Polyhedral monocarbaborane chemistry. Some C-phenylated seven, eight, nine, ten, eleven and twelve-vertex species

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Polyhedral monocarbaborane chemistry. Some C-phenylated seven, eight,

nine, ten, eleven and twelve-vertex species†

Andreas Franken,^aToma´sˇ Jel´ınek,^aRichard G. Taylor,^bDaniel L. Ormsby,‡^aColin A. Kilner,^aWilliam Clegg^c

and John D. Kennedy*^a

Received 11th July 2006, Accepted 4th October 2006

First published as an Advance Article on the web 18th October 2006

DOI: 10.1039/b609908d

Some synthetic and structural systematics for monocarbaboranes, using the C-phenylated motif as the

example, are investigated. The 10-vertex [6-Ph-nido-6-CB₉H₁₁]⁻anion 1, from reaction of PhCHO with

B₁₀H₁₄in KOH/H₂O, is a useful entry synthon into C-phenyl monocarbaborane chemistry. Treatment

of anion 1 with Na/thf yields the 10-vertex [1-Ph-closo-1-CB₉H₉]⁻anion 2a, whereas treatment of

anion 1 with iodine in alkaline solution yields the isomeric 10-vertex [2-Ph-closo-2-CB₉H₉]⁻anion 2b,

which isomerises quantitatively to 2a on heating under reﬂux in DME. Thermolysis of anion 1 yields

the 9-vertex [4-Ph-closo-4-CB₈H₈]⁻anion 5, whereas treatment of anion 1 with FeCl₃/HCl gives neutral

9-vertex [4-Ph-arachno-4-CB₈H₁₃] 3. Compound 3 gives neutral 9-vertex [1-Ph-nido-1-CB₈H₁₁] 4 in

reﬂuxing toluene, and gives the 7-vertex [2-Ph-closo-2-CB₆H₆]⁻anion 7 and the 8-vertex

[1-Ph-closo-1-CB₇H₇]⁻anion 6 in reﬂuxing toluene with NEt₃. Reaction of 1 with [BH₃(thf)] yields the

11-vertex [7-Ph-nido-7-CB₁₀H₁₂]⁻anion 8 which can be converted to the 12-vertex

[1-Ph-closo-1-CB₁₁H₁₁]⁻anion 10 using [BH₃(SMe₂)]; alternatively, anion 1 yields anion 10 directly on

treatment with [BH₃(NEt₃)]. Treatment of anion 8 with I₂/KOH yields the 11-vertex

[2-Ph-closo-2-CB₁₀H₁₀]⁻anion 9. The structures of anions 1, 2a, 2b, 5, 6, 7, 8, 9 and 10 have been

established by single-crystal X-ray diffraction analyses of their [NEt₄]⁺salts, and those of neutral 3 and

4 estimated by DFT calculations at the B3LYP/6-31G* level; similar calculations have also been

applied to the new anionic closo species 2a, 2b, 5, 6, 7, 9 and 10. Crystals of the [NEt₄]⁺salt of the

[2-Ph-closo-2-CB₆H₆]⁻anion 7 required synchrotron X-radiation for sufﬁcient diffraction intensity for

molecular-structure elucidation. The syntheses are in principle generally applicable to give extensive

derivative C-aryl and C-alkyl chemistries.

accord with this perception, we here present results on C-

Introduction

phenylated nido {CB₈}, {CB₉} and {CB₁₀} cluster compounds, and

Because the previously perceived easiest routes into carbaborane

chemistry have conveniently derived from two-carbon additions of

alkynes into pentaborane or decaborane clusters,^1–3carbaborane

chemistry has been dominated by the dicarbaboranes-boron —

containing cluster compounds that also have two carbon atoms

in the cluster.^4,5The discovery by Brellochs⁶that formaldehyde

thence C-phenylated closo {CB₆}, {CB₇}, {CB₈} {CB₉}, {CB₁₀}

and {CB₁₁} species derived from these; a C-phenylated arachno

{CB₈} species is also described. Speciﬁcaly the species formed

are the ten-vertex anions [6-Ph-nido-6-CB₉H₁₁]⁻1, [1-Ph-closo-

1-CB₉H₉]⁻2a and [2-Ph-closo-2-CB₉H₉]⁻2b, neutral nine-vertex

[4-Ph-arachno-4-CB₈H₁₃] 3 and [1-Ph-nido-1-CB₈H₁₁] 4, the nine-

vertex [4-Ph-closo-4-CB₈H₈]⁻anion 5, the eight-vertex [1-Ph-closo-

1-CB₇H₇]⁻anion 6, the seven-vertex [2-Ph-closo-2-CB₆H₆]⁻anion

7, the eleven-vertex anions [7-Ph-nido-7-CB₁₀H₁₂]⁻8 and [2-Ph-

closo-2-CB₁₀H₁₀]⁻9, and the twelve-vertex [1-Ph-closo-1-CB₁₁H₁₁]⁻

anion 10. Schematic skeletal structures for these species are given

in Scheme 1, ordered by increasing cluster size.

and higher organyl aldehydes can react with nido-decaborane

(B₁₀H₁₄) under aqueous alkali conditions to give good yields of

ten-vertex nido-{CB₉} or arachno-{CB₉} monocarbaboranes —

boron cluster compounds with only one carbon atom in the

cluster — now permits the ready development of a systematic

monocarbaborane chemistry that has hitherto been lacking. In

All derive from the reaction of benzaldehyde with nido-B₁₀H₁₄

to give the C-phenylated ten-vertex [6-Ph-nido-6-CB₉H₁₁]⁻an-

ion (species 1), followed by appropriate cluster-Aufbau, cluster-

closure and/or cluster-dismantling reactions as summarised in

Scheme 2. Some aspects of this and closely related work have

been communicated in a preliminary fashion,^7–19and elements of

the preliminary work have also been mentioned in an overview

article.²⁰Contemporaneous and historical related monocarbab-

orane work should also be noted;^21–29in particular, this present

work can be regarded as complementary to the deﬁnitive work of

^aThe School of Chemistry of the University of Leeds, Leeds, UK LS2 9JT.

E-mail: j.d.kennedy@leeds.ac.uk

^bDow Corning, Central Research (BAJO4), Cardiff Road, Barry, South

Glamorgan, UK CF63 2YL

^cThe School of Natural Sciences (Chemistry) of the University of Newcastle,

Newcastle upon Tyne, UK NE1 7RU

† Electronic supplementary information (ESI) available: Coordinates

for arachno-PhCB₈H₁₃(compound 3) and nido-PhCB₈H₁₁(compound

4), as derived from DFT B3LYP/6-31G* calculations. See DOI:

10.1039/b609908d

‡ Current address: Accelrys Ltd., 334 Cambridge Science Park, Cam-

bridge, UK CB4 0WN.

ˇ

Brellochs himself, together with St´ıbr and co-workers, on related

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the eleven-vertex [7-H₃N-nido-7-CB₁₀H₁₂] and ten-vertex [6-H₃N-

nido-6-CB₉H₁₁] neutral species.^32,33Separation of these two com-

pounds requires relatively tedious chromatographic procedures,

but has led to subsequent useful monocarbaborane chemistry,⁴

for example, [6-H₃N-nido-6-CB₉H₁₁] has previously been used

to prepare the [closo-1-CB₉H₁₀]⁻anion by treatment with excess

piperidine.³⁴Alternative useful routes to monocarbaboranes have

started from the twelve-vertex {C₂B₁₀} cluster system, and have

involved selective cluster-dismantling reactions.^35,36

The recently developed Brellochs Reaction now provides

an alternative, convenient, entry into monocarbaborane cluster

chemistry starting from nido-B₁₀H₁₄.^6,7,20,23,28This procedure uses

aldehydes as the carbon source. Thus, in aqueous alkaline solu-

tion, nido-B₁₀H₁₄reacts with formaldehyde, HCHO, or aliphatic

aldehydes, RCHO, to yield the arachno ten-vertex anions [6-

CB₉H₁₄]⁻and [6-R-6-CB₉H₁₃]⁻respectively.^{7,8,22–24,28}Under similar

conditions, but using aryl aldehydes, ArCHO, rather than aliphatic

aldehydes, RCHO, the nido rather than the arachno ten-vertex

monocarbaborane anions are generally formed.^8,10,28For example,

benzaldehyde, PhCHO, gives the [6-Ph-nido-6-CB₉H₁₁]⁻anion 1

(eqn (1)–(4)), from which the bulk of the results in this present

report derives. The overall reaction could possibly proceed via

the formation of the nine-vertex [arachno-B₉H₁₄]⁻anion from the

KOH and the B₁₀H₁₄(eqn (1)), and this anion then reacts further

with the aldehyde under aqueous alkaline conditions (eqn (2)).

However, although we have found yields of up to ca. 50% for

the formation of 1 when direct reactions of the [arachno-B₉H₁₄]⁻

anion according to eqn (2) have been attempted under a variety

of conditions, these are not reproducible and the incidence of

1 in the product mixture is erratic. Alternatively, therefore, the

reaction may well proceed via the known²⁵ten-vertex anionic

arachno intermediate [B₁₀H₁₃-6-(OH)]⁻that is also formed from

B₁₀H₁₄in aqueous KOH (eqn (3) and (4)). It is also possible that the

aldehyde may have a role as oxidizing agent in the removal of the

elements of dihydrogen in eqn (2) and (4). Because of the relative

infancy of the exploitation of the Brellochs reaction, a more

detailed study of this aspect of the reaction chemistry remains

as yet incomplete, although conditions for the optimisation of

Scheme 1

routes in and among some non-C-phenylated monocarbaborane

systems,^21–24in particular by use of the Brellochs reaction to give

the [arachno-6-CB₉H₁₄]⁻anion, from which the [closo-2-CB₉H₁₀]⁻,

[closo-1-CB₉H₁₀]⁻, [closo-4-CB₈H₉]⁻, [closo-1-CB₇H₈]⁻and [closo-

2-CB₆H₇]⁻anions, together with neutral [arachno-4-CB₈H₁₄] and

[nido-1-CB₈H₁₂], can then be derived.

Results and discussion

1

Ten-vertex nido and closo species

The traditional route to the classes of monocarbaboranes that

have eight to eleven cluster vertices is via the ten-vertex [endo-

6-NC-arachno-B₁₀H₁₃]²⁻dianion,³⁰which can be formed by the

reaction of the commonly used starting substrate nido-B₁₀H₁₄with

the CN⁻anion in aqueous alkaline solution.³¹Thence, in a second

step, protonation with hydrochloric acid leads to direct insertion

of the CN group into the boron cage, to produce a mixture of

Scheme 2

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the formation of the non-phenylated [arachno-6-CB₉H₁₄]⁻species

from HCHO and nido-B₁₀H₁₄have been investigated.²³

50%). The two anions 2a and 10 may be separated by fractional

crystallization of their Cs⁺salts.⁸A higher isolatable yield of 2a,

uncontaminated by other species, derives from the heating of

the [NEt₄]⁺salt of 1 with elemental sodium in thf solution. In

this case, the crude product 2a is generally contaminated with

residual starting anion 1, but this latter can be removed by its

ready conversion to the neutral nine-vertex arachno derivative [4-

Ph-4-CB₈H₁₃] (compound 3) by treatment with FeCl₃as discussed

below. This conversion leaves the anion 2a intact. Neutral 3 can

then be extracted into a neutral solvent, thence permitting the

isolation of the pure [NEt₄]⁺salt of 2a in 74% yield. The role

of the sodium in this ostensibly simple dihydrogen elimination is

not clear: the reaction does not proceed in the absence of sodium

under otherwise identical conditions; one possibility suggested by

a referee is that an initial reductive opening of the cage to arachno

may facilitate the subsequent, essentially oxidative, dihydrogen

loss. A cleaner, essentially quantitative, route to anion 2a entails

a thermal isomerisation of the [NEt₄]⁺salt of its [2-Ph-closo-2-

CB₉H₉]⁻isomer 2b in reﬂuxing DME solution. This procedure

results in the formation of the [NEt₄]⁺salt of 2a in isolatable yields

of up to 94%. The isomeric anion 2b is obtained from the nido

precursor 1 via oxidation with elemental I₂in aqueous alkaline

solution, from which its [NEt₄]⁺salt can be isolated, again in high

yields, of up to 92%.

B₁₀H₁₄+ 2 H₂O + 2 [OH]⁻→ [B₉H₁₄]⁻+ [B(OH)₄]⁻+ H₂

[B₉H₁₄]⁻+ PhCHO → [PhCB₉H₁₁]⁻1 + H₂+ H₂O

B₁₀H₁₄+ 2 [OH]⁻→ [B₁₀H₁₃-6-(OH)]⁻+ H₂O

(1)

(2)

(3)

[B₁₀H₁₃-6-(OH)]⁻+ PhCHO + 2 H₂O

→ [PhCB₉H₁₁]⁻1 + [B(OH)₄]⁻+ 2 H₂

(4)

The benzaldehyde reaction proceeds in good yield and the

anion 1 may be isolated as its [NEt₄]⁺salt in yields of up to

64%. The species is characterised crystallographically (Fig. 1).

Anion 1 has typical nido ten-vertex geometry in accord with the

Williams–Wade paradigms.^37,38The cluster carbon atom is in a

lower-connectivity ‘prow’ position in the nido ten-vertex boat, also

in accord with long-established behavioural patterns that note that

carbon atoms will preferentially occupy lower-connectivity cluster

positions.^37,39

The molecular structures of the isomeric [PhCB₉H₉]⁻anions 2a

and 2b are both established by single-crystal X-ray diffraction

analyses of their individual [NEt₄]⁺salts (Fig. 2). Both have

the classical bicapped square-antiprismatic structures, as also

exhibited by their non-phenylated ‘parent’ species, the [closo-1-

CB₉H₁₀]⁻anion 2c, and by other related species such as the neutral

ligand adduct [1-(NMe₃)-closo-1-CB₉H₉] 2d.³⁴In anions 2a and 2c,

and in neutral 2d, the cluster carbon atoms are in positions of lower

cluster-connectivity four, whereas in 2b the carbon atom occupies

a position of higher cluster-connectivity ﬁve. That 2b gives 2a

upon thermolysis implies a greater thermodynamic stability for

the isomer 2a in which the carbon atom preferentially occupies

a site of lower cluster connectivity, which is in accord with long-

established general patterns of carbaborane cluster behaviour.^37,39

The identiﬁcation of these two isomeric monocarbaborane species

is of interest because positional isomerism of the heteroatom was

previously unknown in main-group monoheteroborane chemistry,

although well-recognised in other areas, for example in the

dicarbaboranes.^3–5

Fig. 1 Crystallographically determined molecular structure of the

[6-Ph-nido-6-CB₉H₁₁]⁻anion 1 as its [NEt₄]⁺salt [CCDC deposition no.

184220]. The C(6)–C(phenyl) distance is 1.492(2) A, other distances to

˚

C(6) being 1.704(2) from B(2), 1.536(2) from B(5) and 1.539(2) A from

B(7).

Contrary to na¨ıve expectation, solid-state thermolysis of the

[NEt₄]⁺salt of 1 did not result in cluster closure to give a ten-vertex

closo [PhCB₉H₉]⁻anion 2 via a simple dihydrogen loss. Rather,

boron loss and some disproportionation preferentially occurred

(eqn (5)), to give a mixture of the nine-vertex closo anion [4-Ph-

4-CB₈H₈]⁻(species 5; see below, up to ca. 68%) and the eleven-

vertex nido anion [7-Ph-7-CB₁₀H₁₂]⁻(species 8; see below, up to

ca. 22%); only small quantities of one of the two possible closo

[PhCB₉H₉]⁻isomers 2, viz. the [1-Ph-closo-1-CB₉H₉]⁻anion 2a (ca.

5%), were formed. This disproportionation of a ten-vertex species

to give nine-vertex and eleven-vertex species is also contrary to

expectation in view of the generally held perception that ten-

vertex clusters (along with six-vertex and twelve-vertex clusters)

constitute ‘islands of stability’.

2

Nine-vertex arachno, nido and closo species

Previous routes to nine-vertex carbaborane species have typically

involved the removal of a deﬁned number of boron vertices, some-

times together with one cage carbon atom, from the twelve-vertex

[nido-7,9-C₂B₁₀H₁₃]⁻anion.⁴Thus, acid-promoted dismantling of

this last anion in the presence of Lewis bases results in the removal

of one carbon and two boron vertices, with the formation of nine-

vertex monocarbaborane ligand derivatives such as [6-(NMe₃)-

arachno-4-CB₈H₁₂].³⁵We have now found that a convenient

alternative route into the arachno nine-vertex monocarbaborane

systems can derive from the initial ten-vertex product from the

Brellochs reaction, the [6-Ph-nido-6-CB₉H₁₁]⁻anion 1. Thus,

the neutral nine-vertex species [4-Ph-arachno-4-CB₈H₁₃] (com-

pound 3) can be isolated in 78% yield following the treatment

2 [PhCB₉H₁₁]⁻1 → [PhCB₈H₈]⁻5 + [PhCB₁₀H₁₂]⁻8 + H₂(5)

Larger yields of anion 2a (ca. 25%) are obtained from the reac-

tion between the [6-Ph-nido-6-CB₉H₁₁]⁻anion 1 and [BH₃(NEt₃)]

at 210 ^◦C, but now the product 2a is admixed with a twelve-

vertex species, the [1-Ph-closo-1-CB₁₁H₁₁]⁻anion 10 (see below; ca.

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Fig. 3 Molecular structure of [4-Ph-arachno-4-CB₈H₁₃] (compound 3)

as derived from DFT calculations at the B3LYP/6-31G* level. The

˚

C(4)–C(phenyl) distance is 1.450 A, with the other distances from

˚

C(4) being 1.669 to B(1), 1.735 to B(5), and 1.724 A to B(9). The

molecule is ﬂuxional with respect to exchange of character between the

inner-sphere bridging and the endo-terminal hydrogen atoms associated

with the B(6)B(7)B(8) ‘notch’ site on the open face,⁵⁷so the molecule on

time-average appears as if it has a mirror plane in solution, exhibiting a 2 :

2 : 2 : 1 : 1 relative-intensity pattern in its ¹¹B NMR spectrum.

31G* calculations of boron nuclear magnetic shielding, from

which the calculated values reasonably match the observed ones.

These values are summarised in the Experimental section below.

The overall cluster characteristics are of course very similar to

those known for the non-phenylated ‘parent’ [arachno-4-CB₈H₁₄]

species.^4,40That the carbon atom occupies a cluster position of

cluster-connectivity three is again in accord with general patterns

of carbaborane cluster behaviour in which cluster carbon atoms

preferentially occupy cluster sites of lowest connectivities.^37,39

The arachno nine-vertex [4-Ph-4-CB₈H₁₃] compound 3 is pre-

cursor to the C-phenylated eight-vertex and seven-vertex closo

monocarbaborane species 6 and 7, as discussed in Section 3 below.

It is also precursive to its more direct nine-vertex nido congener

[1-Ph-nido-1-CB₈H₁₁] (compound 4). Here, a simple thermolytic

dehydrogenation of the arachno compound 3 in reﬂuxing toluene

readily gives compound 4 as a colourless oil, isolatable in 87%

yield. As with its arachno precursor 3, we have not yet been able to

obtain crystals of 4 suitable for its structural characterization by

single-crystal X-ray crystallography in spite of several attempts.

Its general constitution and molecular weight imply that it would

be expected to be a solid at room temperature; there may

be nucleation problems with this hitherto unreported species.

Again, however, multinuclear NMR spectroscopy reasonably

indicates the constitution and structure (Fig. 4), and the NMR

characteristics are in addition closely related to those of the

unsubstituted ‘parent’ compound [nido-1-CB₈H₁₂].^33,41As with the

arachno species 3, this structural conclusion for the nido species 4 is

also supported by B3LYP/6-31G* DFT calculations, and thence

by consequent DFT-GIAO//B3LYP/6-31G* calculations of ¹¹B

nuclear shielding that match those observed experimentally.

Comparison of the structures of compounds 3 and 4 in Fig. 3

and 4 indicates that a {CB₈} skeletal rearrangement has occurred

in the conversion of 3 to 4. That this type of nine-vertex arachno-

to-nido conversion does not simply involve a connectivity closure

across the open face, and therefore must also involve a skeletal

rearrangement, has been long recognized.^33,41In this regard, it

is thence of interest that the resulting nido nine-vertex structure

Fig. 2 Crystallographically determined molecular structures of (top) the

[1-Ph-closo-1-CB₉H₉]⁻anion 2a as its [NEt₄]⁺salt [CCDC deposition no.

164851] and (bottom) the [2-Ph-closo-2-CB₉H₉]⁻anion 2b as its [NEt₄]⁺salt

[CCDC deposition no. 184221]. In anion 2a the C(1)–C(phenyl) distance is

˚

1.503(4) A, the distances to C(1) from the four adjacent boron atoms being

˚

1.613(4), 1.606(4), 1.598(5) and 1.606(4) A. In anion 2b the C(2)–C(phenyl)

˚

distance is 1.498(2) A, other distances to C(2) being 1.636(3) from B(1),

˚

1.761(3) from B(3), 1.766(3) from B(5), 1.752(3) from B(6) and 1.749(3) A

from B(9).

of the [6-Ph-nido-6-CB₉H₁₁]⁻anion 1 with FeCl₃as oxidizing

agent in aqueous acidic solution. A reasonable stoichiometry can

be written down as in eqn (6). Compound 3 also arises in 22%

yield as a signiﬁcant by-product in the reaction of [BH₃(thf)]

in 1,2-Cl₂C₂H₄with the [6-Ph-nido-6-CB₉H₁₁]⁻anion 1, followed

by treatment with FeCl₃. The last reaction, however, yields the

eleven-vertex [7-Ph-nido-7-CB₁₀H₁₂]⁻anion 8 (see below) as a more

predominant product, isolatable in 44% yield.

[PhCB₉H₁₁]⁻1 + 2 Fe³⁺+ 3 H₂O

→ [PhCB₈H₁₃] 3 + [B(OH)₃] + 2 Fe²⁺+ H⁺

(6)

[4-Ph-arachno-4-CB₈H₁₃] (compound 3) is a white solid, mp 74 ^◦C,

but we have not yet been able to obtain crystals suitable enough for

its structural characterization by single-crystal X-ray crystallog-

raphy. However, multinuclear NMR spectroscopy indicates a con-

ventional Williams–Wade^37,38nine-vertex arachno conﬁguration,³⁶

with the cluster carbon atom in the 4-position, a conclusion

supported by the results of B3LYP/6-31G* DFT structural

calculations (Fig. 3), and consequent DFT-GIAO//B3LYP/6-

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Fig. 4 Molecular structure of [1-Ph-nido-1-CB₈H₁₁] (compound 4)

as derived from DFT calculations at the B3LYP/6-31G* level. The

˚

C(1)–C(phenyl) distance is 1.450 A, with the other distances from C(2)

˚

being 1.605 to B(2), 1.611 to B(3), 1.611 to B(4) and 1.605 A to B(5).

Fig. 5 Crystallographically determined molecular structure of the

[4-Ph-closo-4-CB₈H₈]⁻anion 5 as its [NEt₄]⁺salt [CCDC deposition no.

˚

has the carbon atom at the 1-position of cluster-connectivity

four, rather than being sited at the 9-position of lower cluster-

connectivity three that may have been na¨ıvely expected on the basis

of a simple closure across the B(6)–B(8) gap (Fig. 3) of the arachno

geometry of compound 3. This is now contrary to the more general

observations^37,39that carbon atoms will preferentially migrate to

sites of lower cluster-connectivity.

The nine-vertex [4-Ph-closo-4-CB₈H₈]⁻anion (species 5) is

formed in 68% yield from the thermolysis of the [6-Ph-nido-6-

CB₉H₁₁]⁻anion 1. However, as mentioned above, the reaction

is accompanied by substantial quantities of the by-products [7-

Ph-nido-7-CB₁₀H₁₂]⁻(anion 8) and [1-Ph-closo-1-CB₉H₉]⁻(anion

2a). A cleaner route to 5, and in higher yield, derives from

the neutral [1-Ph-nido-1-CB₈H₁₁] compound 4 that is discussed

in the previous paragraph. Dissolution of compound 4 in thf,

followed by deprotonation with triethylamine and oxidation with

elemental iodine (eqn (7)), yields the [4-Ph-closo-4-CB₈H₈]⁻anion

5, which can be isolated in 72% yield as its uncontaminated [NEt₄]⁺

salt.

164852]. The C(4)–C(phenyl) distance is 1.490(2) A, other distances to

C(4) being 1.623(2) from B(1), 1.622(2) from B(2), 1.620(2) from B(7), and

˚

1.623(2) A from B(8).

3

Eight-vertex and seven-vertex closo species

Eight-vertex and seven-vertex monocarbaborane cluster com-

pounds have also hitherto been rare, and substituted seven-vertex

monocarbaborane cluster compounds essentially unknown. Of

the eight-vertex species known before this present work, the

ﬁrst eight-vertex monocarbaborane, the [1-closo-CB₇H₈]⁻anion, is

synthesisable in a one-boron dismantling process when nine-vertex

[6-(NMe₃)-arachno-4-CB₈H₁₂] is heated with sodium hydride in

thf at 60 ^◦C.^35,36For this last synthesis, the [6-(NMe₃)-arachno-4-

CB₈H₁₂] precursor is obtained via the dismantling of the twelve-

vertex [nido-7,9-C₂B₁₀H₁₃]⁻anion,³⁶as mentioned above. The other

previously known eight-vertex monocarbaborane compound, the

neutral parent arachno species [4-CB₇H₁₃], resulted from an

alternative dismantling of the [nido-7,9-C₂B₁₀H₁₃]⁻anion, now

using aqueous HCl.³⁵

[PhCB₈H₁₀]⁻+ I₂→ [PhCB₈H₈]⁻5 + 2 HI

(7)

We have now found that the closo eight-vertex C-phenylated

species, the [1-Ph-closo-1-CB₇H₇]⁻anion 6, may be prepared by

a similar one-boron dismantling as that used for the parent

unsubstituted [1-closo-CB₇H₈]⁻anion just mentioned. The starting

material is either of the two neutral nine-vertex species [4-Ph-

arachno-4-CB₈H₁₃] (compound 3) or [1-Ph-nido-1-CB₈H₁₁] (com-

pound 4). Here the base used is NEt₃in reﬂuxing toluene, rather

than NaH in thf.

Thus, the nido species 4 and NEt₃give predominantly the eight-

vertex [1-Ph-closo-1-CB₇H₇]⁻anion 6, which may be isolated in

72% yield by precipitation out of the product mixture as its [NEt₄]⁺

salt. This precipitation, followed by subsequent recrystallisation,

clears it from residual quantities of the seven-vertex congener

anion, the [2-Ph-closo-2-CB₆H₆]⁻species 7, which is also present

to the extent of about 5% in the crude product mixture. The

unsubstituted ‘parent’ [closo-1-CB₇H₈]⁻anion can be formed

in an analogous manner from unsubstituted neutral [nido-1-

CB₈H₁₂].²²The eight-vertex C-phenylated closo species 6 has been

characterized by a single-crystal X-ray diffraction analysis of its

A drawing of the molecular structure of the [4-Ph-closo-4-

CB₈H₈]⁻anion 5, as determined crystallographically, is given

in Fig. 5. The basic {CB₈} cluster structure is that of the

tricapped trigonal prism that is expected for a classical nine-

vertex closo cluster.^37,38In accord with general behaviour of

carbaborane cluster species,^37,39the carbon atom takes a position of

lower cluster-connectivity four. In contrast to the ten-vertex closo

system discussed above (anions 2a and 2b), but in accord with

observations on the eight-vertex and seven-vertex closo congeners

6 and 7 dealt with below, we have not so far seen any evidence,

for example by NMR spectroscopy on reaction mixtures and low-

yield products, of the alternative [1-Ph-closo-1-CB₈H₈]⁻isomer

that would have the carbon atom in a position of higher cluster-

connectivity ﬁve. The isolation of such a species, presumably

metastable, will require alternative kinetic routes.^22,24,25Examples

of these closo nine-vertex {CB₈} systems have hitherto been

surprisingly elusive, the unsubstituted ‘parent’ [closo-4-CB₈H₉]⁻

anion having been reported only relatively recently.⁷

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1

in the room-temperature ¹¹B{ H} NMR spectrum, rather than the

ﬁve-line 2 : 2 : 1 : 1 : 1 pattern that would be expected from a

static molecular structure. We surmised the possibility that the

C-phenyl substitution may have increased the activation energy

for this ﬂuxional process, which therefore might fall below the

NMR ‘slow-exchange’ limit at practicable lower temperatures.

However, for the [1-Ph-closo-1-CB₇H₇]⁻anion 6 we failed to

detect any incipient splittings in either of the two ¹¹B resonance

positions down to −90 ^◦C in CD₂Cl₂solution at 160 MHz,

implying an activation energy for the ﬂuxional process of < ca.

30 kJ mol⁻¹.

Use of the [4-Ph-arachno-4-CB₈H₁₃] species 3, rather than the

corresponding nido species 4, in the toluene thermolysis with NEt₃

gives an enhanced proportion of the seven-vertex closo anion [2-

Ph-closo-2-CB₆H₆]⁻(species 7, 37%) at the expense of the eight-

vertex congener anion [1-Ph-closo-1-CB₇H₇]⁻6 (49%). Because of

their very similar chromatographic and solubility properties, we

have not yet been able to separate salts of anion 7 from those of

anion 6 sufﬁciently to give a pure bulk sample of anion 7, but a

clean mixture of the two is evident from NMR spectroscopy. The

NMR experiments on the mixture clearly identify the [PhCB₆H₆]⁻

anion 7, and specify its constitution as the 2-isomer. Crystallisation

of the mixture from a concentrated solution in (CH₃)₂CO that

was overlayered with an approximately ﬁve-fold excess of Et₂O

ultimately gave a mixture of crystals of the [NEt₄]⁺salts of both

6 and 7, both being suitable for single-crystal X-ray diffraction

experiments. A manual separation of individual crystals, followed

by unit-cell measurements on each using conventional sealed-

tube X-radiation, thence distinguished crystals of the salt of 6

[orthorhombic, a = 14.438(3), b = 15.762(3) and c = 16.877(4)

at 150 K] from those of the salt of 7 [monoclinic, a = 9.5758(2),

◦

Fig. 6 Crystallographically determined molecular structures of (top) the

[1-Ph-closo-1-CB₇H₇]⁻anion 6 in its [NEt₄]⁺salt [CCDC deposition no.

172016] and (bottom) the [2-Ph-closo-2-CB₆H₆]⁻anion 7 in its [NEt₄]⁺salt

[CCDC deposition no. 186128]. In anion 6, the C(1)–C(phenyl) distance

˚

b = 16.5323(4), c = 12.2821(3) A and b = 98.011(1) at 150 K].

Crystals of the air-stable [NEt₄]⁺salt of the anion 7 thus isolated

were, however, very small, and required the use of synchrotron

X-radiation (wiggler-generated from 2-GeV electrons at 150–

250 mA, station 9.8, CCLRC, Daresbury, UK)⁴³for sufﬁcient

diffraction intensity for determination of the molecular structure.

The analysis of the diffraction data thence gave a clean result, and

there was no disorder apparent. The [NEt₄]⁺salt of anion 7 was

therefore speciﬁcally characterised (Fig. 6, bottom). Additionally,

the structure of anion 7 obtained from DFT B3LYP/6-31G*

geometry optimizations, from which the predicted ¹¹B NMR chem-

ical shifts that are obtained from DFT-GIAO//B3LYP/6-31G*

calculations reasonably match those observed experimentally, is in

accord with the crystallographically obtained architecture.¹¹There

˚

is 1.484(2) A, with the other distances from C(1) being 1.528(2) to B(2),

˚

1.723(2) to B(3), 1.716(2) to B(4), and 1.622(2) A to B(5). In anion 7 the

˚

C(2)–C(phenyl) distance is 1.492(2) A, with the other distances from C(2)

being 1.744(2) to B(1), 1.559(2) to B(3), 1.555(2) to B(6) and 1.736(2) to

˚

B(7), and interboron distances 1.645(3) to 1.825(3) A. DFT calculations

˚

for anion 7 (see text) give the C(2)–C(phenyl) distance as 1.48410 A, with

the other distances from C(2) being 1.75538 to B(1), 1.55663 to B(3),

1.55663 to B(6) and 1.75538 to B(7), and interboron distances 1.64539 to

˚

1.82992 A.

[NEt₄]⁺salt (Fig. 6, top). As with the unsubstituted parent [1-

CB₇H₈]⁻anion,⁴²and, again in accord with general observations

in carbaborane chemistry,^37,39the carbon atom occupies a cluster

position of lower cluster-connectivity four. In contrast to the

[Ph-closo-CB₉H₉]⁻system above (species 2a and 2b), there is no

evidence for an alternative isomer in which the carbon atom

occupies a position of higher cluster-connectivity ﬁve, but the

isolation of such a species is unlikely in an unconstrained system

in view of the well-recognized ﬂuxionality of the eight-vertex

closo cluster.⁴²In this last regard, and also in common with the

behaviour of the parent [CB₇H₈]⁻anion, species 6 is ﬂuxional with

regard to scrambling within each of the two sets of B(6)B(7)B(8)

and B(2)B(3)B(4)B(5) positions. This phenomenon is addressed

adequately in an account of the [closo-1-CB₇H₈]⁻anion.⁴²It results

inter alia in the observation of a two-line 3 : 4 pattern of resonances

ˇ

are also clear parallels with St´ıbr’s recently reported unsubstituted

parent [closo-2-CB₆H₇]⁻anion, which can be obtained in a yield

of 6% from the closely related reaction of NEt₃and unsubstituted

[nido-1-CB₈H₁₂] in toluene under a 95 : 5 N₂/O₂atmosphere, the

other products being the [closo-1-CB₇H₈]⁻and [closo-4-CB₈H₉]⁻

anions in 35 and 48% yields respectively.²²That both the eight-

vertex and the seven-vertex anions, as well as the nine-vertex

anions, are obtained from these reactions with NEt₃in toluene

suggests that appropriate modiﬁcations of base and reaction

conditions may further enhance either the formation of the

interesting but hitherto elusive seven-vertex anions at the expense

of the other closo anions, or vice versa, but our laboratory has not

yet been able to carry out this type of developmental work on the

C-phenylated system that gives anions 6 and 7.

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4

Eleven-vertex nido and closo species

The unsubstituted ‘parent’ eleven-vertex [nido-7-CB₁₀H₁₃]⁻anion

was originally prepared by heating neutral [7-(NMe₃)-nido-7-

CB₁₀H₁₂] with elemental sodium in reﬂuxing thf for several hours.⁴⁴

The reaction has been held to proceed via the sparsely solu-

ble [nido-7-CB₁₀H₁₁]³⁻trianion, a potentially fascinating species,

which would easily form the [nido-7-CB₁₀H₁₃]⁻monoanion upon

protonation. In terms of the C-phenylated species that are the

subject of this present paper, and as mentioned above in Section

2, the corresponding C-phenylated eleven-vertex [7-Ph-nido-7-

CB₁₀H₁₂]⁻anion 8 results from the solid-state thermolysis of the

[NEt₄]⁺salt of the [6-Ph-nido-6-CB₉H₁₁]⁻anion 1, and may be

isolated from that process as its [NEt₄]⁺salt in 22% yield.

However, a more designed synthesis of the [7-Ph-nido-7-

CB₁₀H₁₂]⁻anion 8 results from the cluster-Aufbau reaction of

anion 1 with [BH₃(thf)]. A stoichiometry can be written as in

eqn (8), where L is thf. However, the formation of 8 is an

intermediate step in a two-step quasi-cascade process that, if

continued to completion, ultimately results in the formation of the

twelve-vertex [1-Ph-closo-1-CB₁₁H₁₁]⁻anion 10, as summarised

in eqn (9), where L is again thf. These ‘intermediate’ reaction

mixtures therefore contain mixtures of species 8 and 10, as well

as the starting anion 1. For the isolation of the desired anion 8,

conditions must therefore be chosen to minimize the presence

of 10 in the product mixture, because the other contaminant,

anion 1, can be removed by conversion to neutral [4-Ph-arachno-

4-CB₈H₁₃] (compound 3) by treatment with aqueous acidic FeCl₃,

as discussed in Section 2 above, followed by extraction into a non-

polar organic solvent.

Fig. 7 Crystallographically determined molecular structure of the

[7-Ph-nido-7-CB₁₀H₁₂]⁻anion 8 in its [NEt₄]⁺salt [CCDC deposition

no. 184222]. The C(7)–C(phenyl) distance is 1.506(2) A, with the other

˚

distances from C(7) being 1.686(3) to B(2), 1.693(3) to B(3), 1.680(3) to

˚

B(8), and 1.656(3) A to B(11).

CB₁₁H₁₁]⁻anion 10 by the route of eqn (8) and (9), as salts of

anion 10 are similarly difﬁcult to clear of any contamination by

anion 8 (Section 5 below). In this last case, however, it is feasible

to minimise the incidence of 8 by driving the reaction of eqn (9) to

completion, as anion 8 is essentially an intermediate in the overall

Aufbau process.

The molecular structure of the ca. 66% of the [7-Ph-nido-7-

CB₁₀H₁₂]⁻anion 8 in the crystal structure was, however, reasonably

successfully reﬁned (Fig. 7). It clearly shows the classical nido

eleven-vertex cluster geometry.^37,38The C-phenylated carbon atom

is in the open face, and there are two interboron bridging hydrogen

atoms on the open face. This cluster conﬁguration is in accord

with that of the unsubstituted parent [nido-7-CB₁₀H₁₃]⁻anion.⁴

In principle, deprotonation at one of the open-face bridging

[nido-PhCB₉H₁₁]⁻1 + [BH₃L]

→ [nido-PhCB₁₀H₁₂]⁻8 + H₂+ L

(8)

2−

hydrogen sites should yield the corresponding {PhCB₁₀H₁₁}

[nido-PhCB₁₀H₁₂]⁻8 + [BH₃L]

dianion, which, via differential dianionic/monoanionic solubility

versus the closo twelve-vertex monoanion 10, would in principle

provide a means of mutual decontamination of the nido eleven-

vertex and closo twelve-vertex systems. However, in common with

its nido eleven-vertex all-boron [nido-B₁₁H₁₃]²⁻equivalent, the nido

[PhCB₁₀H₁₂]⁻anion 8 seems relatively resistant to deprotonation

in aqueous solution. Here it is of interest that formal or real

removal of a proton from a bridging hydrogen position occurs

more readily from the corresponding neutral eleven-vertex nido

C₂B₉H₁₃dicarbaboranes and their C-substituted derivatives.⁴

The [7-Ph-nido-7-CB₁₀H₁₂]⁻anion 8 is amenable to oxidation.

Oxidation is a cluster condensation process,⁴⁵and, in accord

with this perception, reaction of anion 8 with elemental I₂in

aqueous alkaline solution results in cluster closure to give its

eleven-vertex closo congener, the [2-Ph-closo-2-CB₁₀H₁₀]⁻anion 9.

The non-phenylated [closo-2-CB₁₀H₁₁]⁻analogue has long been

known, and can be obtained, for example, from the reaction

with elemental iodine in thf of the sodium salt of what has been

proposed to be the [CB₁₀H₁₁]³⁻trianion.⁴⁶The crystallographically

determined molecular structure of the [2-Ph-closo-2-CB₁₀H₁₀]⁻

anion 9 (Fig. 8) shows the classical Williams–Wade eleven-vertex

closo geometry,^37,38although the ﬂuxionality of this anion, as well

as that of the closo eight-vertex [PhCB₇H₇]⁻anion 6 described

above, does emphasise that there can be a fragility associated with

the application of the Williams–Wade structure/electron-counting

→ [closo-PhCB₁₁H₁₁]⁻10 + 2 H₂+ L

(9)

The best conditions that we have so far found for the generation

of the [7-Ph-nido-7-CB₁₀H₁₂]⁻anion 8 in terms of the reaction

of eqn (8) involve the heating of the [NEt₄]⁺salt of the [6-Ph-

nido-6-CB₉H₁₁]⁻anion 1 in thf solution with [BH₃(thf)] at reﬂux

temperature for 48 h, followed by treatment with FeCl₃. This

yields neutral compound 3 in a mole fraction of ca. 22%, with

the target eleven-vertex nido anion 8 thence isolatable in 40%

yield as its [NEt₄]⁺salt, albeit contaminated with ca. 10 mol%

of the closo twelve-vertex anion 10. It is difﬁcult to free anion 8

from anion 10, as the two anions are quasi-isomorphous and have

very similar chromatographic properties. In this regard, it is noted

that the [NEt₄]⁺salts are in fact isomorphous crystallographically.

Both have the space group P2₁/c, with Z = 4, and the unit-cell

dimensions are essentially identical, the corresponding pairs of

linear and angular unit-cell dimensions all being within 1% of

each other. For the crystal of the [NEt₄]⁺salt of anion 8 that was

selected for the single-crystal work, the collected diffraction data

did in fact reﬁne for a mixture: there was an approximately 2 :

1 occupancy ratio of nido eleven-vertex anion 8 to closo twelve-

vertex anion 10 in the asymmetric unit, with 35% of the positions

of the H(10,11) and H(8,9) bridging hydrogen atoms in anion 8

(Fig. 7) being taken up by a BH(exo) unit of anion 10. The converse

problem can sometimes arise in the preparation of [1-Ph-closo-1-

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basicity, and therefore have great utility in the investigation

of, and in the application of, very acidic systems.^51–53Because

of these ‘least-coordinating’ anionic properties of the {CB₁₁}

monoanionic moieties, the twelve-vertex [closo-CB₁₁H₁₂]⁻anion

and its substituted derivatives have received relatively much more

attention than other polyhedral monocarbaboranes over the last

several years. However, C-aryl substitution has been elusive,²⁶and

only recently have syntheses of the C-phenylated [1-Ph-closo-1-

CB₁₁H₁₁]⁻anion 10 been reported, for example via a phenylcarbene

insertion into the eleven-vertex [nido-B₁₁H₁₃]²⁻dianion that is

engendered by the treatment of this dianion with NaH in thf

solution, followed by treatment with a mixture of PhCHCl₂and

EtOH.^54,56From this last reaction sequence, however, the product

10 is prone to contamination by its B-chlorinated derivatives.

An alternative route, well established for dicarbaboranes,⁵via C-

metallation of the parent unsubstituted [closo-CB₁₁H₁₂]⁻anion

followed by aryl coupling, might offer possibilities, but, as far

as we are aware, this has not yet been reported. As part of

our present work, we have established that the [1-Ph-closo-1-

CB₁₁H₁₁]⁻anion 10 can now also be prepared by cluster-Aufbau on

the ten-vertex [6-Ph-nido-6-CB₉H₁₁]⁻anion 1, using [BH₃(NEt₃)]

or [BH₃(SMe₂)]. This process proceeds essentially according to

a combination of eqn (8) and (9) above, which sum to give a

stoichiometry as in eqn (10).

Fig. 8 Crystallographically determined molecular structure of the

[2-Ph-closo-2-CB₁₀H₁₀]⁻anion 9 in its [NEt₄]⁺salt [CCDC deposition

no. 172017]. The C(2)–C(phenyl) distance is 1.492(3) A, with the other

˚

distances from C(2) being 1.643(3) to B(1), 1.584(3) to B(4), 1.593(2) to

˚

B(5), and 1.676(3) A to B(8).

‘rules’ for certain cluster species.⁴⁷The structure shows that the

carbon atom adopts the 2-position of cluster-connectivity four:

this is the site of lowest cluster-connectivity, again in accord with

long-established patterns of carbaborane cluster behaviour.^37,39

As just mentioned, and as also in the ‘parent’ [closo-CB₁₀H₁₁]⁻

monoanion, and in the all-boron analogue, the [closo-B₁₁H₁₁]²⁻

dianion, the eleven-vertex closo anion 9 is ﬂuxional in solution with

respect to interchange among the boron sites. The general mech-

anism and associated considerations are adequately discussed

elsewhere.^48,49In sum, there is a rapid scrambling within each

of the two sets of B(1)B(4)B(5)B(8) and B(3)B(6)B(7)B(10)B(11)

positions. A principal manifestation is the observation of a three-

[nido-PhCB₉H₁₁]⁻1 + 2 [BH₃L]

→ [closo-PhCB₁₁H₁₁]⁻10 + 3 H₂+ 2 L

(10)

In accord with eqn (10), therefore, the overall reaction between

the ten-vertex nido anion 1 and [BH₃(NEt₃)] in the absence of

solvent gives the closo twelve-vertex anion 10. NMR spectroscopy

indicated that anion 10 is formed in a (non-isolated) yield of

ca. 50%, but, as mentioned in Section 1 above, these particular

reaction conditions also engender a competing cluster closure to

give the ten-vertex [1-Ph-closo-1-CB₉H₉]⁻anion 2a. Fractional

crystallisations of the Cs⁺salts of 2a and 10 are therefore required

to separate and purify the two products. More conveniently, a

cleaner conversion to give 10 in much higher yield is afforded

by the use of [BH₃(SMe₂)] in 1,2-Cl₂C₂H₄as solvent (eqn (10),

where L is SMe₂), from which the [NEt₄]⁺salt of desired anion

10 is isolatable in 92% yield. As mentioned above, however, care

must be taken to drive the reaction to completion, otherwise the

desired twelve-vertex product anion 10 will be contaminated with

the eleven-vertex intermediate [7-Ph-nido-7-CB₁₀H₁₂]⁻anion 8,

which, as discussed above, is difﬁcult to clear from samples of

salts of anion 10. It may be noted here that we have also used

the route to generate the para-brominated [1-(p-BrC₆H₄)-closo-1-

CB₁₁H₁₁]⁻anion via the [6-(p-BrC₆H₄)-nido-6-CB₉H₁₁]⁻anion, this

intermediate [6-(p-BrC₆H₄)-nido-6-CB₉H₁₁]⁻anion being obtained

from p-BrC₆H₄CHO and B₁₀H₁₄in an exactly analogous manner

to the formation of compound 1 described in this present

paper. The synthesis and structure (CCDC 184223) of the [1-(p-

BrC₆H₄)-closo-1-CB₁₁H₁₁]⁻anion are as reported in preliminary

and summary communications elsewhere.^17,20

1

line 1 : 4 : 5 pattern of resonances in the room-temperature ¹¹B{ H}

NMR spectrum, rather than the six-line 2 : 2 : 2 : 1 : 1 : 1 pattern

that would be expected from a static molecular structure. As with

the similarly ﬂuxional closo [PhCB₇H₇]⁻anion 6 discussed above

(Section 3), therefore, but in contrast to the closo [PhCB₉H₉]⁻

systems (anions 2a and 2b) (Section 1), isolation of other C-

positioned isomers is unlikely in this {CB₁₀} system, unless it is

constrained by stabilising features such as bridging exo-chains,

etc. Again, as with the ﬂuxional eight-vertex anion 6, the slow-

exchange limit for the ﬂuxionality of the eleven-vertex anion 9 did

not seem to be approached in the 160 MHz ¹¹B NMR spectrum

on cooling to −90 ^◦C in CD₂Cl₂solution, implying an activation

energy for the ﬂuxional process of < ca. 30 kJ mol⁻¹.

5

The closo twelve-vertex system

Compounds of the twelve-vertex closo family, represented by the

parent [closo-1-CB₁₁H₁₂]⁻anion, have been previously prepared

via the cluster-expansion reaction between [7-(NMe₃)-nido-7-

CB₁₀H₁₂] and triethylamine-borane at the elevated temperatures of

180–200 ^◦C to produce the [1-(NHMe₂)-closo-1-CB₁₁H₁₁]⁻anion,

which can then, in a second step, be methylated to give neutral [1-

(NMe₃)-closo-1-CB₁₁H₁₁]; the NMe₃ligand can then be removed

in liquid ammonia with sodium to yield the parent unsubstituted

[closo-1-CB₁₁H₁₂]⁻anion.⁵⁰The monoanionic {CB₁₁} closo twelve-

vertex system is of particular contemporary interest.^{26,27,51–55}Be-

cause of the globular nature of the cluster, and the delocalisation

of the unit charge over the cluster surface, these monoanionic

monocarbaborane species are of very low Lewis and Brønsted

The molecular structure of the [1-Ph-closo-1-CB₁₁H₁₁]⁻anion

10 (Fig. 9) is based on a classical icosahedral twelve-vertex

closo cluster geometry.^37,38The cluster deviates somewhat from

regular icosahedral symmetry principally by the accommodation

of the smaller carbon atom C(1). It is of interest that, in

attempts to prepare pure examples of [NEt₄]⁺[closo-PhCB₁₁H₁₁]⁻

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species [PhCB₈H₁₃] and [PhCB₈H₁₁], compounds 3 and 4 respec-

tively, in a crystalline form suitable for conﬁrmatory single-crystal

X-ray work. However, their DFT-GIAO//B3LYP/6-31G* GIAO-

calculated ¹¹B nuclear magnetic shieldings, based on molecular

structures calculated at the B3LYP/6-31G* level (Fig. 3 and 4

above), matched the observed ¹¹B NMR chemical-shift values

sufﬁciently closely to give conﬁrmation that these are reasonable

representations of their true molecular conﬁgurations. Values for

the shieldings so calculated, together with the experimentally

observed ¹¹B NMR chemical shift values, are given in the Experi-

mental section below. In these comparisons, it may be noted that

the arachno [PhCB₈H₁₃] compound 3 is ﬂuxional with respect to

exchange of character between the inner-sphere bridging and the

endo-terminal hydrogen atoms associated with the B(6)B(7)B(8)

‘notch’ site on the open face; associated with this there is a

more subtle exchange in the asymmetry of the bridging character

between H(5,6) and H(8,9). A consequence of this ﬂuxionality is

that the molecule on time-average appears as if it has a mirror

plane through B(1)B(4)B(7) in solution, thus exhibiting a 2 : 2 :

2 : 1 : 1 relative-intensity pattern in the ¹¹B NMR spectrum.

This type of ﬂuxionality, together with other closely related open-

face inner-sphere hydrogen-atom ﬂuxionalities in arachno nine-

vertex systems, is well recognised, and has recently been dealt with

in detail using the anionic [arachno-B₉H₁₄]⁻borons-only cluster

system as a model.⁵⁷

Fig. 9 Crystallographically determined molecular structure of the

[1-Ph-closo-1-CB₁₁H₁₁]⁻anion 10 in its CH₂Cl₂solvate 10a of its [NEt₄]⁺

salt [CCDC deposition no. 164850]. The C(1)–C(phenyl) distance is

˚

1.512(3) A, with the distances from C(1) to its ﬁve adjacent boron atoms

˚

ranging from 1.715(3) to 1.730(4) A. For its non-solvated P2₁/c morph

10b [CCDC deposition number 194221], the C(1)–C(phenyl) distance is

˚

1.506(3) A, with the distances from C(1) to its ﬁve adjacent boron atoms

˚

ranging from 1.695(3) to 1.763(4) A. For the non-solvated Pbca morph

The molecular structures of all seven new closo anions 2a, 2b,

5, 6, 7, 9 and 10, as well as the nido anions 1 and 8, were all

structurally characterised by single-crystal X-ray work, using the

[NEt₄]⁺cation as a standard counterion. As variously mentioned

in the text, these all conformed to classical boron-containing

cluster geometries as initially classiﬁed by Williams,³⁴and were

all in accord with the classical ‘Wade’s Rules’ electron-counts.³⁸

Examination of the detailed dimensions obtained crystallograph-

ically suggested that there could be a small though real increase in

the C(cluster)–C(phenyl) distances in the anions [PhCB_nH_n]⁻as n

10c [CCDC deposition number 194222], the C(1)–C(phenyl) distance is

1.510(3) A, with the distances from C(1) to its ﬁve adjacent boron atoms

˚

ranging from 1.699(4) to 1.739(4) A.

for crystallographic purposes, this salt of 10 seemed prone to

form more than one variety of crystal. Crystals from acetone-

diethyl ether were either monoclinic, space group P2₁/c with Z =

4 (type 10b), or orthorhombic, space group Pbca, with Z = 8

(type 10c). From dichloromethane–hexane a CH₂Cl₂monosolvate

is formed, space group P2₁/c (type 10a). Of these, type 10b is, as

mentioned above near eqn 9, prone to form co-crystals with the

quasi-isomorphous [NEt₄]⁺salt of the [nido-PhCB₁₀H₁₂]⁻anion 8

if both anions 8 and 10 are present in solution.

˚

increases from 6 to 11, ranging from 1.484(2) A for smaller values

˚

of n (anions 6 and 7) to 1.512(3) A where n = 11 (anion 10). In

view of crystallographic uncertainties, it was of interest to calculate

the molecular structures at a sufﬁciently high level in order to see

whether such changes were mimicked in the calculated structures,

and whether they were therefore indeed structurally signiﬁcant.

Structures for the closo species 2a, 2b, 5, 6, 7, 9, and 10 were

therefore also calculated at the B3LYP/6-31G* level. In Table 1

DFT calculations and some structural comparisons

As mentioned in the text above, we have not been successful in

obtaining samples of the neutral arachno and nido nine-vertex

a

b

˚

Table 1 Measured and calculated C(cluster)–C(phenyl) interatomic distances in A for the closo anions 2a, 2b, 5, 6, 7, 9 and 10, ordered by increasing

cluster size

Compound

Measured distance^a

Calculated distance^b

Cluster connectivity of C atom

[2-Ph-closo-2-CB₆H₆]⁻anion 7

[1-Ph-closo-1-CB₇H₇]⁻anion 6

[4-Ph-closo-4-CB₈H₈]⁻anion 5

[1-Ph-closo-1-CB₉H₉]⁻anion 2a

[2-Ph-closo-2-CB₉H₉]⁻anion 2b

[2-Ph-closo-2-CB₁₀H₁₀]⁻anion 9

[1-Ph-closo-1-CB₁₁H₁₁]⁻anion 10a^c

[1-Ph-closo-1-CB₁₁H₁₁]⁻anion 10b^d

[1-Ph-closo-1-CB₁₁H₁₁]⁻anion 10c^e

1.492(2)

1.484(2)

1.490(2)

1.503(4)

1.498(2)

1.492(3)

1.512(3)

1.506(3)

1.510(3)

1.4841

1.4802

1.4873

1.4862

1.4986

1.4930

1.5147

4

5

4

5

^aAs measured in the solid state by single-crystal X-ray diffraction analysis on the [NEt₄]⁺salts at 150(2) K. ^bDFT calculations for gas-phase molecules

at the B3LYP/6-31G* level. ^cCH₂Cl₂solvate. ^dP2₁/c polymorph. ^ePbca morph.

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it can be seen that these calculated results do indeed indicate a

general increase in the intercarbon distances as n increases from

6 to 11, although the increase with n is by no means monotonic.

This phenomenon presumably represents differential electronic

effects exercised by the {CB_nH_n} clusters on the phenyl units

as n changes. Within this context it may also be noted that the

anions with the carbon atoms of higher cluster connectivities

(anions 2b and 9) have the longer C(cluster)–C(phenyl) distances.

The effects of such differences may become more apparent

when comparisons of reactivity are made among the aromatic

residues of such species and related families of compounds as

the monocarbaborane residue varies. Here it is of interest that

a converse effect, viz. apparent variations of reactivity of the

closo monocarbaborane clusters as their C-substituents change,

has been noted qualitatively when the reactivity to halogenation

of [HCB_nH_n]⁻vs. [PhCB_nH_n]⁻units has been compared in the two

cases where n = 9 and n = 11.²⁰It will ultimately be of interest

quantitatively to investigate these relative reactivities.

unsubstituted ‘parent’ compounds [arachno-4-CB₈H₁₄] and [nido-

1-CB₈H₁₂].⁵⁸

Preparation of the [6-Ph-nido-6-CB₉H₁₁]⁻anion 1 from B₁₀H₁₄and

PhCHO in alkaline solution

A solution of KOH (15 g, 0.25 mol) in H₂O (150 ml) and EtOH

(70 ml) was added to B₁₀H₁₄(6 g, 49 mmol) and cooled in

a bath at 0 ^◦C. PhCHO (40 ml, 400 mmol) was then added

batchwise over a period of 6 h. The EtOH was removed using

a rotary thin-ﬁlm evaporator and H₂O (200 ml) was added. The

alkaline solution was extracted three times with Et₂O (75 ml),

and H₂O (200 ml) was added to the combined ether extracts.

The Et₂O was removed under reduced pressure and [NEt₄]⁺Cl⁻

(10 g, 60 mmol) was added to the remaining aqueous layer, and

stirred. After addition of EtOH (50 ml) and further stirring,

the resulting colourless precipitate was ﬁltered off, washed with

water (3 × 40 ml) and dried in vacuo to yield the [NEt₄]⁺salt

of the [6-Ph-nido-6-CB₉H₁₁]⁻anion 1 as a white crystalline solid

(10.3 g, 31.4 mmol, 64%). Measured NMR data for the [6-

Ph-nido-6-CB₉H₁₁]⁻anion 1, [NEt₄]⁺salt in (CD₃)₂CO at 294–

299 K, ordered as assignment d(¹¹B)/ppm [d(¹H)/ppm], are as

follows: BH(5,7) +1.6 [+3.38], BH(9) −2.5 [+2.94], BH(1,3) −4.7

[+2.50], BH(8,10) −12.1 [+2.00], BH(2) −25.7 [+0.61], BH(4)

−37.8 [+0.42] with d(¹H) for lH(8,9) and lH(9,10) at −3.32 ppm;

additionally d(¹H)(Ph) centred at ca. + 7.29 (5H, compact over-

lapping multiplet), and d(¹H)(Et) at +3.49 (8H, quartet), +1.33

(12H, triplet), also d(¹³C)(Ph) +141.5 (1C), +127.7 (2C), +127.2

(2C), +124.4 (1C) and with d(¹³C)(cluster) +136.2 and d(¹³C)(Et)

+52.5 and +7.1 ppm. Calculated gas-phase d(¹¹B) values by DFT-

GIAO//B3LYP/6-31G* are as follows: BH(5,7) +1.3, BH(9) −5.7,

BH(1,3) −4.5, BH(8,10) −11.9 [+2.00], BH(2) −25.4, BH(4)

−37.7 ppm, in reasonable agreement with the experimentally

observed room-temperature solution-phase values.

Summary and conclusions

The Brellochs reaction,⁶here of benzaldehyde with nido-B₁₀H₁₄to

give entry into the C-phenylated monocarbaborane system via the

[6-Ph-nido-6-CB₉H₁₁]⁻anion 1, leads to a whole new portfolio of

C-phenylated monocarbaborane skeletons (see Scheme 1 in the

Introduction above). Most are obtainable in reasonable yield in

one, two or three steps from the B₁₀H₁₄starting substrate. The re-

action systems described are in principle extendable to an extensive

derivative C-aryl chemistry via choice of appropriate substituted

aromatic aldehyde in the initial reaction with B₁₀H₁₄. This principle

also of course applies to substituted aliphatic residues as well

as aromatic residues.^6,8Thus, a useful variety of functionalised

monocarbaborane cluster species are thence available.^{13,15,16,18,20,28,29}

Together, these considerations augur excellently for an increasingly

rapid development of monocarbaborane chemistry to match the

more extensively known chemistries in the ﬂanking areas of

dicarbaboranes on one hand and, on the other hand, the binary

boron hydrides that have no carbon atoms within their clusters.

Preparation of neutral [4-Ph-arachno-4-CB₈H₁₃] (compound 3)

from the [6-Ph-nido-6-CB₉H₁₁]⁻anion 1 with FeCl₃in aqueous

HCl solution

The [NEt₄]⁺salt of the [6-Ph-nido-6-CB₉H₁₁]⁻anion 1 (2.0 g,

6.2 mmol) was added to aqueous HCl (10%, 50 ml). The aqueous

layer was extracted several times with Et₂O (3 × 40 ml), the extracts

combined and the Et₂O removed using a rotary ﬁlm evaporator.

The residual colourless oil was stirred in a mixture of aqueous HCl

(10%, 50 ml), n-hexane (50 ml) and [FeCl₃(OH₂)₆] (6.0 g, 22 mmol)

for 3 h at room temperature. The n-hexane layer was then sepa-

rated, and the organic volatile components were removed from

it in vacuo to give neutral [4-Ph-arachno-4-CB₈H₁₃] (compound 3;

0.91 g, 4.8 mmol, 78%) as a white solid, mp 73–75 ^◦C. Measured

NMR data for [4-Ph-arachno-4-CB₈H₁₃] 3, in CDCl₃at 294–

299 K, ordered as assignment d(¹¹B)/ppm [d(¹H)/ppm], are as

follows: BH(7) +18.7 [+4.29], BH(1) −1.5 [+3.53], BH(5,9) −3.2

[+3.01], BH(6,8) −32.6 [+1.70] and BH(2,3) −40.9 [−0.60], with

d(¹H) for lH(5,6)/lH(8,9) at −2.53 ppm, for lH(6,7)/lH(7,8) at

−3.06 and H_endoC(4) +0.14 ppm; additionally d(¹H)(Ph) centred

at ca. + 7.29 (5H, compact overlapping multiplet), and d(¹³C)(Ph)

+138.5 (1C), +128.8 (2C), +128.4 (2C), +124.1 (1C), and

d(¹³C)(cluster) +13.4 ppm. Calculated gas-phase d(¹¹B) values by

DFT-GIAO//B3LYP/6-31G* are as follows: BH(7) +24.8, BH(1)

Experimental

General

Solvents were dried and distilled under dry dinitrogen. Reactions

were carried out under dry dinitrogen, and subsequent manip-

ulations were carried out in air. Decaborane (nido-B₁₀H₁₄) and

other reagents were obtained commercially and used as supplied.

Criteria of identity and purity for the new anionic species consisted

of unequivocal location and identiﬁcation of all atoms via single-

crystal X-ray diffraction analysis, backed up by clean multinuclear

NMR spectra for all carbon, hydrogen and boron positions: all

NMR spectra clearly showed that pure compounds were present.

For the two neutral species [4-Ph-arachno-4-CB₈H₁₃] 3 and [1-

Ph-nido-1-CB₈H₁₁] 4, for which single-crystal X-ray work was

precluded, clean multinuclear NMR spectra in conjunction with

DFT structural/NMR calculations that matched the experimental

NMR ﬁndings were taken as criteria of identity and purity,

supported by the results of mass spectrometry. Also for 3 and

4 there are clear parallels with the reported NMR data for the

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+2.5, BH(5,9) −4.1, BH(6,8) −25.9 and BH(2,3) −37.5 ppm, in

reasonable agreement with the experimentally observed room-

temperature solution-phase values. The molecule is ﬂuxional

with respect to exchange of character among the inner-sphere

bridging and the endo-terminal hydrogen atoms associated with

the B(6)B(7)B(8) ‘notch’ site on the open face,⁵⁷so the molecule

on time-average appears as if it has a mirror plane in solution,

exhibiting a 2 : 2 : 2 : 1 : 1 relative intensity pattern in its ¹¹B NMR

spectrum. The 70-eV EI (electron-impact ionisation) mass spec-

trum shows a highest-mass isotopomer envelope corresponding

to a combination of the isotopomer envelopes of the molecular

ion M⁺and a more abundant fragment corresponding to [M −

4H]⁺arising from the loss of two H₂molecules.

in (CH₃)₂CO that was overlayered with an approximately ﬁve-

fold excess of Et₂O. It was necessary manually to pick out

individual crystals as described in Section 3 of the Results and

discussion section above. Measured NMR data for [NEt₄]⁺salt

of the ﬂuxional [1-Ph-closo-1-CB₇H₇]⁻anion 6 in (CD₃)₂CO

at 294–299 K, ordered as assignment d(¹¹B)/ppm [d(¹H)/ppm],

are as follows: BH(6,7,8) +3.7 [+3.18] and BH(2,3,4,5) −1.3

[+2.61]; additionally d(¹H)(Ph) ca. + 7.76 to + 6.93 (5H, compact

overlapping multiplet), and d(¹H)(Et) at +3.49 (8H, quartet),

+1.33 (12H, triplet), also d(¹³C)(Ph) +141.5 (1C), +127.7 (2C),

+127.2 (2C) and +124.1 (1C), with d(¹³C)(cluster) +41.2 and

d(¹³C)(Et) 52.5 and +7.1 ppm. Calculated gas-phase d(¹¹B) values

for the rigid anion by DFT-GIAO//B3LYP/6-31G* are as follows:

B(2,4) +5.2, B(3,5) −11.4, B(6) −9.5 and B(7,8) +7.2 ppm,

which with ﬂuxionality would average at BH(6,7,8) +1.6 and

BH(2,3,4,5) −3.1, in reasonable agreement with the experimentally

observed room-temperature solution-phase values, Dd(¹¹B) −2.1

and −1.8 ppm, respectively. Measured NMR data for the [NEt₄]⁺

salt of the [2-Ph-closo-2-CB₆H₆]⁻anion 7 in (CD₃)₂CO at 294–

299 K, ordered as assignment d(¹¹B)/ppm [d(¹H)/ppm], are as

follows: BH(3,6) +6.4 [+4.11], BH(4,5) +0.5 [+3.55], BH(1,7)

−19.5 [-0.16]; additionally d(¹H)(Ph) ca. +7.39 to +7.03 (5H,

compact overlapping multiplet), and d(¹H)(Et) at +3.47 (8H,

quartet), +1.39 (12H, triplet), also d(¹³C)(Ph) +143.0 (1C), +127.7

(2C), +127.2 (2C) and +124.3 (1C), with d(¹³C)(cluster) +47.4

and d(¹³C)(Et) +52.5 and +7.1 ppm. Calculated gas-phase d(¹¹B)

values by DFT-GIAO//B3LYP/6-31G* are as follows: B(3,6)

+2.9, B(4,5) −2.4 and B(1,7) −20.0 ppm, in reasonable agreement

with the experimentally observed room-temperature solution-

phase values.

Thermolytic preparation of [1-Ph-nido-1-CB₈H₁₁] 4 from

[4-Ph-arachno-4-CB₈H₁₃] 3 in toluene

A solution of [1-Ph-arachno-1-CB₈H₁₃] 3 (prepared as above, 0.5 g,

2.7 mmol) in toluene (20 ml) was heated under reﬂux for 18 h. The

reaction mixture was allowed to cool down to ambient temperature

and the toluene was removed using a rotary thin-ﬁlm evaporator.

The residue was dissolved in n-hexane (50 ml) and the solution

was ﬁltered. The ﬁltrate was evaporated in vacuo to give neutral

[1-Ph-nido-1-CB₈H₁₁] (compound 4; 0.44 g, 2.35 mmol, 87%) as

a colourless oil. Measured NMR data for [1-Ph-nido-1-CB₈H₁₁]

4, in CDCl₃at 294–299 K, ordered as assignment d(¹¹B)/ppm

[d(¹H)/ppm], are as follows: BH(9) +5.9 [+4.07], BH(3,4) −0.9

[+3.20], BH(6,8) −11.1 [+2.54], BH(2,9) −24.0 [+2.41], BH(7)

−52.4 [+0.08]; with d(¹H) for lH(6,9)/lH(8,9) at −2.34 ppm,

and for lH(2,5) at −4.15 ppm; additionally d(¹H)(Ph) centred at

ca. + 7.29 (5H, compact overlapping multiplet), and d(¹³C)(Ph)

+141.0 (1C), +128.9 (2C), +128.8 (2C), +127.8 (1C) and

d(¹³C)(cluster) +52.1 ppm. Calculated d(¹¹B) values by

DFT-GIAO//B3LYP/6-31G* are as follows: BH(9) +3.0, BH(3,4)

−1.6, BH(6,8) −10.4, BH(2,5) −24.0, BH(7) −52.2 ppm. The 70-

eV EI (electron-impact ionisation) mass spectrum shows a highest-

mass isotopomer envelope corresponding to a combination of

the isotopomer envelopes of the molecular ion M⁺and a more

abundant fragment at [M − 2H]⁺corresponding to the loss of one

H₂molecule.

Thermolysis of the [NEt₄]⁺salt of the [6-Ph-nido-6-CB₉H₁₁]⁻anion

1: formation of the [4-Ph-closo-4-CB₈H₈]⁻anion 5, the [1-Ph-

closo-1-CB₉H₉]⁻anion 2a, and the [7-Ph-nido-7-CB₁₀H₁₂]⁻anion 8

The [NEt₄]⁺salt of [6-Ph-nido-6-CB₉H₁₁]⁻1 (1 g, 3 mmol) was

◦

heated in vacuo at 200 C for 4 h. The resulting colourless solid

was dissolved in MeCN (40 ml) and ﬁltered. The ﬁltrate was

evaporated using a rotary thin-ﬁlm evaporator, and the viscous

residue was dissolved in a 10 : 90 mixture of MeCN and CH₂Cl₂

and separated by column chromatography (silica gel, 30 cm ×

2.5 cm) using the same 10 : 90 mixture of MeCN and CH₂Cl₂

as the liquid phase Three fractions were obtained, containing

the [NEt₄]⁺salts of the [4-Ph-closo-4-CB₈H₈]⁻anion 5 (R_F0.21;

0.66 g, 2.1 mmol, 68%), the [1-Ph-closo-1-CB₉H₉]⁻anion 2a (R_F

0.19; 50 mg, 0.15 mmol, 5%) and the [7-Ph-nido-7-CB₁₀H₁₂]⁻

anion 8 (R_F0.10; 0.23 g, 0.67 mmol, 22%), R_Fvalues being

given for analytical TLC on silica gel [Silufol, Kavalier (Prague),

silica gel on aluminium foil; again using the same MeCN–CH₂Cl₂

(10 : 90) solvent; detection by UV at 254 nm]. In each of the

three cases the solvent mixture was evaporated to dryness to

give the yields just stated. Crystals suitable for single-crystal X-

ray diffraction analyses for the salts of anions 5 and 2a were

obtained by liquid–liquid diffusion from concentrated solutions

in CH₂Cl₂that were each overlayered with an approximately

ﬁve-fold excess of Et₂O. Crystals suitable for single-crystal X-ray

diffraction analysis for the [NEt₄]⁺salt of anion 8 were obtained by

liquid–liquid diffusion from a concentrated solution in (CH₃)₂CO

that was overlayered with an approximately ﬁve-fold excess of

Formation of the [NEt₄]⁺salts of the [1-Ph-closo-1-CB₇H₇]⁻anion

6 and the [2-Ph-closo-2-CB₆H₆]⁻anion 7 from the reaction of the

[4-Ph-arachno-4-CB₈H₁₃] compound 3 with NEt₃in toluene

A solution of [4-Ph-arachno-4-CB₈H₁₁] 3 (prepared as above, 0.5 g,

2.7 mmol) in toluene (15 ml) and NEt₃(5 ml) was heated under

reﬂux for 14 h. The reaction mixture was allowed to cool down

to ambient temperature and the volatile organic solvents were

removed using a rotary thin-ﬁlm evaporator. H₂O (50 ml) and

HCl (5%, 20 ml) were then added. The solution was stirred,

and a solution of [NEt₄]⁺Cl⁻(1.0 g, 6.0 mmol) in H₂O (30 ml)

was added slowly. After further stirring, the resulting precipitate

was ﬁltered off and dried in vacuo, to yield a mixture of the

[NEt₄]⁺salts of [1-Ph-closo-1-CB₇H₇]⁻and [2-Ph-closo-2-CB₆H₆]⁻

(anions 6 and 7 respectively, molar ratio 5 : 4 by integrated

¹¹B NMR spectroscopy; combined yield 0.70 g, 86%). Crystals

suitable for the single-crystal X-ray diffraction analyses were

obtained by liquid–liquid diffusion from a concentrated solution

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Et₂O. Measured NMR data for the [NEt₄]⁺salt of the [1-Ph-

closo-1-CB₉H₉]⁻anion 2a, in (CD₃)₂CO at 294–299 K, ordered as

assignment d(¹¹B)/ppm [d(¹H)/ppm], are as follows: BH(10) +27.1

[+5.49], BH(2,3,4,5) −16.1 [+1.79], BH(6,7,8,9) −24.4 [+0.84,

additionally d(¹H)(Ph) centred at ca. +7.29 (5H, compact overlap-

ping multiplet), d(¹³C)(cluster) +68.5 and d(¹H)(Et) at +3.49 (8H,

quartet), +1.33 (12H, triplet), also d(¹³C)(Ph) +141.5 (1C), +127.7

(2C), +127.1 (2C), +124.4 (1C), and d(¹³C)(Et) +52.5 and 7.1 ppm.

Calculated d(¹¹B) values by DFT-GIAO//B3LYP/6-31G* are as

follows: B(10) +28.9, B(2,3,4,5) −14.9 and B(6,7,8,9) −23.4 ppm,

in reasonable agreement with the experimentally observed room-

temperature solution-phase values. Measured NMR data for the

[NEt₄]⁺salt of the [4-Ph-closo-4-CB₈H₈]⁻anion 5, in (CD₃)₂CO

at 294–299 K, ordered as assignment d(¹¹B)/ppm [d(¹H)/ppm],

are as follows: BH(5,6) +13.4 [+4.44], BH(1,2,7,8) −10.8 [+1.71],

BH(3,9) −17.2 [+0.88], additionally d(¹H)(Ph) centred at ca. +

7.29 (5H, compact overlapping multiplet) and d(¹H)(Et) at +3.49

(8H, quartet), +1.33 (12H, triplet), also d(¹³C)(Ph) +141.9 (1C),

+127.5 (2C), +127.0 (2C), +124.1 (1C), d(¹³C)(cluster) +64.3 and

d(¹³C)(Et) +52.5, +7.1 ppm. Calculated d(¹¹B) values by DFT-

GIAO//B3LYP/6-31G* are as follows: B(5,6) +13.9, B(1,2,7,8)

−10.3 and B(3,9) −18.0 ppm, in reasonable agreement with

the experimentally observed room-temperature solution-phase

values. Measured NMR data for the [NEt₄]⁺salt of the [7-Ph-

nido-7-CB₁₀H₁₂]⁻anion 8, in (CD₃)₂CO at 294–299 K, ordered

as assignment d(¹¹B)/ppm [d(¹H)/ppm], are as follows: BH(5)

−0.8 [+2.32], BH(2,3) −9.1 [+2.09], BH(8,11) −10.0 [+1.98],

BH(9,10) −22.6 [+1.63], BH(1) −25.2 [+1.76], BH(4,6) −31.5

[+0.45]; with d(¹H) for lH(8,9)/lH(10,11) at −3.33 ppm; addi-

tionally d(¹H)(Ph) centred at ca. + 7.29 (5H, compact overlapping

multiplet), d(¹³C)(cluster) +55.4 and d(¹H)(Et) at +3.49 (8H,

quartet), +1.33 (12H, triplet), also d(¹³C)(Ph) +141.7 (1C), +127.8

(2C), +127.1 (2C), +124.5 (1C) and d(¹³C)(Et) +52.5, +7.0 ppm.

Calculated gas-phase d(¹¹B) values by DFT-GIAO//B3LYP/6-

31G* are as follows: BH(5) +0.3, BH(2,3) −9.5, BH(8,11) −10.2,

BH(9,10) −22.7, BH(1) −26.1, BH(4,6) −30.8 ppm, in reasonable

agreement with the experimentally observed room-temperature

solution-phase values.

was ﬁltered off and dried in vacuo, to yield the [NEt₄]⁺salt of the

[4-Ph-closo-4-CB₈H₈]⁻anion 5 (1.13 g, 3.6 mmol, 72%).

Preparation of the [NEt₄]⁺salt of the [2-Ph-closo-2-CB₉H₉]⁻anion

2b from the reaction of the [6-Ph-nido-6-CB₉H₁₁]⁻anion 1 with

elemental iodine in alkaline solution

A solution of the [NEt₄]⁺salt of the [6-Ph-nido-6-CB₉H₁₁]⁻anion

1 (0.98 g, 3 mmol) in aqueous HCl (10%, 50 ml) was extracted

with Et₂O (3 × 50 ml). The combined ethereal layers were then

shaken with aqueous KOH (ca. 1.0 M; 50 ml), and the Et₂O then

evaporated from the mixture using a rotary thin-ﬁlm evaporator.

Elemental I₂(2 g, 7.9 mmol) was then added to the resulting

alkaline aqueous solution, which was then stirred for 3 h at

ambient temperature. [NEt₄]⁺Cl⁻(0.83 g, 5 mmol) was then slowly

added to the stirred solution, and, after further stirring, the

resulting colourless precipitate was ﬁltered off, washed with H₂O

(3 × 30 ml) and dried in vacuo, to yield the [NEt₄]⁺salt of the [2-Ph-

closo-2-CB₉H₉]⁻anion 2b as a white microcrystalline solid (0.93 g,

2.9 mmol, 92%). A sample of this [NEt₄]⁺salt was dissolved to give

a saturated solution in acetone, which was then overlayered with

diethyl ether and then allowed to stand at ambient temperature.

Mutual diffusion (ca. 48 h) thence gave colourless crystals suitable

for the X-ray diffraction analysis. Measured NMR data for the

[2-Ph-closo-2-CB₉H₉]⁻anion 2b, [NEt₄]⁺salt in (CD₃)₂CO at

294–299 K, ordered as assignment d(¹¹B)/ppm [d(¹H)/ppm in

square brackets] are as follows: BH(10) +1.7 [+3.84], BH(1) −2.8

[+3.41], BH(4) −21.1 [+1.27], BH(6,9) −25.6 [+0.93], BH(3,5)

−28.6 [+1.72], BH(7,8) −28.6 [+0.63]; additionally d(¹H)(Ph) at

+6.96 to +6.85 (unresolved multiplets, 5H) and d(¹H)(Et) at +3.44

(8H, quartet), +1.37 (12H, triplet) with d(¹³C)(Ph) at +144.2

(1C), +127.0 (2C), +126.2 (2C), +124.2 (1C), d(¹³C)(cluster) +49.0

and d(¹³C)(Et) +52.5 and +7.1 ppm. Calculated gas-phase d(¹¹B)

values by DFT-GIAO//B3LYP/6-31G* are as follows: B(10) +0.8,

B(1) −5.3, B(4) −20.1, B(6,9) −25.0, B(3,5) −29.3 and B(7,8)

−27.6 ppm, in reasonable agreement with the experimentally

observed room-temperature solution-phase values.

Preparation of the [NEt₄]⁺salt of the [1-Ph-closo-1-CB₉H₉]⁻anion

2a from the reaction of the [6-Ph-nido-6-CB₉H₁₁]⁻anion 1 with

elemental sodium in thf solution

High-yield preparation of the [NEt₄]⁺salt of the

[4-Ph-closo-4-CB₈H₈]⁻anion 5 from the reaction of neutral

[4-Ph-arachno-4-CB₈H₁₃] 3 with NEt₃

The [NEt₄]⁺salt of the [6-Ph-nido-6-CB₉H₁₁]⁻anion 1 (4.0 g,

12.2 mmol) was dissolved in thf (40 ml), and pieces of sodium

metal (5 g, 0.2 mol) were then added. The colourless mixture

was then heated under reﬂux for 73 h, and then allowed to cool

to ambient temperature. The solid pieces of excess sodium were

removed, EtOH (20 ml) and H₂O (100 ml) were added, and the thf

was removed under reduced pressure (rotary thin-ﬁlm evaporator).

The resulting aqueous solution was acidiﬁed with aqueous HCl

(10%, 50 ml) and extracted with Et₂O (4 × 40 ml). The extracts

were combined and the Et₂O removed from them using a rotary

thin-ﬁlm evaporator. The residual colourless oil was stirred with

a mixture of aqueous HCl (10%, 100 ml), n-hexane (100 ml) and

[FeCl₃(OH₂)₆] (12.0 g, 44 mmol) for 3 h at ambient temperature.

The n-hexane layer was then separated, and the organic volatile

components were removed from it, initially using a rotary thin-

ﬁlm evaporator, and then ﬁnally in vacuo, to give neutral [4-Ph-

arachno-4-CB₈H₁₃] 3 as a colourless oil (0.41 g, 2.2 mmol, 18%).

A solution of [4-Ph-arachno-4-CB₈H₁₃] 3 (prepared as above,

0.94 g, 5.0 mmol) in toluene (15 ml) was heated under reﬂux for

14 h. The reaction mixture was allowed to cool down to ambient

temperature and the toluene was removed using a rotary thin-ﬁlm

evaporator. The oily residue was dissolved in dry thf (20 ml), and

cooled in a bath at −78 ^◦C. NEt₃(5 ml, 36 mmol) was added to the

stirred solution, and then a solution of I₂(1.27 g, 5 mmol) in thf

(25 ml) was slowly added dropwise with stirring, all with cooling at

−78 ^◦C. The reaction mixture was then allowed to warm slowly up

to ambient temperature. The thf and NEt₃were removed using a

rotary thin-ﬁlm evaporator, and then H₂O (50 ml), Na₂SO₃(0.63 g,

5 mmol) and aqueous HCl (5%; 50 ml) were added. The solution

was extracted with Et₂O (3 × 30 ml), and H₂O (50 ml) was added to

the combined ether extracts. The Et₂O was removed under reduced

pressure and [NEt₄]⁺Cl⁻(0.83 g, 5 mmol) was added to the stirred

aqueous solution. After further stirring, the resulting precipitate

5764 | Dalton Trans., 2006, 5753–5769

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The remaining aqueous layer was extracted with Et₂O (3 × 60 ml),

and the combined ethereal extracts were evaporated using a rotary

thin-ﬁlm evaporator, to yield a colourless oil. This oily residue was

then dissolved in H₂O (100 ml) and after a the slow addition of

[NEt₄]⁺Cl⁻(3.0 g, 18 mmol) to the stirred solution, a colourless

precipitate developed. After further stirring, this white precipitate

was ﬁltered off and dried in vacuo to yield of the [NEt₄]⁺salt of

the ten-vertex [1-Ph-closo-1-CB₉H₉]⁻anion 2a as a colourless solid

(2.94 g, 9.0 mmol, 74%).

a mixture of the [NEt₄]⁺salts of [1-Ph-closo-1-CB₁₁H₁₁]⁻and [1-

Ph-closo-1-CB₉H₉]⁻(anions 10 and 2a respectively; molar ratio

2 : 1; combined yield 1.45 g, 76%). Samples of these [NEt₄]⁺

salts of 2a and 10 suitable for single crystal X-ray diffraction

analysis were obtained by fractional crystallisation from CH₂Cl₂–

Et₂O. However, bulk separation of 2a and 10 is more conveniently

achieved by ion-exchange of [NEt₄]⁺for Cs⁺, followed by fractional

crystallisation of the Cs⁺salts. Thus, a solution of the 2 : 1 mixture

of the two [NEt₄]⁺salts (1.20 g) was dissolved in an 80 : 20

ethanol–water mixture (60 ml) and passed through a column of

ion-exchange resin (Zerolit 325 in the protonated form, column

dimension 2.5 cm × 30 cm) using the same 80 : 20 ethanol–

water proportions in the eluting liquid. Evaporation of ethanol,

using a rotary ﬁlm evaporator, from the resulting acidic solution,

followed by neutralisation with Cs⁺[OH]⁻to pH 7, afforded a white

precipitate of the [Cs]⁺salts of 10 and 2a. This mixture was then

separated by fractional crystallisation from hot water, starting with

ca. 15 ml H₂O: the Cs⁺salt of the [1-Ph-closo-1-CB₉H₉]⁻anion 2a

is much less soluble in cold H₂O (factor ca. × 12) than the Cs⁺salt

of the [1-Ph-closo-1-CB₁₁H₁₁]⁻anion 10.

Preparation of the [7-Ph-nido-7-CB₁₀H₁₂]⁻anion 8 from the

[6-Ph-nido-6-CB₉H₁₁]⁻anion 1

A sample of the [NEt₄]⁺salt of the [6-Ph-nido-6-CB₉H₁₁]⁻anion

1 (6.0 g, 18.3 mmol) was dissolved in thf (40 ml), a solution

of [BH₃(thf)] in thf (1.0 M, 75 ml, corresponding to 75 mmol

of {BH₃}) was added, and the mixture heated under reﬂux for

48 h. After cooling down to 0 ^◦C, H₂O (100 ml) was added

slowly, and then aqueous HCl (10%, 20 ml). Following removal

of the thf under reduced pressure, the resulting aqueous solution

was extracted with Et₂O (3 × 60 ml), and the combined ether

layers were evaporated using a rotary thin-ﬁlm evaporator to give

a colourless oil. A mixture of [FeCl₃(OH₂)₆] (18 g, 67 mmol),

aqueous HCl (10%, 120 ml) and n-hexane (100 ml) was then

added to the residual oil, and the resulting two-phase system was

stirred for 3 h. The n-hexane layer was then separated off, and

the aqueous layer then extracted with n-hexane (3 × 40 ml). The

combined n-hexane layers were evaporated, ﬁrstly by rotary thin-

ﬁlm evaporator, and then in vacuo, to yield a colourless solid,

[4-Ph-arachno-4-CB₈H₁₃] compound 3 (750 mg, 4.0 mmol, 22%).

The remaining aqueous layer was extracted with Et₂O (3 × 60 ml)

and H₂O (200 ml) was added to the combined Et₂O extracts. The

Et₂O was evaporated using a rotary thin-ﬁlm evaporator. The

remaining aqueous solution was ﬁltered, and [NEt₄]⁺Cl⁻(2.0 g,

12 mmol) was slowly added to the stirred ﬁltrate. After further

stirring, this white precipitate was ﬁltered off, and dried in vacuo,

to yield a colourless mixture of the [NEt₄]⁺salts of the eleven-vertex

[7-Ph-nido-7-CB₁₀H₁₂]⁻anion 8 and the twelve-vertex [1-Ph-closo-

1-CB₁₁H₁₁]⁻anion 10 (3.14 g), molar ratio ca. 9 : 1 by integrated

¹¹B NMR spectroscopy, corresponding to ca. 7.3 mmol (40%) of

8 and ca. 0.8 mmol (ca. 5%) of 10.

Preparation of the [NEt₄]⁺salt of the [2-Ph-closo-2-CB₁₀H₁₀]⁻

anion 9 from the reaction of the [7-Ph-nido-7-CB₁₀H₁₂]⁻anion 8

with iodine

Aqueous HCl (10%, 40 ml) was added to a sample of the mixture

of the [NEt₄]⁺salts of the [7-Ph-nido-7-CB₁₀H₁₂]⁻anion 8 and [1-

Ph-closo-1-CB₁₁H₁₁]⁻anion 10 (prepared as above, 0.68 g). The

mixture was extracted with Et₂O (3 × 30 ml), and the combined

ethereal layers were evaporated using a rotary thin-ﬁlm evaporator.

The residual pale yellow oil was dissolved in aqueous KOH (5%,

50 ml), elemental I₂(1.0 g, 3.9 mmol) was added, and the reaction

mixture was stirred for 1 h at ambient temperature. The reaction

mixture was then ﬁltered, and a solution of [NEt₄]⁺Cl⁻(0.50 g,

3.0 mmol) in H₂O (50 ml) was slowly added to the stirred ﬁltrate.

After further stirring, the resulting white precipitate was ﬁltered

off, and dried in vacuo to yield a mixture of the [NEt₄]⁺salts of the

[2-Ph-closo-2-CB₁₀H₁₀]⁻anion 9 and the [1-Ph-closo-1-CB₁₁H₁₁]⁻

anion 10. The white solid was crystallised from hot mixture of

H₂O and EtOH (proportions approximately 1 : 1) to yield a

pure sample of the [NEt₄]⁺salt of the [2-Ph-closo-2-CB₁₀H₁₀]⁻

anion 9 as a white crystalline solid (0.44 g, 1.3 mmol, 65%).

Crystals suitable for single-crystal X-ray diffraction analysis were

obtained by liquid–liquid diffusion from a concentrated solution

in (CH₃)₂CO that was overlayered with a ca. ﬁve-fold excess of

Et₂O. Measured NMR data for the [NEt₄]⁺salt of the ﬂuxional

[closo-PhCB₁₀H₁₀]⁻anion 9, in (CD₃)₂CO at 294–299 K, ordered

as assignment d(¹¹B)/ppm [d(¹H)/ppm] are as follows: BH(9)

−4.5 [+1.59], BH(3,6,7,10,11) −10.4 [+2.10], BH(1,4,5,8) −12.3

[+1.88]; additionally d(¹H)(Ph) ca. +7.77 to +6.93 (5H, compact

overlapping multiplet), and d(¹H)(Et) at +3.49 (8H, quartet), +1.40

(12H, triplet), also d(¹³C)(Ph) +141.4 (1C), +127.6 (2C), +127.2

(2C), +124.3 (1C), d(¹³C)(cluster) +72.4 and d(¹³C)(Et) +52.5,

7.1 ppm. Gas-phase d(¹¹B) values for the rigid anion, as calculated

by DFT-GIAO//B3LYP/6-31G*, are as follows: B(1) −21.6, B(3)

+20.2, B(4,5) −5.1, B(6,7) −18.5, B(8) −18.0, B(9) −3.9 and

B(10,11) −18.6 ppm; with ﬂuxionality these would average as,

BH(3,6,7,10,11) −10.8, BH(1,4,5,8) −12.4 [+1.88], which, with

Reaction of the [6-Ph-nido-6-CB₉H₁₁]⁻anion 1 with [BH₃L]; a

second route for the isolation of salts of the [1-Ph-closo-1-CB₉H₉]⁻

anion 2a and the [1-Ph-closo-1-CB₁₁H₁₁]⁻anion 10

[BH₃(NEt₃)] converts the ten-vertex [6-Ph-nido-6-CB₉H₁₁]⁻anion

1, now more directly, to the eleven-vertex [1-Ph-closo-1-CB₁₁H₁₁]⁻

anion 10. Thus, the [NEt₄]⁺salt of the [6-Ph-nido-6-CB₉H₁₁]⁻

anion 1 (1.65 g, 5 mmol) was suspended in [BH₃(NEt₃)] (2.3 g,

◦

20 mmol) and the mixture heated in an oil bath at 210 C under

dinitrogen for 6 h. After cooling down to ambient temperature, the

mixture was extracted several times with benzene (3 × 30 ml), and

the remaining solid residue was decomposed by heating under

reﬂux for 3 h in a 50 : 50 mixture of ethanol and concentrated

aqueous hydrochloric acid (50 ml). The ethanol was removed

using a rotary ﬁlm evaporator, H₂O (50 ml) was added to the

resulting mixture with stirring, and, after further stirring, the

resulting precipitate was ﬁltered off and dried in vacuo, to yield

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BH(9) at −4.5 ppm, are in good agreement with the experimentally

92%). Crystals suitable for single-crystal X-ray diffraction analysis

were obtained by liquid–liquid diffusion from a concentrated

solution in (CH₃)₂CO that was overlayered with an approximately

ﬁve-fold excess of Et₂O. Measured NMR data for the [NEt₄]⁺salt

of [1-Ph-closo-1-CB₁₁H₁₁]⁻10, in CD₃CN at 294–299 K, ordered

as assignment d(¹¹B)/ppm [d(¹H)/ppm], are as follows: BH(12)

−8.0 [+2.13], BH(2,3,4,5,6) and BH(7,8,9,10,11) coincident at

−12.9 [+2.03 and +1.74]; additionally d(¹H)(Ph) centred at ca. +

7.29 (5H, compact overlapping multiplet), and d(¹H)(Et) at +3.49

(8H, quartet), +1.33 (12H, triplet), also d(¹³C)(Ph) +141.7 (1C),

+127.8 (2C), +127.3 (2C), +124.4 (1C), d(¹³C)(cluster) +69.0 and

d(¹³C)(Et) +52.5 and 7.1 ppm. Calculated gas-phase d(¹¹B) values

by DFT-GIAO//B3LYP/6-31G* are as follows: B(12) −6.7,

B(2,3,4,5,6) −14.1 and B(7,8,9,10,11) −13.3 ppm, in reasonable

agreement with the experimentally observed room-temperature

solution-phase values.

observed room-temperature solution-phase values.

Preparation of the [NEt₄]⁺salt of the [1-Ph-closo-1-CB₁₁H₁₁]⁻

anion 10 from the reaction of the [6-Ph-nido-6-CB₉H₁₁]⁻anion 1

with BH₃(SMe₂) in 1,2-Cl₂C₂H₄

A sample of the [NEt₄]⁺salt of the [6-Ph-nido-6-CB₉H₁₁]⁻anion

1 (6.0 g, 18.4 mmol) was dissolved in 1,2-Cl₂C₂H₄(60 ml),

[BH₃(SMe₂)] (30 ml, mmol) was added and the mixture was then

heated under reﬂux for 48 h. After cooling down to 0 ^◦C, H₂O

(50 ml) was added slowly, and then aqueous HCl (10%, 150 ml).

Following removal of the 1,2-Cl₂C₂H₄under reduced pressure

(rotary thin-ﬁlm evaporator), the remaining aqueous layer was

extracted with Et₂O (3 × 75 ml), and the combined ethereal

extracts were evaporated using a rotary thin-ﬁlm evaporator, to

yield a colourless oil. This oily residue was then dissolved in H₂O

(100 ml) and, after the slow addition of [NEt₄]⁺Cl⁻(4.0 g, 24 mmol)

to the stirred solution, a colourless precipitate developed. After

further stirring, this precipitate was ﬁltered off, and dried in

vacuo, to yield the [NEt₄]⁺salt of the twelve-vertex [1-Ph-closo-

1-CB₁₁H₁₁]⁻anion 10 as a white crystalline solid (5.9 g, 17 mmol,

X-Ray crystallography

Crystallographic data for the [NEt₄]⁺salts of the [6-Ph-nido-6-

CB₉H₁₁]⁻anion 1, the [1-Ph-closo-1-CB₉H₉]⁻anion 2a, the [2-Ph-

closo-2-CB₉H₉]⁻anion 2b, the [4-Ph-closo-4-CB₈H₈]⁻anion 5, the

+

˚

Table 2 Crystallographic data for the [NEt₄] salts of the anions. All data were collected with Mo-Karadiation (k = 0.71073 A) on a Nonius KappaCCD

˚

diffractometer, except for the salt of anion 7, for which data were collected with synchrotron radiation (k = 0.6883 A) on a Bruker SMART diffractometer.

All data were measured at 150 K

Anion

1

2a

2b

5

6

7

Formula

M

C₁₅H₃₆B₉N

327.7

CH₂Cl₂–hexane

C₁₅H₃₄B₉N

325.7

CH₂Cl₂–hexane

Colourless prism

C₁₅H₃₄B₉N

325.7

Me₂O–Et₂O

Colourless rod

C₁₅H₃₃B₈N

313.9

CH₂Cl₂–hexane

Colourless prism

C₁₅H₃₂B₇N

302.1

Me₂CO–Et₂O

Colourless prism

C₁₅H₃₁B₆N

290.3

Me₂CO–Et₂O

Colourless rod

Crystallization

Crystal description Colourless prism

Crystal size–mm

Crystal system

Space group

0.74 × 0.52 × 0.37 0.19 × 0.14 × 0.07 0.43 × 0.31 × 0.25 0.51 × 0.18 × 0.12 0.41 × 0.28 × 0.23 0.30 × 0.03 × 0.03

Monoclinic

C2/c

Monoclinic

C2/c

Orthorhombic

P2₁2₁2₁

Monoclinic

P2₁/n

Monoclinic

P2₁/n

Orthorhombic

Pbca

˚

a/A

18.6342(4)

10.2871(2)

23.4709(5)

109.019(1)

4253.57(15)

8

24.4404(16)

10.5641(6)

16.7478(14)

99.099(3)

4269.7(5)

8

10.0208(2)

13.6557(3)

15.4382(3)

11.6480(3)

10.5204(3)

17.5709(6)

107.974(1)

2048.08(11)

4

9.5758(2)

16.5323(4)

12.2821(3)

98.011(1)

1925.40(8)

4

14.438(3)

15.762(3)

16.877(4)

˚

b/A

˚

c/A

b/^◦

V/A

Z

3

˚

2112.58(87)

4

4139

3840.6(14)

8

4073

Unique data

4128

4147

4011

3772

R (on F, F²> 2r)

0.060

0.083

0.229

164851

0.045

0.123

184221

0.055

0.170

164852

0.059

0.152

172016

0.070

0.179

186128

R_w(on F², all data) 0.176

CCDC ref.

184220

Anion

8 and 10b

9

10 in 10a

10 in 10b

10 in 10c

Formula

M

C₁₅H_36.65B_10.35

343.0

Me₂CO–Et₂O

N

C₁₅H₃₅B₁₀N

3337.5

Me₂CO–Et₂O

Colourless block

C₁₆H₃₈B₁₁Cl₂N

434.3

CH₂Cl₂–Et₂O

Colourless plate

C₁₅H₃₆B₁₁N

349.4

Me₂CO–Et₂O

Colourless plate

C₁₅H₃₆B₁₁N

349.4

Me₂CO–Et₂O

Colourless rod

Crystallization

Crystal description Colourless needle

Crystal size/mm

Crystal system

Space group

0.65 × 0.15 × 0.06 0.48 × 0.40 × 0.36 0.55 × 0.51 × 0.22 0.40 × 0.15 × 0.05 0.26 × 0.05 × 0.04

Monoclinic

P2₁/c

Monoclinic

C2/c

Monoclinic

P2₁/c

Monoclinic

P2₁/c

Orthorhombic

P2₁2₁2₁

˚

a/A

12.9967(5)

10.1842(3)

17.1010(8)

100.303(1)

2227.01(15)

4

17.4206(4)

13.9339(4)

19.5909(5)

113.534(1)

4359.9(2)

8

15.7334(5)

10.8812(4)

15.7928(6)

110.935(1)

2525.22(16)

4

13.0151(3)

10.2266(3)

16.9186(5)

100.558(1)

2213.74(11)

4

10.2352(2)

16.9027(5)

25.6191(9)

˚

b/A

˚

c/A

b/^◦

V/A

Z

3

˚

4432.2(2)

8

4353

Unique data

4350

4216

4857

4335

R (on F, F²> 2r)

0.063

0.073

0.202

172017

0.065

0.173

164850

0.069

0.184

194221

0.062

0.165

194222

R_w(on F², all data) 0.177

CCDC ref.

184222

5766 | Dalton Trans., 2006, 5753–5769

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[1-Ph-closo-1-CB₇H₇]⁻anion 6, the [2-Ph-closo-2-CB₆H₆]⁻anion

7, the [7-Ph-nido-7-CB₁₀H₁₂]⁻anion 8, the [2-Ph-closo-2-CB₁₀H₁₀]⁻

anion 9, the CH₂Cl₂-solvated [NEt₄]⁺salt 10a of the [1-Ph-closo-

1-CB₁₁H₁₁]⁻anion 10, and two different non-solvated morphs,

P2₁/c 10b and Pbca 10c, of the [NEt₄]⁺salt of [1-Ph-closo-1-

CB₁₁H₁₁]⁻anion 10, are summarised in Table 2. Data have been

presented in association with preliminary communications,^8,10,11

but are also recorded here for cohesive convenience of reference. It

may be noted that the [NEt₄]⁺salt of the [7-Ph-nido-7-CB₁₀H₁₂]⁻

anion 8, space group P2₁/c, contains ca. 35% of the [1-Ph-closo-1-

CB₁₁H₁₁]⁻anion 10, of which the molecular structure is quasi-

isomorphous with that of anion 8, and of which the crystal

structure is crystallographically isomorphous with that of the

P2₁/c polymorph 10b of the [NEt₄]⁺salt of anion 10. An additional

noteworthy crystallographic point is that crystals of the [NEt₄]⁺

salt of the anion [2-Ph-closo-2-CB₆H₆]⁻anion 7 were too tiny to

give sufﬁcient diffraction intensity using a conventional sealed-

tube X-ray source, and so a high-intensity synchrotron X-ray

source was used.^11,43,59Methods and programs were otherwise

standard.^60–65The crystallographic data are summarised in Table 2.

˚

All data were collected with Mo-Karadiation (k = 0.71073 A) on a

Nonius KappaCCD diffractometer, except for the salt of anion 7,

for which data were collected with synchrotron wiggler-generated

radiation (station 9.8, Daresbury;^52–54k = 0.6883 A) on a Bruker

˚

SMART diffractometer. All data were measured at 150 K.

Of crystallographic interest is the packing of anions within

the anionic layers of the crystal structures of the [NEt₄]⁺salts

of the three smaller of the C-phenyl monocarbaborane anions,

speciﬁcally the [4-Ph-closo-4-CB₈H₈]⁻anion 5, the [1-Ph-closo-1-

CB₇H₇]⁻anion 6 and the [2-Ph-closo-2-CB₆H₆]⁻anion 7. Although

not isomorphous, they have similar packing arrangements. Each of

these three salts crystallises with alternate layers of [NEt₄]⁺cations

and [PhCB_nH_n]⁻anions. This layering behaviour is typiﬁed by the

salt of the [2-Ph-closo-2-CB₆H₆]⁻anion 7 (Fig. 10). If this type

of phenomenon can be engineered with cations and anions that

have functional groups with cross-linking capability, then the path

may be open via thermal or irradiative curing to tailored boron–

carbon–nitrogen ceramic materials layered at the atomic level.⁴⁵

CCDC reference numbers for the [NEt₄]⁺salts: [6-Ph-nido-6-

CB₉H₁₁]⁻1 (184220), [1-Ph-closo-1-CB₉H₉]⁻2a (164851), [2-Ph-

closo-2-CB₉H₉]⁻2b (184221), [4-Ph-closo-4-CB₈H₈]⁻5 (164852),

[1-Ph-closo-1-CB₇H₇]⁻

6

(172016), [2-Ph-closo-2-CB₆H₆]⁻

(184222), [2-Ph-closo-2-

7

Fig. 10 Fragments of the Pbca orthorhombic crystal lattice of the [NEt₄]⁺

salt of the [2-Ph-closo-2-CB₆H₆]⁻anion 7. The top two diagrams show suc-

cessive views of ac planes, showing a cationic [NEt₄]⁺layer (top diagram)

and an anionic [2-Ph-closo-2-CB₆H₆]⁻layer (centre diagram). The lowest

diagram views the lattice perpendicular to bc, and shows the alternating

cation–anion layered structure, with cations depicted in light grey and

anions in dark grey. The [NEt₄]⁺salts of the [4-Ph-closo-4-CB₈H₈]⁻and

[1-Ph-closo-1-CB₇H₇]⁻anions, species 5 and 6 respectively, also have very

similar layered packings, although the three species are not isomorphous:

both 5 and 6 are monoclinic P2₁/n, but have different cell dimensions, with

(186128), [7-Ph-closo-7-CB₁₀H₁₂]⁻

8

CB₁₀H₁₀]⁻9 (172017), [1-Ph-closo-1-CB₁₁H₁₁]⁻(CH₂Cl₂solvate,

P2₁/c) 10a (164850), [PhCB₁₁H₁₁]⁻(non-solvate, P2₁/c) 10b

(194221) and [PhCB₁₁H₁₁]⁻(non-solvate, Pbca) 10c (194222).

For crystallographic data in CIF or other electronic format see

DOI: 10.1039/b609908d

DFT calculations

◦

˚

a = 11.6480(3), b = 10.5204(3), c = 17.5709(6) A and b = 107.9740(10)

Structures were optimized with the 6-31G* basis sets by the

standard Gaussian-98 package for ab initio methods.⁶⁶Final

optimizations, frequency analyses to conﬁrm true minima, and

GIAO nuclear-shielding predictions were performed by B3LYP

methodology, also as incorporated in the Gaussian-98 package,

and with the 6-31G* basis set, i.e. B3LYP/6-31G*. The gas-phase

nuclear-shielding predictions (i.e. DFT-GIAO//B3LYP/6-31G*)

were calibrated to the ¹¹B NMR chemical-shift scale d(¹¹B) via a

˚

for 5 and with a = 9.5758(2), b = 16.5323(4), c = 12.2821(3) A and b =

98.0110(10)^◦for 6; the orthorhombic salt of 7 has a = 14.438(3), b =

˚

15.762(3) and c = 16.877(4) A.

prediction on diborane, which is taken to have a gas-phase d(¹¹B)

value of −16.6 ppm with respect to conventionally used standard⁶⁷

[BF₃(OEt)₂].

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The Royal Society of Chemistry 2006

Dalton Trans., 2006, 5753–5769 | 5767

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View Article Online

Nuclear magnetic resonance spectroscopy

19 N. J. Bullen, A. Franken, C. A. Kilner, S. J. Teat, W. Clegg and J. D.

Kennedy, J. Organomet. Chem., 2005, 690, 2815.

20 A. Franken, C. A. Kilner, M. Thornton-Pett and J. D. Kennedy,

(J. Plesˇek, Special Edition), Collect. Czech. Chem. Commun., 2002, 67,

869.

NMR spectroscopy was performed at 294–299 K and at ca. 5.9 T

and 11.75 T (ﬁelds corresponding to 250 and 500 MHz ¹H frequen-

cies respectively) using commercially available instrumentation

and using techniques and procedures as adequately described

and enunciated elsewhere.^67–72Chemical shifts d are given in ppm

relative to N = 100 MHz for d(¹H) ( 0.05 ppm) (nominally TMS),

N = 32.083972 MHz for d(¹¹B) ( 0.5 ppm) (nominally Et₂O·BF₃

in CDCl₃)⁶⁷and N = 25.145004 MHz for d(¹³C) ( 0.5 ppm)

(nominally TMS). N is as deﬁned by McFarlane.⁷³

ˇ

21 (a) B. St´ıbr, Abstracts Eleventh International Meeting on Boron Chem-

istry (IMEBORON XI), Moscow, Russia, July 28–August 2, 2002,

ˇ

Abstract no. IB-2, p. 22; (b) T. Jel´ınek, J. D. Kennedy, B. St´ıbr and

A. Franken, loc. cit, Abstract no. CA-2, p. 27; (c) J. Bacˇkovsky´, B.

ˇ

St´ıbr, and J. Plesˇek, loc. cit, Abstract no. P-1, p. 79; D. Hnyk and B.

ˇ

St´ıbr, loc. cit, Abstract no. P-19, p. 97.

ˇ

22 B. St´ıbr and B. Wrackmeyer, J. Organomet. Chem., 2002, 657, 3.

ˇ

23 (a) B. St´ıbr, O. L. Tok, W. Milius, M. Bakardiev, J. Holub, D. Hnyk and

B. Wrackmeyer, Angew. Chem., Int. Ed., 2002, 41, 12; (b) B. Brellochs,

ˇ

J. Bacˇkovsky´, B. St´ıbr, T. Jel´ınek, J. Holub, M. Bakardjiev, D. Hnyk,

Acknowledgements

M. Hoffmann, I. Cisarˇova´ and B. Wrackmeyer, Eur. J. Inorg. Chem.,

2004, 3605.

ˇ

We thank the UK DTI for support, Dr Ian Moss for his good

ofﬁces, and Dr Mark Thornton-Pett for crystallographic help in

the early stages of this work. We also acknowledge instrumental

support from the UK EPSRC (grants nos. J/56929, GR/L/49505

and R/61949), and synchrotron beam-time awarded by the UK

CCLRC.

24 B. St´ıbr, Pure Appl. Chem., 2003, 75, 1295.

25 E. L. Muetterties and F. Klanberg, Inorg. Chem., 1966, 5, 315.

26 (a) K. Vyakaraman, L. Eriksson, M. G. Fete, P. J. Schreiber, S. Ko¨rbe,

ˇ

M. Kv´ıcˇalova´, D. B. Soweres, R. U. Rymer, H. Dvisˇova´, J. Castul´ık,

J. Kv´ıcˇala, B. K. McConnell and J. Michl, Abstracts Third European

Symposium on Boron Chemistry, EUROBORON 3, Prhonice, The Czech

Republic, 12–16 September 2004, Abstract no. O-1; (b) Z. Janousˇek, U.

Lehmann, J. Castul´ık, I. Cisarova´ and J. Michl, loc. cit., Abstract no.

O-3.

27 Z. Janousˇek, C. L. Hilton, P. J. Schreiber and J. Michl, (J. Plesˇek, Special

Edition), Collect. Czech. Chem. Commun., 2002, 67, 1025.

28 (a) I. Sivaev, S. Sjo¨berg and V. Bregadze, Abstracts Eleventh Inter-

national Meeting on Boron Chemistry (IMEBORON XI), Moscow,

Russia, July 28–August 2, 2002, Abstract no. CA-15, p. 69; (b) I. B.

Sivaev, S. Sjoberg and V. I. Bregadze, in Boron Chemistry at the

Beginning of the 21st Century, ed. Yu. N. Bubnov, Editorial URSS,

Moscow, 2003, p. 334; (c) I. B. Sivaev, Z. A. Starikova, P. V. Petrovskii,

V. I. Bregadze and S. Sjo¨berg, J. Organomet. Chem., 2005, 690,

2790.

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Polyhedral monocarbaborane chemistry. Some C-phenylated seven, eight, nine, ten, eleven and twelve-vertex species

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