Developing nitrosocarborane chemistry

Article abstract of DOI:10.1039/c5dt04831a

The new nitrosocarboranes [1-NO-2-R-1,2-closo-C₂B₁₀H₁₀] [R = CH₂Cl (1), CH₃OCH₂ (2) p-MeC₆H₄ (3), SiMe₃ (4) and SiMe₂^tBu (5)] and [1-NO-7-Ph-1,7-closo-C₂B₁₀H₁₀] (6) were synthesised by reaction of the appropriate lithiocarborane in diethyl ether with NOCl in petroleum ether followed by quenching the reaction with aqueous NaHCO₃. These bright-blue compounds were characterised spectroscopically and, in several cases, crystallographically including structural determinations of 2 and 6 using crystals grown in situ on the diffractometer from liquid samples. In all cases the nitroso group bonds to the carborane as a 1e substituent (bent C-N-O sequence) and has little or no influence on <δ¹¹B>, the weighted average ¹¹B chemical shift, relative to that in the parent (monosubstituted) carborane. Mono- and dinitroso derivatives of 1,1′-bis(m-carborane), compounds 7 and 8 respectively, were similarly synthesised but attempts to prepare the mononitroso 1,1′-bis(o-carborane) by the same protocol led only to the hydroxylamine species [1-(1′-1′,2′-closo-C₂B₁₀H₁₁)-2-N(H)OH-1,2-closo-C₂B₁₀H₁₀] (9); the desired compound [1-(1′-1′,2′-closo-C₂B₁₀H₁₁)-2-NO-1,2-closo-C₂B₁₀H₁₀] (10) was only realised by switching to a non-aqueous work-up. The involvement of water in effecting the net reduction of the NO function in 10 to N(H)OH in 9 was confirmed by a series of experiments involving [1-N(H)OH-2-Ph-1,2-closo-C₂B₁₀H₁₀] (11), [1-(1′-2′-D-1′,2′-closo-C₂B₁₀H₁₀)-2-D-1,2-closo-C₂B₁₀H₁₀] (12) and [1-(1′-2′-D-1′,2′-closo-C₂B₁₀H₁₀)-2-N(H)OH-1,2-closo-C₂B₁₀H₁₀] (13). It is suggested that during aqueous work-up a water molecule, H-bonded to the acidic C2′H of 10, is "delivered" to the adjacent C2NO unit. The ability of the NO group in nitrosocarboranes to undergo Diels-Alder cycloaddition reactions with cyclic 1,3-dienes was established via the syntheses of [1-(NOC₁₀H₁₄)-1,2-closo-C₂B₁₀H₁₁] (14) and [1-(NOC₆H₈)-2-Ph-1,2-closo-C₂B₁₀H₁₀] (15). This strategy was then utilised to prepare derivatives of the elusive dinitroso compounds of [1,2-closo-C₂B₁₀H₁₂] and 1,1′-bis(o-carborane) leading to the sterically-crowded products [1,2-(NOC₆H₈)₂-1,2-closo-C₂B₁₀H₁₀] (16, prepared as meso and racemic diastereoisomers), [1-{1′-2′-(NOC₆H₈)-1′,2′-closo-C₂B₁₀H₁₀}-2-(NOC₆H₈)-1,2-closo-C₂B₁₀H₁₀] (17) and [1-(1′-1′,2′-closo-C₂B₁₀H₁₁)-2-(NOC₆H₈)-1,2-closo-C₂B₁₀H₁₀] (18).

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Developing nitrosocarborane chemistry†
Samuel L. Powley, Louise Schaefer, Wing. Y. Man, David Ellis, Georgina M. Rosair and
Alan J. Welch*
Cite this: DOI: 10.1039/c5dt04831a
The new nitrosocarboranes [1-NO-2-R-1,2-closo-C2B10H10] [R = CH2Cl (1), CH3OCH2 (2) p-MeC6H4 (3),
t
SiMe3 (4) and SiMe2 Bu (5)] and [1-NO-7-Ph-1,7-closo-C2B10H10] (6) were synthesised by reaction of the
appropriate lithiocarborane in diethyl ether with NOCl in petroleum ether followed by quenching the
reaction with aqueous NaHCO3. These bright-blue compounds were characterised spectroscopically and,
in several cases, crystallographically including structural determinations of 2 and 6 using crystals grown
in situ on the diffractometer from liquid samples. In all cases the nitroso group bonds to the carborane as
a 1e substituent (bent C–N–O sequence) and has little or no influence on <δ11B>, the weighted average
11B chemical shift, relative to that in the parent (monosubstituted) carborane. Mono- and dinitroso derivatives
of 1,1’-bis(m-carborane), compounds 7 and 8 respectively, were similarly synthesised but attempts to
prepare the mononitroso 1,1’-bis(o-carborane) by the same protocol led only to the hydroxylamine
species [1-(1’-1’,2’-closo-C2B10H11)-2-N(H)OH-1,2-closo-C2B10H10] (9); the desired compound [1-(1’-
1’,2’-closo-C2B10H11)-2-NO-1,2-closo-C2B10H10] (10) was only realised by switching to a non-aqueous
work-up. The involvement of water in effecting the net reduction of the NO function in 10 to N(H)OH in
9 was confirmed by a series of experiments involving [1-N(H)OH-2-Ph-1,2-closo-C2B10H10] (11), [1-(1’-2’-
D-1’,2’-closo-C2B10H10)-2-D-1,2-closo-C2B10H10] (12) and [1-(1’-2’-D-1’,2’-closo-C2B10H10)-2-N(H)OH-
1,2-closo-C2B10H10] (13). It is suggested that during aqueous work-up a water molecule, H-bonded
to the acidic C2’H of 10, is “delivered” to the adjacent C2NO unit. The ability of the NO group in nitroso-
carboranes to undergo Diels–Alder cycloaddition reactions with cyclic 1,3-dienes was established via the
syntheses of [1-(NOC10H14)-1,2-closo-C2B10H11] (14) and [1-(NOC6H8)-2-Ph-1,2-closo-C2B10H10] (15).
This strategy was then utilised to prepare derivatives of the elusive dinitroso compounds of [1,2-closo-
C2B10H12] and 1,1’-bis(o-carborane) leading to the sterically-crowded products [1,2-(NOC6H8)2-1,2-
closo-C2B10H10] (16, prepared as meso and racemic diastereoisomers), [1-{1’-2’-(NOC6H8)-1’,2’-closo-
C2B10H10}-2-(NOC6H8)-1,2-closo-C2B10H10] (17) and [1-(1’-1’,2’-closo-C2B10H11)-2-(NOC6H8)-1,2-closo-
C2B10H10] (18).
Received 10th December 2015,
Accepted 11th January 2016
DOI: 10.1039/c5dt04831a
www.rsc.org/dalton
closo-C2B10H10] and [1-NO-1,7-closo-C2B10H11] in a paper which
also included a brief reference to the syntheses of [1-NO-7-Me-
Introduction
Nitrosocarboranes (strictly C-nitrosocarboranes since the NO 1,7-closo-C2B10H10] and the dinitrosocarborane [1,7-(NO)2-1,7-
group in principle could be attached to either a cage C or a closo-C2B10H10] (III).4 The area then lay dormant for more than
cage B atom, although only the former are currently known) 30 years until Fox et al. reported detailed synthetic and
were first reported in 1964 when Kauffman and co-workers pre- spectroscopic studies of [1-NO-2-Me-1,2-closo-C2B10H10] and
pared [1-NO-1,2-closo-C2B10H11] (I) and [1-NO-2-Me-1,2-closo- [1-NO-2-Ph-1,2-closo-C2B10H10] (IV) as well as a crystallographic
C2B10H10] by reaction of the appropriate lithiocarborane with determination of the latter, the first such structural study of a
NOCl.1 Subsequently, Zakharkin and colleagues described nitrosocarborane.5
both [1-NO-1,12-closo-C2B10H11]2 and [1-NO-1,7-closo-C2B10H11
]
It is well established in transition-metal chemistry that the
(II)3 and then reported several reactions of [1-NO-2-Me-1,2- NO group can formally act as either a 1e ligand (bent M–N–O
sequence) or a 3e ligand (linear M–N–O) to the metal centre6
and that facile switching between these two modes is poss-
Institute of Chemical Sciences, Heriot-Watt University, Edinburgh, EH14 4AS, UK.
ible.7 Since the structures (and consequent reactivity) of
E-mail: a.j.welch@hw.ac.uk; Tel: +44 (0)131 451 3217
boranes, heteroboranes, carboranes and heterocarboranes
†CCDC 1440360–1440371. For crystallographic data in CIF or other electronic
format see DOI: 10.1039/c5dt04831a
are dependent on their numbers of skeletal electron pairs,8
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Table 1 11B NMR spectroscopic data for 1–6 and related compounds
substituents to these clusters that have the capacity to alter by
two the number of electrons they provide to the polyhedron
are naturally of interest. Our ability to exploit this potential
switching between 1e and 3e donation of the NO substituent is
likely to rely on ready access to a range of nitrosocarboranes
which is wider than that currently reported. Hence in this con-
tribution we describe the synthesis and characterisation of a
number of new nitrosocarboranes and related compounds
including, in some cases, their Diels–Alder adducts with cyclic
1,3-dienes. Many of the species reported have been studied
crystallographically including two examples where diffraction-
quality crystals were grown by in situ cooling.
11B NMR
Compound
patterna
<δ11B>
[1,2-closo-C2B10H12
[1-NO-1,2-closo-C2B10H11] (I)
[1-CH2Cl-1,2-closo-C2B10H11
[1-NO-2-CH2Cl-1,2-closo-C2B10H10] (1)
[1-CH2OCH3-1,2-closo-C2B10H11
[1-NO-2-CH2OCH3-1,2-closo-C2B10H10] (2)
[1-(p-MeC6H4)-1,2-closo-C2B10H11
[1-NO-2-(p-MeC6H4)-1,2-closo-C2B10H10
(3)
]
2 : 2 : 4 : 2
1 : 1 : 2 : 6
–10.7
–10.5
–9.7
]
1 : 1 : 2 : 2 : 4
1 : 1 : 2 : 2 : 4
1 : 1 : 2 : 2 : 4
1 : 1 : 2 : 2 : 4
1 : 1 : 2 : 4 : 2
1 : 1 : 2 : 2 : 4
–9.8
]
–10.3
–10.2
–9.3
]
]
–9.7
[1-SiMe3-1,2-closo-C2B10H11
]
1 : 1 : 2 : 2 : 2 : 2
2 : 2 : 6
1 : 1 : 2 : 2 : 2 : 2
1 : 1 : 2 : 2 : 4
–9.0
–9.0
–8.8
–8.7
[1-NO-2-SiMe3-1,2-closo-C2B10H10] (4)
t
[1-SiMe2 Bu-1,2-closo-C2B10H11
]
t
[1-NO-2-SiMe2 Bu-1,2-closo-C2B10H10] (5)
Results and discussion
[1,7-closo-C2B10H12
[1-NO-1,7-closo-C2B10H11] (II)
[1,7-(NO)2-1,7-closo-C2B10H10] (III)
[1-Ph-1,7-closo-C2B10H11
]
2 : 2 : 4 : 2
1 : 1 : 2 : 4 : 2
2 : 2 : 4 : 2
1 : 1 : 4 : 2 : 2
1 : 1 : 4 : 2 : 2
–12.1
–12.4
–12.9
–11.0
–11.5
The simple nitrosocarboranes [1-NO-1,2-closo-C2B10H11] (I),
[1-NO-2-CH2Cl-1,2-closo-C2B10H10] (1), [1-NO-2-CH3OCH2-1,2-
closo-C2B10H10] (2), [1-NO-2-(p-MeC6H4)-1,2-closo-C2B10H10] (3),
]
[1-NO-7-Ph-1,7-closo-C2B10H10] (6)
a From high frequency to low frequency, in CDCl3.
t
[1-NO-2-SiMe3-1,2-closo-C2B10H10] (4), [1-NO-2-SiMe2 Bu-1,2-
closo-C2B10H10] (5), [1-NO-1,7-closo-C2B10H11] (II), [1-NO-7-Ph-
1,7-closo-C2B10H10] (6), and [1,7-(NO)2-1,7-closo-C2B10H10] (III)
were either made (compounds 1–6) or remade (compounds I–
III) by the established method of deprotonation of the parent
carborane with n-BuLi in Et2O followed by reaction of the
lithiocarborane with a large excess of NOCl in 40–60 °C
petroleum ether (petrol). Work-up involved quenching the
reaction mixture with aqueous NaHCO3 and retention of the
non-aqueous phase, with the product subsequently purified
by column chromatography on silica. The bright-blue nitro-
socarboranes, afforded in moderate yields except for III, are
generally isolated as relatively volatile solids although 2 and 6
are both liquids at room temperature.
t
The structural identity of [1-NO-2-SiMe2 Bu-1,2-closo-
C2B10H10] (5) was established by a crystallographic study using
crystals grown by slow evaporation of a petrol solution, and a
perspective view of a single molecule is shown in Fig. 1. The
determination is highly precise with e.s.d.s on B–B connectivi-
ties of only 0.0013–0.0014 Å. The C1–N1–O1 sequence is bent,
Although compound I–III are all literature compounds1–3
relatively little data, and certainly no NMR spectroscopic data,
were reported so we have re-synthesised them for comparison
with the new nitrosocarboranes 1–6. All compounds show a
strong absorption for the NO group in the range
1560–1570 cm−1 which suggests the likelihood of a bent C–N–O
sequence by comparison with that for the crystallographically-
established compound [1-NO-2-Ph-1,2-closo-C2B10H10] (νmax
=
1560 cm−1),5 although it has been pointed out that the NO
stretching frequency is unreliable in distinguishing between
bent and linear M–N–O fragments in transition-metal nitrosyls.9
Table 1 summarises the 11B NMR patterns and <δ11B>, the
weighted average 11B chemical shifts, for I–III and 1–6,
together with those of their immediate precursors. Replace-
ment of a CH by a CNO has little effect on the average chemi-
cal shift. Within each of the six pairs of compounds with 1,2-
C2B10 structures the maximum variation in <δ11B> is only
0.4 ppm, for the (p-MeC6H4) species. For the 1,7-C2B10 species
there is perhaps more evidence of a systematic change in
<δ11B> in that each successive NO group seems to result in an
increase in shielding with the average 11B resonance falling by
ca. 0.4–0.5 ppm. Further studies to determine the generality of
this embryonic pattern appear warranted.
Fig. 1 Perspective view of compound 5. In this and all subsequent
figures the displacement ellipsoids are drawn at the 50% probability
level, except for H atoms.
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112.98(7)°, confirming that the NO group in 5 is a 1e substitu-
t
ent, and O1 lies effectively trans to the SiMe2 Bu group with
the torsion angle O1–N1–C1–C2 being −176.44(7)°. A very
similar orientation was observed by Fox and co-workers in [1-
NO-2-Ph-1,2-closo-C2B10H10]].5 Relative to this latter species,
the only other nitrosocarborane to have been crystallographi-
cally-studied, the C1–N1 and N1–O1 distances in 5 are some-
what longer, 1.5143(11) vs. 1.421(7) Å, and somewhat shorter,
1.1736(10) vs. 1.241(7) Å, respectively.
As noted, the compounds [1-NO-2-CH3OCH2-1,2-closo-
C2B10H10] (2) and [1-NO-7-Ph-1,7-closo-C2B10H10] (6) are liquids
at room temperature, and we were attracted by the challenge of
trying to grow single crystals of each of them in situ on the
diffractometer. This was successful in both cases but particu-
larly so for compound 2, where a crystal was grown that
diffracted well to θmax = 25° (Mo-Kα radiation) and afforded a
reasonably precise determination (e.s.d.s on B–B connectivities
0.006–0.008 Å). As shown in Fig. 2 the C1–N1–O1 unit is bent,
112.4(3) and 112.1(4)° for the two crystallographically-indepen-
dent molecules, and the NO group adopts a similar orientation
with respect to the substituent on C2 as was observed in [1-
NO-2-Ph-1,2-closo-C2B10H10] and in 5 [O1–N1–C1–C2 = 179.7(3)
and 178.1(4)°]. C1–N1 and N1–O1 are 1.494(5) and 1.185(4) Å
for one independent molecule and 1.501(5) and 1.183(5) Å for
the other.
Fig. 3 Perspective view of compound 6. The NO group is disordered
between vertices 1 (54%) and 10 (46%) so that shown is the major
component. The large thermal ellipsoids reflect the relatively imprecise
determination but the structure of the molecule is nevertheless un-
ambiguously established.
The structural study of 6 was of lower precision (e.s.d.s on
B–B connectivities 0.02–0.03 Å), in part because of the rela-
tively poor quality of crystal that could be grown (not single
and significant diffraction to only θmax = 22°) and in part graphic determinations of 2 and 6 represent the first such
because there is 54 : 46 disorder between the {C1N1O1} and studies of carboranes in which single crystals were grown by
cooling samples which are liquid at room temperature.
An area of considerable current activity in carborane chem-
{B10H10} fragments (that shown in Fig. 3 is the major com-
ponent). Nevertheless the structural identity of 6 as 1-NO-7-Ph-
1,7-closo-C2B10H10 with a bent C1–N1–O1 unit, 117(3)°, is fully istry is that of bis(carboranes),10 molecules composed of two
carborane units connected by a 2c–2e bond, and accordingly
we have explored both mono-NO and di-NO derivatives of 1,1′-
bis(o-carborane) and 1,1′-bis(m-carborane) (Fig. 4).
confirmed by this study. As far as we are aware the crystallo-
The deprotonation of 1,1′-bis(m-carborane) with three
equivalents of n-BuLi followed by reaction with excess NOCl
and the standard aqueous work-up results in the formation of
two bright-blue products separated by column chromato-
graphy on silica, the mononitroso [1-(1′-1′,7′-closo-C2B10H11)-7-
NO-1,7-closo-C2B10H10] (7) and the dinitroso [1-(1′-7′-NO-1′,7′-
closo-C2B10H10)-7-NO-1,7-closo-C2B10H10] (8), in 10% and 42%
yields, respectively. Both compounds were initially character-
ised by microanalysis, mass spectrometry and NMR spectro-
scopy, and ultimately compound
8 was subjected to
crystallographic analysis. The molecule has crystallographi-
cally-imposed Ci symmetry about the centre of the C1–C1′
bond and the O atom is disordered over four sites with that
shown in Fig. 5 the major (60%) component. The C7–N71 and
N71–O73 distances are 1.502(2) and 1.089(3) Å and the C7–
N71–O73 angle is 120.7(2)°.
An attempt to add one NO group to 1,1′-bis(o-carborane)
was, however, not successful. Following deprotonation of 1,1′-
bis(o-carborane) with one equivalent of n-BuLi in Et2O and
addition of the suspension thus formed to an excess of NOCl
Fig. 2 Perspective view of compound 2.
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Fig. 4 (Left) 1,1’-Bis(o-carborane) and (right) 1,1’-bis(m-carborane).
Fig. 5 Perspective view of compound 8. The molecule sits on a crystallo-
graphic inversion centre located in the middle of the C1–C1’ bond. The
O atom is disordered over four sites with that shown the major one.
Fig. 6 Perspective view of compound 9. The molecule has crystallo-
graphically-imposed Ci symmetry with the inversion centre located in
the middle of the C1–C1’ bond. The N(H)OH group is disordered
between C2 and C2’.
in petrol a dark-green solution was produced, but on adding
this to aqueous NaHCO3 the blue organic layer decolourised
after a short time. Work-up subsequently afforded a low (IV). By the standard aqueous procedure IV was isolated in
yield of the white hydroxylamine derivative [1-(1′-1′,2′-closo- 52% yield. Alternatively, following synthesis in Et2O/petrol the
C2B10H11)-2-N(H)OH-1,2-closo-C2B10H10] (9). An accurate mass solvent was evaporated and the product extracted into petrol
determination confirmed the molecular formula and in the 1H and purified by column chromatography on florisil (using
NMR spectrum are doublets at δ 6.11 (NH) and 5.51 (OH) ppm petrol as eluent) affording IV in 42% yield. In both cases the
1
(identified as such by a H,15N HMQC experiment which also identity of the product was checked by comparison of its 11B
revealed a 15N chemical shift of 133 ppm in 9) each integrating NMR spectrum against an authentic sample.5
for one H atom, together with a broad singlet at δ 3.98 ppm
Thereafter the reaction that led to the isolation of 9 was
(cage CH). Finally, the molecular structure of 9 was established repeated substituting the aqueous work-up by this non-
by a crystallographic study (Fig. 6). The molecule resides on a aqueous protocol, leading to the isolation of the desired com-
crystallographic inversion centre necessitating 1 : 1 disorder pound [1-(1′-1′,2′-closo-C2B10H11)-2-NO-1,2-closo-C2B10H10] (10)
between the C2N(H)OH unit and the C2′H′ fragment. The in effectively the same yield achieved for 9. Compound 10 dis-
N21–O22 distance is 1.427(3) Å and the C1–C2 distance 1.7677 plays a clear NO stretch in the IR spectrum (1573 cm−1) and no
(18) Å, the latter being significantly longer than that in 2 evidence of NH or OH resonances in the 1H NMR spectrum,
[1.661(5) and 1.646(5) Å, two independent molecules] and 5 and an accurate mass determination is fully consistent with no
[1.6597(11) Å] and in 1,1′-bis(o-carborane) itself [1.6950(8) and reduction of the nitroso group having occurred. Final confir-
1.6991(8) Å; C2 and B3 are equally disordered]11 as the result mation was provided by a crystallographic study (Fig. 7). Crys-
of intramolecular steric crowding between N(H)OH and tallographically compounds 9 and 10 are almost isomorphous
C2B10H11 substituents.
and the NO group in 10 [C2–N21 1.441 (3), N21–O22 1.195(4)
The formation of a hydroxylamine from a nitrosyl is for- Å, C2–N21–O22 113.8(3)°] is equally disordered between C2
mally reduction12 and prompts consideration of the source of and C2′.
that reduction. Our first suspicions centred on the aqueous
This strongly suggests that the source of the reduction of
work-up that has been used in the synthesis of all nitroso- the NO function in 10 to the hydroxylamine unit in 9 is water,
carboranes to date so initially we trialled a non-aqueous work-up but it also suggests that the aqueous work-up of [1-(1′-1′,2′-
of a well-characterised species, [1-NO-2-Ph-1,2-closo-C2B10H10
]
closo-C2B10H11)-2-NO-1,2-closo-C2B10H10] is somehow different
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Fig. 7 Perspective view of compound 10. The NO group is disordered
between C2 and C2’ and there is a crystallographic inversion centre in
the mid-point of the C1–C1’ bond.
to the aqueous work-up of the other nitrosocarboranes pre-
pared to date since these latter species are not converted to
hydroxylamines. It must be remembered, however, that
although the work-up is aqueous it is also biphasic with the
nitrosocarborane in the non-aqueous phase and therefore
generally protected from the effects of water. Support for this
arises from an experiment in which IV in THF was treated with
Fig. 8 Perspective view of compound 11. Only one of the two crystallo-
graphically-independent molecules is shown.
water. The blue solution immediately begins to fade and the exchange with solvent) compound 13 was clearly the major
product.
These experiments conclusively establish that in the syn-
colourless hydroxylamine product [1-N(H)OH-2-Ph-1,2-closo-
C2B10H10] (11) was ultimately isolated. The identity of the com-
pound was established by accurate mass measurement, by the thesis of compound 9 the nitroso group in [1-(1′-1′,2′-closo-
presence of NH (δ 5.70 ppm) and OH (δ 5.18 ppm) resonances C2B10H11)-2-NO-1,2-closo-C2B10H10] is converted to hydroxyl-
1
in the H NMR spectrum (each integrating for 1H against the amine by water as part of the aqueous work-up. We propose
aromatic 5H) and ultimately by a crystallographic study that the unique structure of the 1,1′-bis(o-carborane) frame-
(Fig. 8). There are two crystallographically-independent mole- work is responsible for this phenomenon. Even though the
cules in the asymmetric fraction of the unit cell which are system is formally biphasic we believe that a water molecule
practically identical. The N–O bond distances are 1.4305(13) becomes H-bonded to the acidic C2′H atom and is thus “deli-
and 1.4323(12) Å, close to the corresponding distance in 9.
vered” to the adjacent C2NO unit effecting the reduction. This
Although it seems likely that it is water which is converting is a possibility unique to 1,1′-bis(o-carborane). Thus, for
the NO group in 10 to hydroxylamine we also considered the example, in the case of the synthesis of the mononitroso bis
possibility of H transfer from the adjacent C2′ atom to the (m-carborane) compound 7 involving aqueous work-up, even if
C2NO unit (were this to be the source of the reduction it is an H2O molecule is similarly H-bonded to C7′H atom it never
feasible that C2′ would subsequently pick up an H from the gets to be adjacent to the nitroso function.
solvent). To test this possibility we first prepared 12, the C,C′-
Two glaring omissions from the list of nitrosocarboranes in
dideutero analogue of 1,1′-bis(o-carborane) by double deproto- this and previous papers are the dinitroso derivative of 1,2-
nation of 1,1′-bis(o-carborane) and addition of D2O. Com- closo-C2B10H12 and the dinitroso derivative of 1,1′-bis(o-carbor-
pound 12 was afforded in >90% yield and its identity ane). We have attempted to synthesise both of these on mul-
established by an accurate mass measurement (calculated tiple occasions with both aqueous and non-aqueous work-up
288.3850, found 288.3850). Compound 12 was then used in protocols but were never able to isolate blue products
the targeted synthesis of its mononitroso derivative involving (although initial blue species are observed). This is presumably
aqueous work-up, resulting in the isolation of the hydroxyl- due to coupling of the two adjacent NO groups into either a
amine product [1-(1′-2′-D-1′,2′-closo-C2B10H10)-2-N(H)OH-1,2- furoxan or a cis-azodioxy unit as is well established in organo-
closo-C2B10H10] (13). This was identified by 1H NMR spectro- dinitroso chemistry.13 Indeed, the NO groups in the putative
scopy (NH at δ 6.10 ppm, OH at δ 5.50 ppm, no cage CH) compound [1,2-(NO)2-1,2-closo-C2B10H10] are ideally oriented
and by an accurate mass measurement (calculated 318.3846, for furoxan coupling and those in the putative [1-(1′-2′-NO-
found 318.3852). Although a small amount of the C2′H ana- 1′,2′-closo-C2B10H10)-2-NO-1,2-closo-C2B10H10] are perfectly set
logue 9 was also detected (presumably resulting from H/D up for cis-azodioxy coupling (Fig. 9).
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Fig. 9 (left) Possible formation of a furoxan from [1,2-(NO)2-1,2-closo-C2B10H10] and (right) possible formation of a cis-azodioxy compound from
[1-(1’-2’-NO-1’,2’-closo-C2B10H10)-2-NO-1,2-closo-C2B10H10].
Since it is well-established that organonitroso compounds equivalent and a representative example is shown in the
readily undergo Diels–Alder (DA) cycloaddition with cyclic 1,3- Figure. Clearly the cycloaddition has been successful. Also suc-
dienes14 we have explored the possibility of using this to “trap cessful was the formation of a DA cycloadduct between IV and
out” the dinitroso derivatives of o-carborane and 1,1′-bis(o-car- 1,3-cyclohexadiene, the resulting product [1-(NOC6H8)-2-Ph-
borane). Since DA cycloadducts of nitrosocarboranes have not 1,2-closo-C2B10H10] (15) being isolated in ca. 30% yield and
previously been reported we initially targeted those of the fully characterised spectroscopically.
simple compounds I and IV. Reaction of I with α-terpinene
With these sighting experiments completed we turned to
(1-isopropyl-4-methyl-1,3-cyclohexadiene) in CH2Cl2 (DCM) [1,2-closo-C2B10H12]. Double deprotonation followed by reac-
resulted in decolourisation and near-quantitative formation of tion with NOCl afforded a dark-green solution in Et2O/petrol.
the cycloadduct [1-(NOC10H16)-1,2-closo-C2B10H11] (14) charac- Following removal of solvent and excess NOCl the green solid
1
terised by mass spectrometry, H and 11B NMR spectroscopies was extracted into DCM, filtered, and the filtrate treated with
and by a crystallographic study (Fig. 10). Somewhat unusually an excess of 1,3-cyclohexadiene at which point the colour
there are four crystallographically-independent molecules began to fade affording the off-white product [1,2-(NOC6H8)2-
of 14 in the asymmetric fraction of the unit cell, but all are 1,2-closo-C2B10H10] (16) as an almost equal mixture of meso
(16a) and racemic (16b) diastereoisomers, separated by pre-
parative thin-layer chromatography (TLC). Both diastereo-
isomers were characterised by mass spectrometry, 1H and
11B NMR spectroscopies and crystallographic studies (Fig. 11).
Both molecules have crystallographically-imposed sym-
metry, Cs for meso 16a and C2 for racemic 16b. One interesting
feature of both structures is the extended C1–C2 distance,
1.947(2) Å in 16a and 1.9485(19) Å in 16b. Undoubtedly this
stretching arises from steric congestion between the two bulky
substituents and is reminiscent of the deformations of the
o-carborane cage that we have previously observed with two di-
methylferrocenyl or pentamethyleneferrocenyl substituents.15
Similarly, treatment of 1,1′-bis(o-carborane) with two
equivalents of n-BuLi and then NOCl afforded a blue-green
product to which was quickly added an excess of 1,3-cyclohexa-
diene resulting in rapid decolourisation. Following work-up
involving TLC compound 17, the DA adduct of [1-(1′-2′-
NO-1′,2′-closo-C2B10H10)-2-NO-1,2-closo-C2B10H10], was isolated
together with a small amount of compound 18, the DA adduct
of the mononitroso species [1-(1′-1′,2′-closo-C2B10H11)-2-
NO-1,2-closo-C2B10H10]. As was the case with compound 16 it
was be expected that compound 17 would be formed as both
meso and racemic diastereoisomers however only a single band
was observed by TLC. The crystallographically-determined
structure has imposed Ci symmetry and shows disorder of the
NOC6H8 substituent, with both enantiomers of the chiral N1
Fig. 10 Perspective view of one of four crystallographically-indepen-
dent molecules of compound 14.
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Fig. 11 Perspective views of (left) the meso and (right) the racemic isomers of compound 16.
centre present in the ratio 75 : 25 (Fig. 12 shows only the major lised, up to the limit of 2 : 1 respectively. Irrespective of this
component of this disorder). This makes the precise interpret- ambiguity, however, the key conclusion of the isolation of the
ation of the crystallographic result ambiguous. The crystals DA adducts 16 and 17 is confirmation that the initial reactions
could be composed of only the (disordered) meso form between NOCl and doubly-deprotonated [1,2-closo-C2B10H12]
(implying that this disatereoisomer selectively crystallised) or and 1,1′-bis(o-carborane) do indeed produce the elusive
alternatively the meso and racemic forms may have co-crystal- dinitroso compounds [1,2-(NO)2-1,2-closo-C2B10H10
] and
[1-(1′-2′-NO-1′,2′-closo-C2B10H10)-2-NO-1,2-closo-C2B10H10].
A minor co-product in the synthesis of compound 17 was a
small amount of the monosubstituted species [1-(1′-1′,2′-closo-
C2B10H11)-2-(NOC6H8)-1,2-closo-C2B10H10] (18), clearly the
result of DA addition of 1,3-cyclohexadiene to the nitrosocarbo-
rane [1-(1′-1′,2′-closo-C2B10H11)-2-NO-1,2-closo-C2B10H10] formed
by incomplete deprotonation of 1,1′-bis(o-carborane), and a per-
spective view of a single molecule of 18 is shown in Fig. 13.
Fig. 12 Perspective view of compound 17. The NOC6H8 substituent is
disordered and that shown is the major component. There is a crystallo-
graphic inversion centre located at the mid-point of the C1–C1’ bond.
Fig. 13 Perspective view of compound 18.
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Both 17 and 18 feature long C1–C2 connectivities, 1.946(2) and
[1-NO-1,2-closo-C2B10H11] (I). [1,2-closo-C2B10H12] (500 mg,
1.961(6) Å, respectively, as a result of steric crowding between 3.47 mmol) was dissolved in degassed diethyl ether (30 mL).
the substituents on these atoms, as was noted for 16.
n-BuLi (3.50 mmol) was added and the solution was stirred
under N2 for 3 hours. The solution was added dropwise to a
large excess of nitrosyl chloride in petrol (30 mL) at −78 °C
and the resulting blue-green solution was stirred for
30 minutes, then added to a saturated solution of NaHCO3 in
water (100 mL). The resulting blue organic phase was separ-
ated, washed with water (60 mL), concentrated and applied to
a silica column. Elution with petrol afforded a blue mobile
band which yielded [1-NO-1,2-closo-C2B10H11] (I) as a volatile
blue solid on removal of solvent (146 mg, 24.3%). C2H11B10NO
requires C 13.9, H 6.40, N 8.09. Found C 14.1, H 6.62, H
7.94%. 11B{1H} NMR, δ −2.6 (1B), −3.6 (1B), −9.3 (2B), −13.4
(6B). 1H NMR, δ 4.45 (s, 1H, cage CH). IR, νmax 2600 (BH), 1570
(NO) cm−1. EIMS, envelope centred on m/z 173 (M+).
[1-NO-2-CH2Cl-1,2-closo-C2B10H10] (1). Similarly, a diethyl
ether solution of [1-CH2Cl-1,2-closo-C2B10H11] (500 mg,
2.26 mmol) was deprotonated with n-butyllithium and added
to a petrol solution of an excess of nitrosyl chloride. Following
work-up, 1 was isolated as a blue solid (258 mg, 44.9%).
C3H12B10ClNO requires C 16.3, H 5.46, N 6.32. Found C 17.2,
H 5.76, N 5.98%. 11B{1H} NMR, δ −2.6 (1B), −4.6 (1B), −9.8
(2B), −10.8 (2B), −12.5 (4B). 1H NMR, δ 4.90 (s, 2H, CH2Cl). IR,
νmax 2599 (BH), 1570 (NO) cm−1. EIMS, envelope centred on
Conclusions
The range of known nitrosocarboranes has been considerably
expanded and spectroscopic properties of a number of such
compounds previously synthesised have been reported. Single
crystals of two nitrosocarboranes which are liquids at room
temperature have been grown in situ on the diffractometer
allowing their structures to be determined. The conversion of
nitrosocarboranes to hydroxylaminocarboranes has been
shown to be effected by water. Dinitroso derivatives of
[1,2-closo-C2B10H12] and 1,1′-bis(o-carborane) remain unknown
since presumably such species are ideally suited to rearrange
into a furoxan and a cis-azodioxy compound respectively, but
nevertheless these nitroso compounds have been isolated in
derivatised form by converting them to stable Diels–Alder
cycloadducts.
Experimental
Synthesis
All experiments were performed under dry, oxygen-free, N2 m/z 221/222 (M+).
using standard Schlenk techniques, with some subsequent
[1-NO-2-CH2OCH3-1,2-closo-C2B10H10] (2). Similarly, [1-
manipulation in the open laboratory. Solvents were freshly dis- CH3OCH2-1,2-closo-C2B10H11] (450 mg, 2.39 mmol) in ether
tilled over Na wire (diethyl ether, petrol) or CaH2 (DCM) or (20 mL) was treated with n-BuLi (3.75 mmol) and added drop-
stored over 4 Å molecular sieves (CDCl3) and were degassed wise to an excess of nitrosyl chloride in petrol (20 mL) at
(3 × freeze–pump–thaw cycles) before use. Preparative thin-layer −78 °C. Work-up proceeded exactly as for I yielding [1-NO-2-
chromatography (TLC) employed 20 × 20 cm Kieselgel F254 CH2OCH3-1,2-closo-C2B10H10] (2) as a blue liquid (200 mg,
glass plates. NMR spectra at 300.1/400.1 MHz (1H) or 96.3/ 38.5%). C4H15B10NO requires C 22.1, H 6.96, N 6.45. Found C
128.4 MHz (11B) were recorded on a Bruker AVIII 300/400 22.9, H 7.17, N 5.75%. 11B{1H} NMR, δ −2.6 (1B), −4.7 (1B),
spectrometer respectively at ambient temperature from CDCl3 −10.6 (2B), −11.2 (2B), −12.8 (4B). 1H NMR, δ 4.66 (s, 2H,
solutions. EI mass spectra were recorded using a Finnigan CH2O), 3.46 (s, 3H, OCH3). IR, νmax 2608 (BH), 1568 (NO) cm−1
.
(Thermo) LCQ Classic ion trap mass spectrometer at the Uni- EIMS, envelope centred on m/z 217 (M+). Diffraction-quality
versity of Edinburgh. Elemental analyses were carried out crystals grown by slow cooling a sample of 2 in a Lindemann
using an Exeter CE-440 elemental analyser. Infrared spectra tube.
were recorded as DCM solutions on a PerkinElmer Spectrum
[1-NO-2-(p-MeC6H4)-1,2-closo-C2B10H10] (3). Similarly, [1-
100 FT-IR spectrometer. [1-R-1,2-closo-C2B10H11] (R = CH2Cl,16 (p-MeC6H4)-1,2-closo-C2B10H11] (503 mg, 2.15 mmol) in ether
19
t
CH2OCH3,17 p-MeC6H4,18 SiMe3
closo-C2B10H11],21 1,1′-bis(o-carborane)10a and 1,1′-bis(m-car- chloride in petrol (20 mL) at −78 °C. Work-up afforded [1-
borane)22 were prepared by literature methods or variants NO-2-(p-MeC6H4)-1,2-closo-C2B10H10] (3) as
blue solid
or SiMe2 Bu20), [1-Ph-1,7- (20 mL) was deprotonated and added to an excess of nitrosyl
a
thereof. All other reagents were supplied commercially.
(90 mg, 15.6%). C9H17B10NO requires C 41.1, H 6.51, N 5.32.
Nitrosyl chloride. Following the general method of Morton Found C 40.3, H 6.64, N 5.76%. 11B{1H} NMR, δ −1.9 (1B), −4.6
and Wilcox,23 NaNO2 (10.5 g, 0.15 mol) was dissolved in water (1B), −9.8 (2B), −10.4 (2B), −12.5 (4B). H NMR, δ 7.70 (app d,
1
(25 mL) and added dropwise to 12M hydrochloric acid (60 mL, 2H, C6H4), 7.20 (app d, 2H, C6H4), 2.38 (s, 3H, CH3). IR, νmax
0.72 mol). The resulting yellow-brown gas was carried by a flow 2607 (BH), 2585 (BH), 1564 (NO) cm−1. EIMS, envelope centred
of N2 through drying tubes containing (in order) NaNO2, KCl on m/z 263 (M+).
and CaCl2. The gas was condensed using liquid N2 into a
[1-NO-2-SiMe3-1,2-closo-C2B10H10] (4). In an analogous
bespoke vessel where it was stored at room temperature under fashion, a solution of [1-Me3Si-1,2-closo-C2B10H11] (400 mg,
its own pressure. When required, a small quantity of the gas 1.85 mmol) was dissolved in ether (20 mL) and deprotonated.
was transferred under reduced pressure and condensed using The resulting yellow solution was added dropwise to an excess
liquid N2 into the reaction vessel containing dry, degassed, of nitrosyl chloride in petrol (20 mL) which was cooled to
petrol.
−78 °C. Work-up gave 1-NO-2-SiMe3-1,2-closo-C2B10H10 (4) as a
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blue solid (193 mg, 42.6%). C5H19B10NOSi requires 24.5, in dry, degassed, diethyl ether (20 mL) and treated with
H 7.80, N 5.71. Found C 24.7, H 7.96, N 5.10%. 11B{1H} NMR, n-butyllithium solution (2.10 mmol). The solution was stirred
δ −1.7 (2B), −8.2 (2B), −11.7 (6B). 1H NMR, δ 0.39 [s, 9H, for 30 minutes at room temperature and heated at reflux for a
Si(CH3)]. IR, νmax 2603 (BH), 2582 (BH), 1567 (NO) cm−1. EIMS, further 30 minutes. The carborane solution was added drop-
envelope centred on m/z 245 (M+).
wise to an excess of nitrosyl chloride in dry, degassed, petrol
t
[1-NO-2-SiMe2 Bu-1,2-closo-C2B10H10] (5). Correspondingly, (20 mL) at −78 °C. The reaction mixture was quenched with
t
[1-SiMe2 Bu-1,2-closo-C2B10H11] was dissolved in ether (20 mL), NaHCO3 in water, and the resulting blue organic layer was
treated with n-BuLi (1.63 mmol) and added dropwise to nitro- separated, washed with water and dried. Purification by
syl chloride in petrol (20 mL) at −78 °C. After work-up, [1- column chromatography with petrol eluent on silica gave two
t
NO-2-SiMe2 Bu-1,2-closo-C2B10H10] (5) was isolated as a blue blue bands, 7 (23 mg, 10.0%) at Rf 0.67 and 8 (101 mg, 42.5%)
solid (236 mg, 53.1%). C8H25B10NOSi requires C 33.4, H 8.77, at Rf 0.79.
N 4.87. Found C 33.8, H 8.97, N 4.56%. 11B{1H} NMR, δ −0.8
7: 11B{1H} NMR, δ −3.2 (1B), −4.8 (1B), −7.6 (2B), −10.5
1
(1B), −2.0 (1B), −7.9 (2B), −10.8 (2B), −11.8 (4B). 1H NMR, (6B), −11.5 (2B), −13.6 (4B), −14.5 (4B). H NMR, δ 3.00 (br s,
δ 1.04 [s, 9H, C(CH3)], 0.38 [s, 6H, Si(CH3)2]. IR, νmax 2603 (BH), 1H, cage CH). IR, νmax 2614 (BH), 1564 (NO) cm−1. EIMS, enve-
2580 (BH), 1566 (NO) cm−1. EIMS, envelope centred on m/z lope centred on m/z 315 (M+).
288 (M+). Diffraction-quality crystals grown by slow evaporation
of a concentrated petrol solution of 5.
8: C4H20B20N2O2 requires C 14.0, H 5.85, N 8.13. Found C
14.1, H 5.98, N 8.12%. 11B{1H} NMR, δ −4.9 (2B), −7.6 (2B),
[1-NO-1,7-closo-C2B10H11] (II). n-Butyl lithium (2.78 mmol) −10.2 (4B), −11.3 (4B), −13.5 (4B), −14.9 (4B). IR, νmax 2618
was added to [1,7-closo-C2B10H12] (400 mg, 2.78 mmol) in (BH), 1565 (NO) cm−1. EIMS, envelope centred on m/z 344
diethyl ether (10 mL) at 0 °C. The reaction was conducted as (M+). Diffraction-quality crystals of 8 were grown from cooling
described previously to yield II as a blue solid (216 mg, a concentrated ethanol-hexane solution.
44.9%). 11B{1H} NMR, δ −6.6 (1B), −8.4 (1B), −11.1 (2B), −13.2
[1-(1′-1′,2′-closo-C2B10H11)-2-N(H)OH-1,2-closo-C2B10H10]
1
(4B), −17.0 (2B). H NMR, δ 3.11 (br s, 1H, cage CH). IR, νmax (9). A solution of 1,1′-bis(o-carborane) (100 mg, 0.35 mmol) in
2610 (BH), 1562 (NO) cm−1. EIMS, envelope centred on m/z diethyl ether (15 mL) was treated with n-BuLi (0.38 mmol) and
173 (M+).
added dropwise to nitrosyl chloride in petrol (15 mL) at
[1-NO-7-Ph-1,7-closo-C2B10H10] (6). Analogously, [1-Ph-1,7- −78 °C. The dark green solution was added to a saturated
closo-C2B10H11] (850 mg, 3.86 mmol) in diethyl ether was aqueous NaHCO3 solution, giving a blue organic layer which
treated with n-butyllithium solution (4.25 mmol) and added to decolourised after 2 minutes. The organic phase was separated
a petrol solution of an excess of nitrosyl chloride to give, fol- and washed with water (30 mL) resulting in a pale yellow solu-
lowing workup, the product, 6, as a blue liquid (594 mg, tion. This was dried with MgSO4, filtered and reduced to give a
61.7%). C8H15B10NO requires C 38.5, H 6.06, N 5.62. Found C yellow solid. Purification by elution through a short silica plug
37.9, H 6.04, N 4.62%. 11B{1H} NMR, δ −5.5 (1B), −8.7 (1B), with DCM eluent followed by preparative TLC (DCM/petrol
−11.0 (4B), −13.1 (2B), −15.3 (2B). 1H NMR, δ 7.28 (m, 5H, Ph). 3 : 1) gave 9 as a white solid (6 mg, 5.4%) at Rf 0.44.
IR, νmax 2613 (BH), 1562 (NO) cm−1. EIMS, envelope centred C4H23B20NO requires C 15.1, H 7.30, N 4.41. Found C 16.1, H
on m/z 249 (M+). Diffraction-quality crystals were grown from 7.04, N 3.66%. 11B{1H} NMR, δ −1.9 (3B), −6.2 (1B), −9 to −13
cooling 6 in a Lindemann tube.
(overlapping resonances with maxima at −9.5, −10.5, −12.8,
[1,7-(NO)2-1,7-closo-C2B10H10] (III). A solution of [1,7-closo- 16B). 1H NMR, δ 6.11 (br d, 1H, NH), 5.51 (d, 1H, OH), 3.98
C2B10H12] (200 mg, 1.39 mmol) in dry, degassed, diethyl ether (br s, 1H, cage CH). EIMS, centred on m/z 317. Accurate mass:
(20 mL) was treated with n-BuLi (3.00 mmol) at −78 °C. The calculated 317.3783, found: 317.3780. Cooling a petrol solution
solution was warmed to room temperature and then heated at of 9 yielded diffraction-quality crystals.
reflux for 60 minutes to give a cloudy white suspension. This
[1-NO-2-Ph-1,2-closo-C2B10H10]
(IV). [1-Ph-1,2-closo-
was added dropwise to a solution of an excess of nitrosyl chlor- C2B10H10] (250 mg, 1.13 mmol) in dry, degassed, diethyl ether
ide in 10 mL of dry, degassed, petrol at −78 °C. The resulting (15 mL) was treated with n-BuLi (1.25 mmol) and the resulting
green solution was stirred for 10 minutes and then quenched yellow solution was added dropwise to an excess of nitrosyl
by addition to a saturated solution of NaHCO3 (50 mL). The chloride in dry, degassed, petrol (15 mL) at −78 °C to give a
green-blue organic layer was separated, washed with 30 mL dark green solution.
water, dried and reduced to dryness in vacuo. The residue was
Aqueous work-up. Quenching the reaction mixture with
extracted into petrol and filtered. The blue filtrate was reduced NaHCO3 in water and work-up as described previously, includ-
in volume and purified by preparative thin layer chromato- ing purification by silica column chromatography (petrol
graphy (petrol eluent) to give III as a volatile blue solid eluent) gave IV in 52.2% yield.
(4.1 mg, 1.5%) at Rf 0.90. 11B{1H} NMR, δ −8.3 (2B), −11.8 (2B),
Non-aqueous work-up. Warming to room temperature and
evaporation of the excess nitrosyl chloride and solvents gave a
light blue residue. This was extracted into petrol and filtered.
−13.6 (4B), −17.4 (2B). IR, νmax 2618 (BH), 1565 (NO) cm−1
EIMS, envelopes centred on m/z 231 (M+ + NO) and 201 (M+).
.
[1-(1′-1′,7′-closo-C2B10H11)-7-NO-1,7-closo-C2B10H10] (7) and Removal of solvent from the filtrate and purification by
[1-(1′-7′-NO-1′,7′-closo-C2B10H10)-7-NO-1,7-closo-C2B10H10 column chromatography on florisil (petrol eluent) gave IV in
]
(8). 1,1′-Bis(m-carborane) (200 mg, 0.70 mmol) was dissolved 42.2% yield.
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[1-(1′-1′,2′-closo-C2B10H11)-2-NO-1,2-closo-C2B10H10] (10). A (overlapping resonances with maxima at −9.5, −10.5, −12.8,
1
solution of 1,1′-bis(o-carborane) (180 mg, 0.63 mmol) was dis- 16B). H NMR, δ 6.10 (br d, 1H, NH), 5.50 (d, 1H, OH). EIMS,
solved in dry, degassed, diethyl ether (15 mL) and treated with envelope centred on m/z 318 (M+). Accurate mass: calculated
n-BuLi (0.7 mmol) at 0 °C. The cloudy white suspension was 318.3846, found 318.3852.
stirred at room temperature for 45 minutes then added drop-
[1-(NOC10H16)-1,2-closo-C2B10H11
]
(14). [1-NO-1,2-closo-
wise to an excess of nitrosyl chloride in petrol (15 mL) at C2B10H11] (40 mg, 0.23 mmol) was dissolved in DCM (5 mL)
−78 °C. The resulting dark green solution was stirred for and α-terpinene (0.04 mL, 0.25 mmol) was added. The solution
60 minutes at low temperature. After warming to room temp- was stirred until it decolourised. Removal of solvent in vacuo
erature, the nitrosyl chloride and solvents were removed under and purification by preparative TLC (1 : 2 CH2Cl2/petrol) gave
reduced pressure to leave a light blue solid. This was extracted 14 as a white solid at Rf 0.74 (69 mg, 96.9%). C12H27B10NO
into petrol (20 mL) and filtered. The blue filtrate was reduced requires C 46.6, H 8.79, N 4.53. Found C 46.6, H 8.99,
and purified by chromatography on florisil with petrol as the N 4.43%. 11B{1H} NMR δ −4.0 (1B), −6.0 (1B), −10.7 (2B),
eluent. A single blue mobile band was collected which, follow- −11.7 (4B), −14.1 (1B), −14.8 (1B). 1H NMR δ 6.48 (d, 1H,
ing removal of solvent, gave 10 as a blue solid (10 mg, 5.0%). CvCH), 6.24 (d, 1H, CvCH), 3.91 (br s, 1H, cage CH), 1.91
C4H21B20NO requires C 15.2, H 6.71, N 4.44. Found C 14.6, H (m, 2H, CH2), 1.76 (m, 1H, CH), 1.69 (s, 3H, CH3), 1.30 (m, 2H,
6.72, N 4.04%. 11B{1H} NMR, δ −1.6 (2B), −2.8 (2B), −8.9 (8B), CH2), 1.03 (d, 3H, CH3), 1.00 (d, 3H, CH3). EIMS, envelope
1
−9.6 (4B), −12.7 (4B). H NMR, δ 4.25 (br s, 1H, cage CH). IR, centred on m/z 309 (M+). Diffraction-quality crystals were
νmax 2590 (BH), 1573 (NO) cm−1. EIMS, envelope centred on grown by cooling a DCM/petrol solution.
m/z 315 (M+). Accurate mass: calculated 315.3627, found:
[1-(NOC6H8)-2-Ph-1,2-closo-C2B10H10
]
(15). [1-NO-2-Ph-1,2-
315.3620. Diffraction-quality crystals of 10 were obtained from closo-C2B10H10] (224 mg, 0.90 mmol) was dissolved in 5 DCM
cooling a concentrated petrol solution.
(5 mL) and 1,3-cyclohexadiene (0.1 mL, 1.05 mmol) was
[1-N(H)OH-2-Ph-1,2-closo-C2B10H10]
(11). [1-NO-2-Ph-1,2- added. The blue solution decolourised to pale yellow over
closo-C2B10H10] (40 mg, 0.16 mmol) was dissolved in THF 10 minutes stirring. The solvent was removed in vacuo to leave
(5 mL) and water (1 mL) was added. The blue solution began a pale yellow oil which crystallised overnight under a nitrogen
decolourising immediately and, after stirring overnight, had atmosphere. The crude residue was adsorbed onto silica and
become very pale yellow. The solvent was removed and the washed with petrol to remove any unreacted starting material
residue was extracted into diethyl ether and filtered. Reduction and then 1 : 1 DCM/petrol to extract the product. Removal of
of the volume of the filtrate and purification by TLC (DCM/ solvent from the eluent gave the 15 as a white solid (113 mg,
petrol 1 : 1) gave 11 as a colourless band at Rf 0.30 (5 mg, 30.6%). C14H23B10NO requires C 51.0, H 7.04, N 4.25. Found C
12.4%). C8H17B10NO requires C 38.2, H 6.82, N 5.57. Found C 50.8, H 7.11, N 4.08%. 11B{1H} NMR, δ −4.5 (1B), −5.2 (1B),
1
38.3, H 6.43, N 5.31%. 11B{1H} NMR, δ −3.8 (1B), −5.7 (1B), −9.3 (1B), −11.2 (4B), −12.8 (3B). H{11B} NMR δ 7.64 (m, 2H,
−10.6 (4B), −12.1 + shoulder (4B). 1H NMR, δ 7.70 (m, 2H, Ph), Ph), 7.41 (m, 1H, Ph), 7.35 (m, 2H, Ph), 6.58 (m, 1H, CvCH),
7.44 (m, 3H, Ph), 5.70 (br d, 1H, NH), 5.18 (d, 1H, OH). EIMS, 6.43 (m, 1H, CvCH), 4.41 (m, 1H, bridgehead CH), 4.07 (m,
envelope centred on m/z 252 (M+). Accurate mass: calculated 1H, bridgehead CH), 1.75 (m, 1H, CH2), 1.55 (m, 1H, CH2),
251.2312, found: 251.2314. Diffraction-quality crystals of 11 1.27 (m, 1H, CH2), 1.17 (m, 1H, CH2). EIMS, envelope centred
were grown by cooling a concentrated petrol solution.
on m/z 329 (M+).
[1-(1′-2′-D-1′,2′-closo-C2B10H10)-2-D-1,2-closo-C2B10H10]
[1,2-(NOC6H8)2-1,2-closo-C2B10H10] (16). [1,2-closo-C2B10H12]
(12). 1,1′-Bis(o-carborane) (200 mg, 0.70 mmol) was dissolved (100 mg, 0.69 mmol) was dissolved in dry, degassed, diethyl
in 15 mL of dry, degassed, diethyl ether and treated with ether (15 mL) and treated with n-BuLi (1.5 mmol) at 0 °C. The
n-BuLi (1.5 mmol) at 0 °C. The white suspension was stirred at white suspension was stirred for 20 minutes at room tempera-
room temperature for 1 hour before D2O (1 mL, 55 mmol) was ture then added dropwise to an excess of nitrosyl chloride in
added and the mixture agitated before being allowed to settle. dry, degassed, petrol (15 mL) at −78 °C. The resulting dark-
H2O (15 mL) was added and the layers separated. The aqueous green solution was stirred for 15 minutes at −78 °C and
phase was extracted with diethyl ether (15 mL) and the com- warmed to room temperature before both the excess nitrosyl
bined organic phases were dried over MgSO4, filtered and chloride and solvents were removed in vacuo. The residue was
reduced in vacuo to give 12 as a white solid (186 mg, 92.1%). extracted into DCM and filtered. The blue filtrate was treated
EIMS, envelope centred on m/z 288 (M+). Accurate mass calcu- with 1,3-cyclohexadiene (0.25 mL, 3.7 mmol) and stirred for
lated: 288.3850. Found: 288.3850.
[1-(1′-2′-D-1′,2′-closo-C2B10H10)-2-N(H)OH-1,2-closo-C2B10H10
1 hour. The yellow-brown solution was reduced in volume and
purified by preparative TLC (silica, 1 : 1 DCM/petrol eluent) to
]
(13). Compound 12 (170 mg, 0.59 mmol) was treated with give the desired product as two diastereoisomers 16a (meso) at
n-BuLi, nitrosylated and worked up exactly as for 9. Purification Rf 0.44 (16 mg, 6.4%) and 16b (racemic) at Rf 0.59 (15 mg,
of the crude mixture by thin layer chromatography (DCM/ 6.0%), both off-white solids.
petrol 3 : 1) afforded 13 as a colourless band at Rf 0.51 (20 mg,
16a: C14H26B10N2O2 requires C 46.4, H 7.23, N 7.73. Found
10.7%), together with minor contamination of 9. C 46.3, H 7.49, N 8.02%. 11B{1H} NMR δ −7.0 (2B), −10.8 (2B),
C4H22B20DNO requires C 15.1, H 7.60, N 4.40. Found C 15.0, H −13.6 (4B), −16.8 (2B). 1H NMR δ 6.63 (m, 2H, CvCH), 6.51
7.40, N 4.05%. 11B{1H} NMR, δ −1.9 (3B), −6.2 (1B), −9 to −13 (m, 2H, CvCH), 4.57 (m, 2H, bridgehead CH), 4.19 (m, 2H,
Dalton Trans.
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bridgehead CH), 2.24 (m, 2H, CH2), 2.14 (m, 2H, CH2), 1.52
[1-{1′-2′-(NOC6H8)-1′,2′-closo-C2B10H10}-2-(NOC6H8)-1,2-
(m, 2H, CH2), 1.34 (m, 2H, CH2). EIMS, envelope centred on closo-C2B10H10
]
(17) and [1-(1′-1′,2′-closo-C2B10H11)-2-
(18). 1,1′-Bis(o-carborane)
m/z 362 (M+).
(NOC6H8)-1,2-closo-C2B10H10
]
16b: C14H26B10N2O2 requires C 46.4, H 7.23, N 7.73. Found (100 mg, 0.35 mmol) was dissolved in dry, degassed, ether
C 46.9, H 7.48, N 7.41%. 11B{1H} NMR δ −7.1 (2B), −12.3 + (15 mL) and treated with n-BuLi (0.76 mmol) at 0 °C. Reaction
1
shoulder (4B), −14.2 (1B), −15.2 (3B). H NMR δ 6.62 (m, 2H, with nitrosyl chloride and 1,3-cyclohexadiene analogously to
CvCH), 6.50 (m, 2H, CvCH), 4.52 (m, 2H, bridgehead CH), 16a/b, and work-up involving preparative TLC (silica, 1 : 2
4.25 (m, 2H, bridgehead CH), 2.20 (m, 2H, CH2), 2.13 (m, 2H, DCM/petrol eluent) gave 17 (Rf 0.66) as a yellow solid (20 mg,
CH2), 1.50 (m, 2H, CH2), 1.37 (m, 2H, CH2). EIMS, envelope 11.3%) together with a small quantity of 18 (Rf 0.84) as a pale
centred on m/z 362 (M+).
Diffraction-quality crystals of 16a and 16b were grown by
slow diffusion of petrol and DCM solutions at −30 °C.
yellow solid.
17: 11B{1H} NMR δ −1 to −12 (overlapping resonances with
1
maxima at −1.09, −6.56, −9.95, −11.77, 20B). H NMR δ 6.63
Table 2 Crystallographic data
2
5
6
8
9
10
Formula
M
Crystal system
Space group
a/Å
b/Å
c/Å
C4H15B10NO2
217.27
Monoclinic
P21/c
23.339(2)
7.5382(8)
14.5428(17)
90
C8H25B10NOSi
287.48
Monoclinic
P21/n
7.0341(7)
19.1795(18)
12.5279(11)
90
C8H15B10NO
249.31
Orthorhombic
Pbca
8.009(2)
15.970(4)
22.139(6)
90
C4H20B20N2O2
344.42
Tetragonal
P42/n
16.3052(5)
16.3052(5)
6.9144(3)
90
C4H23B20NO
317.43
Monoclinic
P21/n
7.2022(8)
9.5053(10)
12.9926(15)
90
C4H21B20NO
315.42
Monoclinic
P21/n
7.2275(19)
9.678(3)
12.993(4)
90
α (°)
β (°)
106.471(6)
90
2453.6(5)
8, 2
98.544(4)
90
1671.4(3)
4, 1
90
90
90
90
97.350(7)
90
882.15(17)
2, 0.5
93.226(9)
90
907.4(4)
2, 0.5
γ (°)
U/Å3
2831.7(13)
8, 1
1838.26(14)
4, 0.5
Z, Z′
F(000)/e
896
1.176
0.067
25.78
608
1.142
0.128
33.21
1024
1.170
0.062
22.07
696
1.244
0.064
29.61
324
1.195
0.055
29.66
320
1.154
0.054
31.29
Dcalc/Mg m–3
μ(Mo-Kα)/mm–1
θmax (°)
Data measured
Unique data, n
Rint
Obs. data (I > 2σ(I))
R, wR2 (obs. data)
S (all data)
Variables
Emax, Emin/e Å–3
61 791
4605
0.0832
2855
0.0927, 0.1910
1.091
44 494
6302
0.0411
5279
0.0341, 0.0878
1.042
21 512
3783
0.1327
1858
0.1302, 0.3038
1.105
202
37 192
2593
0.0433
2457
0.0438, 0.1137
1.145
177
17 862
2481
0.0512
2063
0.0532, 0.1271
1.107
21 179
2950
0.0432
2353
0.0536, 0.1483
1.147
160
370
0.25, −0.31
225
0.52, −0.28
167
0.24, −0.19
0.28, −0.21
0.33, −0.21
0.28, −0.39
13
14
16a
16b
17
18
Formula
M
Crystal system
Space group
a/Å
b/Å
c/Å
C8H17B10NO
251.32
Triclinic
ˉ
P1
10.4155(10)
11.0414(10)
13.1547(12)
67.377(3)
79.186(3)
89.541(3)
1368.2(2)
4, 2
C12H27B10NO
309.44
Triclinic
ˉ
P1
11.9847(18)
16.213(3)
20.179(3)
87.624(6)
88.802(5)
71.721(5)
3719.8(10)
8, 4
C14H26B10N2O2
362.47
Orthorhombic
Pnma
11.409(4)
16.464(5)
10.356(3)
90
C14H26B10N2O2
362.47
Monoclinic
C2/c
17.316(5)
10.390(3)
11.385(4)
90
110.266(19)
90
C16H36B20N2O2
504.67
Monoclinic
P21/n
10.1193(7)
10.6700(7)
12.4753(8)
90
99.648(3)
90
C10H29B20NO
395.54
Monoclinic
P21/n
9.5006(11)
12.0225(15)
19.411(2)
90
93.031(6)
90
2214.1(4)
4, 1
α (°)
β (°)
90
90
γ (°)
U/Å3
1945.1(10)
4, 0.5
1921.6(10)
4, 0.5
1327.94(15)
2, 0.5
Z, Z′
F(000)/e
520
1.220
0.065
27.96
1312
1.105
0.059
26.46
760
1.238
0.071
28.44
760
1.253
0.072
30.93
524
1.262
0.067
27.16
816
1.187
0.057
24.80
Dcalc/Mg m–3
μ(Mo-Kα)/mm–1
θmax (°)
Data measured
Unique data, n
Rint
Obs. data (I > 2σ(I))
R, wR2 (obs. data)
S (all data)
Variables
Emax, Emin/e Å–3
24 224
6469
0.0241
5427
0.0382, 0.1012
0.931
433
30 882
14 855
0.0377
10 171
0.0483, 0.1033
1.024
1009
29 025
2531
0.0713
1848
0.0436, 0.0984
1.039
150
22 471
3025
0.0551
2275
0.0426, 0.1171
1.068
142
19 862
2902
0.0503
2254
0.0570, 0.1325
1.026
257
26 104
3762
0.1410
2065
0.0900, 0.2205
1.054
353
0.43, −0.40
0.35, −0.39
0.21, −0.25
0.31, −0.25
0.38, −0.28
0.48, −0.48
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(m, 2H, CvCH), 6.50 (m, 2H, CvCH), 4.63 (m, 2H, bridgehead Distance Method,30 with both methods affording excellent
CH), 4.38 (m, 2H, bridgehead CH), 2.30 (m, 2H, CH2), 2.14 (m, agreement.
2H, CH2), 1.41 (m, 4H, CH2). EIMS, envelope centred on m/z
In compound 8 the crystallographically-unique O atom is
disordered over four sites and in 6 the C1N1O1 and B10H10
504/505 (M+).
18: 11B{1H} NMR δ −2.02 (2B), −5.97 (1B), −7 to −13 (over- units are mutually disordered. In compounds 9 and 10 the
lapping resonances with maxima at −9.99, −12.94, 17B). 1H imposed Ci symmetry requires the NO group (10) and the N(H)
NMR δ 6.65 (m, 1H, CvCH), 6.50 (m, 1H, CvCH), 4.66 (m, OH group (9) to be disordered between vertices C2 and C2′ and
1H, bridgehead CH), 4.34 (m, 1H, bridgehead CH), 4.24 (br s, the NOC6H8 substituent in compound 17 suffers from internal
1H, cage CH), 2.29 (m, 1H, CH2), 2.07 (m, 1H, CH2), 1.54 (m, disorder between the two possible enantiomers. All other
1H, CH2), 1.43 (m, 1H, CH2). EIMS, envelope centred on m/z structures are fully ordered.
395 (M+).
For all structures except for that of compound 6 (for which
Diffraction-quality crystals of 17 were grown by slow evapo- B–H was fixed at 1.12 Å) H atoms bound to cage B or C atoms
ration of a DCM solution and crystals of 18 were grown from were allowed to refine positionally, although in the disordered
petrol.
9 and 10 B2–H2 was restrained to 1.10(1) Å. In compounds
with hydroxylamine groups (9 and 11) the NH and OH H
atoms were also allowed positional refinement. Other H atoms
Crystallographic studies
Conditions under which single crystals were grown are given were constrained to idealised geometries; Cphenyl–H = Cene–H =
above. Compounds 2 and 6 are liquids at room temperature, 0.95 Å, Cmethyl–H = 0.98 Å, Cmethylene–H = 0.99 Å, Ctertiary–H =
so single crystals of these species were grown in situ in a 1.00 Å. All H displacement parameters, Uiso, were constrained
0.3 mm glass capillary tube on the diffractometer.24 Variable to be 1.2 × Ueq (bound B, C, N or O) except Me H atoms
and carefully controlled cooling was supplied by an Oxford [Uiso(H) = 1.5 × Ueq C(Me)]. Table 2 contains further experi-
Cryosystems series 700 Cryostream. The liquid sample mental details.
(ca. 1.5 cm) was carefully introduced into the capillary to minimise
air gaps and the capillary mounted in a brass pin and sealed
as close to the meniscus as possible so that the capillary tube
would not collide with the Cryostream during data collection.
Acknowledgements
The capillary has to be sufficiently long so that the meniscus AJW thanks Professor Brian G. Gowenlock for first stimulating
could be moved in and out of the cold stream using the height his interest in nitrosocarboranes. We thank the EPSRC for a
adjustment on the goniometer head. In particular it is necess- DTP studentship supporting SLP and for grant EP/I031545/1
ary that the liquid level can be moved far enough beyond the supporting WYM. We also thank Dr Andrew Bond (University
cold stream so that the uppermost layer melts providing a of Cambridge) for his expert and practical advice on in situ
liquid–solid interface where crystal growth could take place. crystallisation.
With the goniometer at χ = 90° (capillary horizontal) the
sample was cooled at 250 K h−1 to determine the approximate
freezing point, warmed to ca. 20 °C above the freezing point
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Finally the samples were cooled slowly to 150 K for data
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Dalton Trans.