Sciences Research Council
(Quota award to JMAR and Postdoctoral Fellowship to
BMK).
References
1 For general reviews of solid-state organic reactions, see G. M. J.
Schmidt, Pure Appl. Chem., 1971, 27, 657; J. M. Thomas, Philos.
Trans. R. Soc. London, 1974, 277, 251; V. Ramamurthy and
K. Venkatesan, Chem. Rev., 1987, 87, 433; F. Toda, Synlett, 1993,
303; N. B. Singh, R. J. Singh and N. P. Singh, Tetrahedron, 1994, 50,
6441; J. R. Scheffer and C. Scott, Science, 2001, 291, 1712.
2 K. S. Feldman and R. F. Campbell, J. Org. Chem., 1995, 60, 1924.
3 G. W. Coates, A. R. Dunn, L. M. Henling, J. W. Ziller,
E. B. Lobkovsky and R. H. Grubbs, J. Am. Chem. Soc., 1998, 120,
3641.
4 G. W. Coates, A. R. Dunn, L. M. Henling, D. A. Dougherty and
R. H. Grubbs, Angew. Chem., Int. Ed. Engl., 1997, 36, 248.
5 G. R. Desiraju, Solid-state Ionics, 1997, 101, 839.
6 For general reviews of borazaaromatics and related compounds, see
P. M. Maitlis, Chem Rev., 1962, 62, 223; M. J. S. Dewar, Prog. Boron
Chem., 1964, 1, 235.
7 For a recent discussion of the term “supramolecular synthon”, see
A. Nangia and G. R. Desiraju, Top. Curr. Chem., 1998, 198, 57.
8 J. M. A. Robinson, B. M. Kariuki, D. Philp and K. D. M. Harris,
Tetrahedron Lett., 1997, 38, 6281.
9 K. D. M. Harris, B. M. Kariuki, C. Lambropoulos, D. Philp and
J. M. A. Robinson, Tetrahedron, 1997, 53, 8599.
10 M. C. Etter, Acc. Chem. Res., 1990, 23, 120; M. C. Etter,
J. C. Macdonald and J. Bernstein, Acta Crystallogr., Sect. B, 1990,
46, 256.
11 R. Alcala and S. Martinez-Carrera, Acta Crystallogr., Sect. B, 1972,
28, 1671.
cmϪ1
;
1H NMR (300 MHz, CD3COCD3, 25 ЊC): δ 7.53 (d,
3
3J(H,H) = 8.5 Hz, 2H, ArH), 7.23 (d, J(H,H) = 8.5 Hz, 2H,
ArH), 6.21 (s, 2H, ArH), 3.81 (s, 4H, NH2), 1.35 (s, 9H, CH3),
1.24 (s, 9H, CH3); 13C NMR {1H} (75 MHz, CD3COCD3, 25
ЊC): δ 151.75, 150.42, 145.97, 134.48, 131.10, 126.95, 111.16,
102.84, 35.01, 34.68, 31.61; EIMS: m/z (relative %): 296 (100)
[M]ϩ (100), 281 (42) [M Ϫ CH3]ϩ. HRMS. Calcd. for C20H28N2
[M]ϩ: 296.2253. Found: 296.2255.
2,7-Di-tert-butyl-5,9-diphenyl-5,9-dibora-4,10-diazapyrene-
4,10-diium-5,9-diuide 13. Dichlorophenylborane (318 mg,
2 mmol) was added in small portions to a solution of the
diamine 12 (296 mg, 1 mmol) in dry xylene. The stirred mixture
was heated at reflux temperature for 90 minutes, cooled, and
AlCl3 (27 mg, 0.2 mmol) added. The mixture was refluxed for
15 hours, cooled, additional AlCl3 added (27 mg, 0.2 mmol)
and the mixture refluxed for a further 4 hours. Standard aque-
ous workup followed by column chromatography (SiO2,
CH2Cl2 : hexane, 4 : 1, v/v) afforded the product as an off-white
foamy solid (102 mg, 22%). Mp 289–291 ЊC; IR (KBr) ν: 3368
(N–H) cmϪ1; 1H NMR (300 MHz, CDCl3, 25 ЊC): δ 8.67 (s, 2H,
ArH), 7.94–7.91 (m, 6H), 7.60–7.50 (m, 6H), 7.17 (s, 2H, ArH),
1.46 (s, 9H, CH3), 1.44 (s, 9H, CH3); 13C NMR {1H} (75 MHz,
CDCl3, 25 ЊC): δ 151.2, 146.8, 139.4, 139.2, 135.6, 133.5, 128.8,
128.3, 111.5, 108.0, 35.1, 35.0, 31.8, 31.7; EIMS: m/z (relative
%): 468 (100) [M]ϩ, 453 (77) [M Ϫ CH3]ϩ. HRMS. Calcd for
C32H35B2N2 [M ϩ H]ϩ: 469.2986. Found: 469.3002.
12 J. Yang, J.-L. Marendaz, S. J. Geib and A. D. Hamilton,
Tetrahedron, 1994, 35, 3665.
13 D. J. Duchamp and R. E. Marsh, Acta Crystallogr., Sect. B, 1969,
25, 5.
14 S. V. Kolotuchin, E. E. Fenlon, S. R. Wilson, C. J. Loweth and
S. C. Zimmerman, Angew. Chem., Int. Ed. Engl., 1995, 34, 2654.
15 There is literature precedent for the synthesis of a borazapyrene
(5,9-diphenyl-5,9-dibora-4,10-diazapyrene-4,10-diium-5,9-diuide):
S. S. Chissick, M. J. S. Dewar and P. M. Maitlis, Tetrahedron Lett.,
1960, 23, 8.
2,7-Di-tert-butyl-5,9-dihydroxy-5,9-dibora-4,10-diazapyrene-
4,10-diium-5,9-diuide 14. Dichloroborane–dimethyl sulfide
complex (943 mg, 6.51 mmol) was added in small portions to a
solution of the diamine 12 (643 mg, 2.17 mmol) in dry xylene.
The stirred mixture was heated at reflux temperature for
2 hours, cooled, AlCl3 (87 mg, 0.3 mmol) added and the mix-
ture heated at reflux temperature for a further 18 hours. The
mixture was washed with water, before addition of hexane
to the organic phase which precipitated 15 as a white powder
(546 mg, 1.57 mmol, 72%, based upon molecular weight of 14).
Recrystallisation of this powder by slow diffusion of hexane
into a solution of 15 in a mixture of hexane and acetone gave
colourless crystals of [142ؒacetone]. Spectroscopic data for [142ؒ
acetone]: Mp >400 ЊC (transforms into 15 at temperatures >100
ЊC); IR (KBr) ν: 3529–3184 (several absorptions, O–H and
N–H) cmϪ1; 1H NMR (300 MHz, CD3COCD3, 25 ЊC): δ 8.55 (s,
16 J. Cornforth, A. F. Sierakowski and T. W. Wallace, J. Chem. Soc.,
Perkin Trans. 1, 1982, 2299.
17 N. Miyarua and A. Suzuki, Chem Rev., 1995, 95, 2457.
18 M. J. S. Dewar, R. B. K. Dewar and Z. L. F. Gaibel, Org. Synth. Coll.
Vol., 1973, 5, 727.
19 Crystal data for 16 at 294(2) K: [C22H30B2N2O2], M = 376.10 g molϪ1
,
monoclinic, space group C2/m, a = 10.997(2), b = 7.012(2),
c = 14.878(4) Å, β = 106.331(14)Њ, V = 1101.0(5) Å3, Z = 2, Dc = 1.134
g cmϪ3, λ = 0.71069 Å, F(000) = 404. A colourless plate-like crystal
of dimensions 0.3 mm × 0.2 mm × 0.1 mm was used. Data
were measured on a Rigaku R-AXIS II rotating anode X-ray
diffractometer using graphite-monochromated Mo-Kα radiation. In
total, 1047 unique reflections were measured (5.70Њ ≤ 2θ ≤ 51.30Њ,
R(int) = 0.058) of which 655 were considered to be observed.
The structure was solved by direct methods (SHELXS86,
G. M. Sheldrick, University of Göttingen, Germany, 1986) and
2172
J. Chem. Soc., Perkin Trans. 2, 2001, 2166–2173

View Article Online
non-hydrogen atom positions were refined (SHELXL93, G. M.
Sheldrick, University of Göttingen, Germany, 1993) with
anisotropic displacement parameters by full-matrix least squares to
give R = 0.068, wR2 = 0.194. Crystallographic data (excluding
structure factors) for this structure have been deposited with the
Cambridge Crystallographic Data Centre, CCDC 165970. See http:/
/www.rsc.org/suppdata/p2/b1/b104794a/ for crystallographic files in
.cif or other electronic format.
were normalised according to the optimised HF/6-31G geometry
of 7 (N–H and O–H distances 1.00 and 0.94 Å respectively).
Crystallographic data (excluding structure factors) for this structure
have been deposited with the Cambridge Crystallographic Data
Centre, CCDC 165969. See http://www.rsc.org/suppdata/p2/b1/
b104794a/ for crystallographic files in .cif or other electronic format.
22 G. A. Jeffrey, An Introduction to Hydrogen Bonding, Oxford
University Press, New York, 1997, pp. 98–107.
20 The four resonances in the aromatic region of the 1H NMR
spectrum of 14 in d6-acetone were assigned by NOE experiments
which were dependent on the identification of the (exchangeable)
resonance arising from the OH proton. Addition of D2O to the
sample resulted in the disappearance of the resonance at δ 7.25, and
this resonance was thus assigned as the OH proton.
23 F. H. Allen, W. D. S. Motherwell, P. R. Raithby, G. P. Shields and
R. Taylor, New J. Chem., 1999, 25.
24 D. Philp and J. F. Stoddart, Angew. Chem., Int. Ed. Engl., 1996, 35,
1154; D. S. Lawrence, T. Jiang and M. Levett, Chem. Rev., 1995, 95,
2229; J. S. Lindsey, New J. Chem., 1991, 15, 153; G. M. Whitesides,
J. P. Mathias and C. T. Seto, Science, 1991, 254, 1312.
25 For examples of covalent self-assembly, see S. J. Rowan, P. A. Brady
and J. K. M. Sanders, Angew. Chem., Int. Ed. Engl., 1996, 35, 2143;
P. A. Brady and J. K. M. Sanders, Chem. Soc. Rev., 1997, 26, 327;
S. J. Rowan and J. K. M. Sanders, J. Org. Chem., 1998, 63, 1536;
J. Ipaktschi, R. Hosseinzadeh and P. Schlaf, Angew. Chem., Int. Ed.,
1999, 38, 1658.
26 P. J. Comina, D. Philp, B. M. Kariuki and K. D. M. Harris, Chem.
Commun., 1999, 2279.
27 I. Boedtker, Bull. Soc. Chim. Fr., 1906, 35, 830.
28 K. S. Chan, X. Zhou, M. T. Au and C. Y. Tam, Tetrahedron, 1995,
51, 3129.
21 Crystal data for [142ؒacetone] at 296(2) K: [(C25H26B2N2O2)2ؒ
C3H6O], M = 754.18 g molϪ1, orthorhombic, space group Pca21,
a = 16.598(2), b = 11.1270(12), c = 24.240(3) Å, V = 4476.8(8) Å3,
Z = 4, Dc = 1.119 g cmϪ3, λ = 0.71069 Å, F(000) = 1616. A colourless
block-like crystal of dimensions 0.5 mm × 0.3 mm × 0.2 mm was
used. Data were measured as described for 16; 8190 unique
reflections were measured (3.36Њ ≤ 2θ ≤ 51.38Њ, R(int) = 0.067) of
which 7860 were considered to be observed. Structure solution and
refinement as described for 16 gave R = 0.056, wR2 = 0.128. Given the
unusual bonding in this system, the N–H and O–H distances were
not normalised using standard distances; rather, these distances
J. Chem. Soc., Perkin Trans. 2, 2001, 2166–2173
2173