Pentahapto-bonded gold heteroborane clusters [3-(R3P)-closo-2,1- AuTeB10H10]- and [3-(R3P)-closo-3,1, 2-AuAs2B9H9]-

Article abstract of DOI:10.1039/b515165a

Gold acetylide compounds [(R₃P)AuCCC(Me)(OH)Et], where R = Ph 1 or cyclohexyl (Cy) 2, were synthesised and 2 was characterised using X-ray diffraction techniques. The solid-state structure of 2 contained a two-coordinate gold atom and a linear

Full text of DOI:10.1039/b515165a

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www.rsc.org/dalton | Dalton Transactions
Pentahapto-bonded gold heteroborane clusters
[3-(R3P)-closo-2,1-AuTeB10H10]− and [3-(R3P)-closo-3,1,2-AuAs2B9H9]−†
George Ferguson,*a John F. Gallagher,b John D. Kennedy,c Anne-Marie Kelleherd and Trevor R. Spalding*d
Received 25th October 2005, Accepted 31st January 2006
First published as an Advance Article on the web 15th February 2006
DOI: 10.1039/b515165a
Gold acetylide compounds [(R3P)AuC≡CC(Me)(OH)Et], where R = Ph 1 or cyclohexyl (Cy) 2, were
synthesised and 2 was characterised using X-ray diffraction techniques. The solid-state structure of 2
contained a two-coordinate gold atom and a linear P–Au–C≡C–C bonding sequence. The reactions
between 1 or 2 and [NR4][nido-7,8-As2B9H10] or [NR4][nido-7-TeB10H10] in ethanol–acetone solvent
afforded the twelve-vertex cluster species [NMe4][3-(R3P)-closo-3,1,2-AuAs2B9H9], where R = Ph 5 or
Cy 6, or [NEt4][3-(R3P)-closo-2,1-AuTeB10H10], where R = Ph 7 or Cy 8, in moderate or low yields (ca.
35% for 5, 6 and 8 and ca. 20% for 7). Compounds 5 and 7 were characterised with X-ray
crystallographic techniques. Although there was crystallographic disorder in the {As2B3} and {TeB4}
rings to which the gold atoms were attached, the structures of 5 and 7 strongly suggested that the gold
atoms were pentahapto bonded to all the atoms in the {As2B3} or {TeB4} rings giving formally closo
cluster geometries with closo cluster electron counts. The solution-phase NMR properties of 5, 6 and 7
were consistent with closo descriptions.
the open-face twelfth hydrogen atom in the [nido-7,8-C2B9H12]−
Introduction
dicarbaborane anion and the {(Ph3P)Au} unit in the [10-endo-
The chemistry of metallaboranes2,3 and metallaheteroboranes4,5 is
extensive and well documented and compounds of virtually all
the transition elements have been studied including the Group 11
elements copper andgold. To date almost allthe gold heteroborane
compounds which have been reported were carbaborane deriva-
tives and the gold reagent used was almost exclusively [(R3P)AuCl].
We wished to investigate the potential use of gold acetylides
{(Ph3P)Au}-nido-7,8-C2B9H11]− anion 3 [diagram I, where X =
H or {(Ph3P)Au}]9 and a second is the l2 bridging position of a
hydrogen atom in [10,11-l-H-9-SMe2-nido-7,8-C2B9H10]10 and the
{(Ph3P)Au} unit in [10,11-l-(Ph3PAu)-9-SMe2-nido-7,8-C2B9H10]
4 [diagram II, where X = H or {(Ph3P)Au}].11 However, a closer
scrutiny of the bonding in [10-endo-(Ph3PAu)-nido-7,8-C2B9H11]−
1
3 shows that the {(Ph3P)Au} unit is not simply g -bound since
of the general form [(R3P)AuC≡CR] as reagents for {(R3P)Au}
there are also significant interactions with the two flanking boron
3
complexes with heteroborane ligands. The compounds synthesised
atoms, so that it is perhaps better described as g -bonded with one
for this purpose were [(R3P)AuC≡CC(Me)(OH)Et] with R = Ph
˚
short Au–B interaction of 2.222(9) A and two longer interactions
9
1 or cyclohexyl 2.
˚
of 2.486(9) and 2.528(9) A (diagram III).
Additionally, we wished to investigate the bonding potential of
{(R3P)Au} units to heteroborane ligands other than carbaboranes.
Numerous authors have commented on the isolobal relationship
+
between the proton and the phosphine gold(I) cation {(Ph3P)Au}
+
and many have suggested that the {(Ph3P)Au} unit could replace
a proton in cluster compounds.6,7 A detailed analysis based on
extended-Hu¨ckel MO calculations of the energies and forms of
+
the frontier orbitals of the {(H3P)Au} -unit has suggested that
the energies of the gold hybrid 6s/6pz valence orbital and that
the 6px/6py orbital pair were well separated and that the empty
From these considerations, it is evident that the gold-to-ligand
+
hybrid 6s/6pz orbital in a {(H3P)Au} unit might be expected to
hapticity of gold carbaborane complexes such as 3 and 4 can
behave in a similar manner to the s orbital of [H+].8 By extension,
3
be regarded as between one and three. An g -bonded gold atom
+
this would also apply to the {(R3P)Au} unit. This rationale has
also exists in gold cyclopentadienyl complexes, for example in
been used to explain certain structural similarities in carbaboranes
[{(Ph3P)Au}C5Ph4H], where the gold atom is bonded to the
˚
and their gold derivatives: one example is the endo-positionings of
cyclopentadienyl ring by one shorter connectivity of 2.15(1) A and
12
˚
two longer ones of 2.67(1) and 2.76(1) A. These observations
aSchool of Chemistry, The University, St. Andrews, Fife, UK KY16 9ST.
E-mail: gf3@st-andrews.ac.uk
contrast with the bonding in related copper compounds, for
example as in [{(Ph3P)Cu}C5H5], where the {(Ph3P)Cu} unit
bDepartment of Chemistry, Dublin City University, Dublin, Ireland
cSchool of Chemistry, University of Leeds, Leeds, UK LS2 9JT
dDepartment of Chemistry, University College Cork, National University of
Ireland, Cork, Ireland. E-mail: t.spalding@ucc.ie
5
is symmetrically g -bound,13 and as in cupracarbaborane com-
pounds that have been described as belonging to the twelve-
5
vertex closo structural group with the copper atoms g -bonded, for
† Metallaheteroborane chemistry. Part 18.1
example in the [3-(Ph3P)-closo-3,1,2-CuC2B9H11]− anion14 and in
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the neutral compound [3-(Ph3P)-4-SMe2-closo-3,1,2-CuC2B9H10].7
Pentahapto bonding is also seen in copper heteroboranes with
arsenic or tellurium heteroatoms, for example as in [3-(Ph3P)-
8-{isoPrOEt}-closo-3,1,2-CuAs2B9H8].15 In the present work we
thought it of interest to explore the possibility of synthesising
a closo gold heteroborane compound with a hapticity greater
than three. Accordingly, we chose ligands with heteroatoms
other than carbon which, on grounds of atomic orbital size and
energy compatibility, may be more conducive to the formation
of more than three interactions with a gold-based fragment
and thus afford compounds with higher hapticity in the gold-
to-heteroborane bonding. The “softer” ligands that we chose
were derived from the [nido-7,8-As2B9H10]− and [nido-7-TeB10H10]−
anions.
Results and discussion
Synthesis and characterisation of phosphine gold acetylides 1 and 2
Both [(R3P)AuC≡CC(Me)(OH)Et] compounds, where R = Ph 1
or cyclohexyl (Cy) 2, were synthesised by the reaction between
a suspension of [AuC≡CC(Me)(OH)Et] in toluene and a toluene
solution of R3P. 16 Yields of 1 and 2 from the gold acetylide were
79% and 75% respectively and the values of mmax(C≡C) for 1 and 2
were 2134 and 2120 cm−1 respectively. The solid-state structure of
2 was confirmed by single-crystal X-ray analysis.
Fig. 1 ORTEP plot of [(Cy3P)AuC≡CC(Me)(OH)Et] 2, showing the
numbering scheme. Only one of the two orientations of the disordered
groups are shown. Ellipsoids are at the 30% probability level. Selected
◦
˚
interatomic distances (A) and angles ( ): Au(1)–P(1) 2.274(3), Au(1)–C(1)
2.005(8), C(1)–C(2) 1.199(5), C(2)–C(3) 1.467(12); P(1)–Au(1)–C(1)
176.7(3), Au(1)–P(1)–C(11A) 109.6(11), Au(1)–P(1)–C(21) 113.2(3),
Au(1)–P(1)–C(31) 110.3(3), Au(1)–C(1)–C(2) 173(5), C(1)–C(2)–C(3)
177(7).
Molecular structure of [(Cy3P)AuC≡CC(Me)(OH)Et] 2
The X-ray analysis showed that there is one molecule in the
asymmetric unit. There is no bonding interaction between ad-
jacent molecules and hence [(Cy3P)AuC≡CC(Me)(OH)Et] 2 is
an example of a simple monomeric gold acetylide with the
acetylide ligand end-on r-bonded to the gold atom, Fig. 1. It
became clear during the structural analysis that several atoms were
disordered: one of the cyclohexyl rings was disordered unequally
over two orientations [A and B with occupancies 0.356(13) and
0.644(13) respectively] and the five carbon atoms of the 3-hydroxy-
3-methyl-1-pentyne moiety were also disordered over two sites
with occupancies 0.36(2) and 0.64(2), essentially identical to those
of the disordered cyclohexane ring. Important bond distances and
angles for the predominant molecule are given in the legend to
Fig. 1.
Synthesis and characterisation of the gold heteroborane
compounds [Me4N] [3-(PR3)-closo-3,1,2-AuAs2B9H9], where R =
Ph 5 or Cy 6, and [Et4N][3-(R3P)-closo-2,1-AuTeB10H10], where
R = Ph 7 or Cy 8
These compounds were synthesised from the reaction between the
corresponding phosphine gold acetylide and heteroborane anion
in an ethanol–acetone 1 : 1 mixture heated at reflux temperature.
Yields of the pale yellow products were low, at ca. 20% for 7,
or moderate, at ca. 35% for 5, 6 and 8. Initial characterisation
by elemental analysis and FT-IR spectroscopy supported the
compound formulations. Further characterisation was obtained
from solution-phase NMR spectroscopy and, for compounds 5
and 7, from single-crystal X-ray crystallography.
The P–Au–C≡C–C chain is essentially linear with P–Au–C
176.7(3)◦, Au–C–C 173(5)◦ and C≡C–C 177(7)◦, this is similar to
[(Ph3P)AuC≡CPh] where P–Au–C 173.5(5)◦, Au–C≡C 175.7(16)◦
and C≡C–C 176.5(18)◦ were reported.17 The Au–P distance
Molecular structures of [Me4N][3-(Ph3P)-closo-3,1,2-AuAs2B9H9]
5 and [Et4N][2-(Ph3P)-closo-2,1-AuTeB10H10] 7
˚
is typical at 2.274(3) A. It is identical to that reported for
[(Ph3P)AuC≡CC6F5],18 and similar to the values of 2.276(5) and
The X-ray diffraction study of [Me4N][3-(Ph3P)-3,1,2-closo-
AuAs2B9H9] 5 shows the gold-to-heteroborane ligand bonding
mode as pentahapto, Fig. 2; see also diagram IV. Thus, the gold
atom in 5 does not behave as an endo-bonded moiety as in
the isoelectronic carbaborane cluster [10-endo-{(Ph3P)Au}-nido-
7,8-C2B9H11]− 3, see diagram I. The geometrical and bonding
considerations leading to these conclusions are summarised in
the following paragraphs, with relevant and other important
structural data listed in the legend of Fig. 2.
˚
2.282(4) A reported for the two independent molecules in the unit
cell of [(Ph3P)AuC≡CPh].17 The Au–C distance of 2.005(8) A
˚
˚
is close to the two Au–C distances of 1.994(6) and 2.006(6) A
reported for the anion [Au(C≡CCH2OH)2]−.19 The C(1)–C(2)
˚
acetylide bond distance is 1.199(5) A, which is similar to the values
˚
of 1.197(16) A reported for neutral [(Ph3P)AuC≡CC6F5] and
−
˚
˚
1.206(6) A and 1.189(6) A observed in the [Au(C≡CCH2OH)2]
anion.19
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˚
˚
A and Au–B(8) 2.378(5) A, may be compared with the Au–B(10)
˚
2.222(9), Au–B(9) 2.528(9) and Au–B(11) 2.486(9) A distances in
the g -gold compound [10-endo-{(Ph3P)Au}-nido-7,8-C2B9H11]− 3
3
(diagram III)9 and the distances Au–B(10) 2.233(6) and Au–B(11)
˚
2.357(6) A in the l2-bridged [10,11-l-{(Ph3P)Au}-9-SMe2-nido-
7,8-C2B9H10] species 4 (diagram II).11 Thus the Au–B(8) distance
in 5 lies between the shortest and longest distances in 3 and 4 as
might be expected if the gold atom in 5 significantly interacts in a
bonding manner with all the boron atoms on the {As2B3} face of
the arsenaborane ligand and gold atom hapticity thereby increases
above three.
Overall, the cage of 5 may be best described as a twelve-
vertex closo-type structure of {AuAs2B9} atoms (diagram IV) and
1
3
not as either an g -bonded gold, diagram V, or g -bonded gold
cluster, diagram VI. Using Wade’s rules,24 the structure of 5 can be
Fig.
2
ORTEP plot of the anion in [Me4N][3-(Ph3P)-closo-3,1,2-
−
AuAs2B9H9] 5 showing the numbering scheme. Ellipsoids are shown at
interpreted in terms of a formally anionic {(Ph3P)Au} unit acting
˚
the 30% probability level. Selected interatomic distances (A) and angles
as two-electron three-orbital cluster vertex affording an electron
count of 26 cluster electrons which is expected for a twelve-vertex
closo cage. An alternative description of 5 with a formally cationic
(◦): Au(3)–P(1) 2.2459(16), Au(3)–B/As(8) 2.378(5), Au(3)–B/As(7)
2.506(3), Au(3)–B/As(4) 2.537(2), Au(3)–As/B(2) 2.6975(12), Au(3)–
As/B(1) 2.7094(12), As/B(1)–As/B(2) 2.4611(15), As/B(1)–B/As(4)
2.383(2), As/B(2)–B/As(7) 2.296(4), B/As(4)–B/As(8) 2.173(5),
B/As(7)–B/As(8) 2.108(7), P(1)–Au(3)–B/As(8) 139.14(15), P(1)–Au(3)–
B/As(7) 132.79(11), P(1)–Au(3)–B/As(4) 131.07(7), P(1)–Au(3)–As/B(2)
125.37(5), P(1)–Au(3)–As/B(1) 126.02(5).
+
{(Ph3P)Au} unit as a zero-electrons one-orbital unit “bridging”
2−
the open face of an anionic {nido-As2B9H9} cluster is not as
satisfactory, bearing in mind the interatomic dimensions discussed
above in comparison to the structures of compounds 3 and 4
(diagrams I and II). Thus, it appears that in 5 the gold atom uses
not only one of its 6s/6pz hybrid orbital, but also both its 6px and
6py orbitals, in cluster bonding to the “soft” heteroborane {As2B9}
ligand. Compound 5 is, on this basis, the first gold heteroborane
reported to contain a pentahapto gold atom.
The structural analysis showed that the two arsenic atoms of
the {AuAs2B9H9} cage were disordered unequally over all five
sites of the {As2B3} face bonded to the gold atom. There were
two major arsenic sites and three minor ones. The discussion
below is with reference to the major atom types. Atom disorder
in metallaheteroborane structures is a common phenomenon. A
similar but less significant disorder to that in 5 was observed in [3,3-
(PMe2Ph)2-closo-3,1,2-PtAs2B9H9].20 The Au–As(1) and Au–As(2)
˚
The As(1)–As(2) distance in 5 is 2.4611(15) A. This is similar to
the As–As distances reported in the structure of [As2B10H8I2]25
of 2.435(2) and 2.421(2) A and in [3,3-(PMe2Ph)2-closo-3,1,2-
PtAs2B9H9] of 2.497(3) A. Other interatomic distances in the
cage and in the {(Ph3P)Au} unit are not unusual. The P–Au–
B angles in 5 are P(1)–Au(3)–B(4) 131.07(7)◦, P(1)–Au(3)–B(8)
139.14(15)◦ and P(1)–Au(3)–B(7) 132.79(11)◦. These are similar to
˚
20
˚
˚
distances in 5 are slightly different at 2.7094(12) A and 2.6975(12)
˚
A respectively, but in view of the disorder this difference may not
be significant. Previously reported two centre-two electron gold–
5
˚
arsenic distances are 2.342(5) A in [(Ph3As)AuBr], 2.334(3) and
the P–Cu–B angles in the g -bonded compound [3-(Ph3P)-8-{iso-
2.331(1) A in two crystalline forms of [(Ph3As)AuCl] and 2.386(1)
PrOEt}-closo-3,1,2-CuAs2B9H8],15 which range from 134.0(2)◦ to
˚
A in [2-NO2C6H4AuAsPh3].
The gold–arsenic distance for a
131.9(2)◦, but they◦are significantly smaller than the B–Au–P
21–23
˚
◦
3
two-centre two-electron bond can be estimated using Pauling’s
angles of 166.80(15) and 144.99(15) reported for the g -bonded
anionic compound [10-endo-{(R3P)Au}-nido-7,8-C2B9H11]− 3.9 A
decrease in the P–Au–B angle in 5 is expected if the {(R3P)Au} unit
˚
radii as 2.40 A. However, comparison of second- and third-row
transition-element-to-arsenic distances in non-cluster compounds
5
versus cluster compounds shows that the distances in clusters are
has become g -bonded.
˚
longer by 0.2 to 0.3 A. Hence, gold–arsenic distances in 5 may be
The analysis of the single-crystal X-ray diffraction data
from the gold telluraborane compound [Et4N][2-Ph3P-closo-2,1-
AuTeB10H10] 7, revealed that it had acloso twelve-vertex{AuTeB10}
geometry (Fig. 3 and diagram VII). There are two independent
anion/cation pairs in the asymmetric unit and it was apparent
from electron-density maps that in each anion the tellurium atom
was disordered over different sites in the {TeB4} ring adjacent
to gold. In one anion the refinement showed that the tellurium
atom was disordered over three sites [Te(1), Te(3), Te(7), with
˚
expected to be in the region of 2.6 to 2.7 A. The derived average
˚
Au–As distance of 2.703 A in 5 is at the high end of this range even
though the occupancy of the As(1) or As(2) sites are 82.5% and
70.6% respectively which might be expected to lead to a shortening
of the Au–As bonds due to the involvement of gold-to-boron
contributions.
The gold-to-B/As distances in the predominantly boron-
occupied sites 4,7 and 8, i.e. Au–B(4) 2.537(2), Au–B(7) 2.506(3)
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those of the related and previously reported closo-type species
[3,3-(R3P)2-closo-3,1,2-MAs2B9H9], where R3 = Ph3 or PhMe2
and M = Pd or Pt.20 The relative intensity patterns in the 11B
spectra were 2:2:1:1:1:2. Whereas most of the 11B resonances
are in the region d(11B) 0–20 ppm, characteristic of closo-type
twelve-vertex species, the B(8) and B(10) positions in 5, which
are those respectively in adjacent and antipodal positions relative
to the metal site, are significantly less deshielded than in the
corresponding positions in the equivalent palladium and platinum
species [3,3-(R3P)2-closo-3,1,2-MAs2B9H9].20,27 Other differences
in 11B data between the analogous gold and platinum compounds
are minor in terms of overall 11B shielding ranges. The adjacent
and antipodal positions are known to be most affected by a change
in character of the metal centre at the 3 position in closo-3,1,2
1
metalladiheteroboranes.28 The overall 11B-{ H} spectrum of 5 is
quite similar to that of the [nido-7,8-As2B9H10]− anion which might
be taken to indicate that the structure of 5 in solution is nido,
3
i.e. with the gold phosphine unit in an g -bonding mode as in
diagram III above. However, the use of the polar solvent, CD3CN
which was necessary for adequate solubility, may have caused the
5
dislocation of the solid-state g -bonding mode of 5 to give a less-
coordinated bonding mode in solution. Therefore, based on the
current NMR data it is not possible to say with any certainty that,
in solution, the gold atom is part of a closo twelve-vertex cluster or
that the connectivity between the {As2B9} cluster fragment and the
gold centre can be better described as either monohapto to B(8)
(diagram V) or trihapto to B(8), B(4) and B(7) (diagram VI). The
NMR spectra of the PCy3 gold arsenaborane, 6, are very similar
to those of the PPh3 species 5, indicating that the overall solution
behaviour and geometry of these complexes are essentially the
same.
Fig. 3 ORTEP plot of the one of the anions in [Et4N][2-(Ph3P)-
closo-2,1-AuTeB10H10] 7 showing the numbering scheme. The crystal-
lographic analysis shows that the tellurium atom is disordered over
three sites Te(1) Te(3)/B(3) and Te(7)/(B7). Ellipsoids are shown at t◦he
˚
30% probability level. Selected interatomic distances (A) and angles ( ):
˚
Au(2)–Te(1) 2.839(3), Au(2)–P(1) 2.237(6) A. The corresponding values
˚
in the other anion are 2.853(3) and 2.263(6) A. The Au–(minor-Te)/B
˚
distance in both anions is 2.294(12)–2.825(13) A. For the major Te
˚
components the Te–B range is 2.28(5)–2.36(3) A.
occupancies 0.911, 0.028, 0.061 respectively], while in the other
only two sites were affected [Te(21) and Te(27), with occupancies
0.956(3) and 0.044(2) respectively]. The two anions showed only
small differences in the orientation of the gold phosphine unit over
the {TeB4} face to which it is bonded. A view of the anion with
the tellurium atom disordered over three sites is shown in Fig. 3.
The tellurium disorder affects the various tellurium–boron and
Te/B–boron distances; ranges are given in the legend to Fig. 3.
There have been no reported structural characterisations of gold
telluraboranes but the gold–tellurium distances of 2.839(3) and
In Part 9 of this series of papers the discussion of the
NMR properties of the closely related palladium and platinum
analogues [3,3-(PhMe2P)2-closo-3,1,2-MAs2B9H9], where M = Pd
or Pt, noted that the compounds showed fluxional behaviour in
solution.20 The isoelectronic platinacarbaborane [3,3-(PhMe2P)2-
closo-3,1,2-PtC2B9H11] was likewise fluxional: based on evidence
from an X-ray analysis, in which two different rotamer “snapshot”
structures were observed, it was concluded that the fluxionality
involved a process of rotation of the {(PhMe2P)2Pt} unit above
the {C2B3} face of the nido-shaped eleven-vertex {C2B9H11} unit,
involving in turn a sequence of changing coordination modes
between the Pt atom and the {C2B3} face from pentahapto to
tetrahapto, and then back to pentahapto, and so on.27 While
it is possible that the {(R3P)Au} units in 5 and 6 are similarly
fluxional above the As2B9-cage in solution in the same sense
as the platinum-based units were above both {As2B9H9} and
{C2B9H11} ligands, this could not be probed in the present study
because the single R3P phosphine ligands used did not provide
a distinguishing spectroscopic “tag”, whereas, in the platinum
species, the incidence of two non-radially bound PMe2Ph ligands
engendered the necessary prochiral features in the 1H spectrum.21,28
The proton and boron NMR spectra of [Et4N][2-(Ph3P)-closo-
2,1-AuTeB10H10] 7 are not inconsistent with the closo molecular
structure as determined by single-crystal X-ray analysis (Fig. 3
and diagram VII). The compound shows a 1:2:2:2:2:1 relative
˚
2.853(3) A in the major anion components of 7 are very long when
compared with the reported gold–tellurium distance of 2.566(1)
26
˚
A in [(Ph3P)AuTeC(SiMe3)3]. The gold–phosphorus distances
˚
of 2.237(6) and 2.263(6) A for the two major components of 7
are similar to reported values for such distances in other cluster
compounds in which gold is bound to the open face of a nido-
˚
shaped eleven-vertex residue, for example 2.249(2) A in the [10-
−
˚
endo-{(Ph3P)Au}-nido-7,8-C2B9H11] anion 3 and 2.2461(19) A in
the [3-(PPh3)-closo-3,1,2-AuAs2B9H9]− anion of 5.
NMR spectroscopy
1
The 11B, 11B-{ H} and 31P NMR spectra of compounds [Me4N][3-
(PPh3)-closo-3,1,2-AuAs2B9H9] 5, [Me4N][3-(Cy3P)-closo-3,1,2-
AuAs2B9H9] 6 and [Et4N][2-(Ph3P)-closo-2,1-AuTeB10H10] 7 were
1
1
recorded. The 11B and H resonances in the spectra of 5 and 6
intensity pattern in the 11B NMR spectrum. The H resonances
1
11
were tentatively assigned on the basis of relative intensities and
comparisons with linewidth and general shielding behaviour with
were assigned to their directly bound boron atoms by H-{ B
selective} decoupling experiments and the assignment of the
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boron resonances to the corresponding boron positions was
made by comparison with previously assigned spectra of closo
cupratelluraboranes.15 The resonance (broad) at d(11B) −11.2 ppm
is assigned to the boron atoms directly adjacent to the tellurium
atom, i.e. B(3) and B(6). Similar comments can be made about the
spectrum of 7 to those made above for the related arsenaborane
compounds 5 and 6. Therefore it is not possible to conclude
definitively that the structure of the gold telluraborane persists
as closo with essentially pentahapto coordination in solution.
Synthesis of [Me4N][3-(PPh3)-closo-3,1,2-AuAs2B9H9] 5. To a
suspension of [Me4N][nido-7,8-As2B9H10] 9 (0.0957 g, 0.289 mmol)
in
a 1 : 1 EtOH–acetone mixture (70 ml) was added
[(Ph3P)AuC≡CC(Me)(OH)Et] 1, (0.161 g, 0.289 mmol). The
mixture was heated at reflux for 2.5 h during which time a pale
yellow colour developed. The reaction solution was filtered and the
solvent removed from the filtrate under reduced pressure (50 ◦C).
Crystallisation from CH2Cl2–hexane afforded pale yellow micro-
crystals of [Me4N][3-(Ph3P)-closo-3,1,2-AuAs2B9H9] 5 (0.085 g,
37.3%). C22H36As2AuB9NP requires C, 33.5; H, 4.6; N, 1.8; found:
C, 33.4; H, 4.6; N, 1.6%. IR: mmax (KBr) 2563(w), 2531(sh), 2505(s),
2488(vs), 2474(s), 2440(sh) (BH) cm−1. Cluster NMR data, ordered
as d(11B)/assignment/[d(1H) of directly attached H(exo) atom]:
−3.6, B(9,12) [+2.80]; −3.6, B(4,7) [+1.65]; −7.2, B(6) [+2.20];
−12.1, B(8) [+3.45]; −16.1, B(10) [+4.27]; −18.7, B(5,11) [+2.32].
31P NMR (CD3CN, 243 K): d +39.1.
Conclusions
The successful solution and refinement of the molecular struc-
tures of [Me4N][3-(PPh3)-closo-3,1,2-AuAs2B9H9] 5 and [Et4N][2-
(Ph3P)-closo-2,1-AuTeB10H10] 7 showed that both compounds
have closo twelve-vertex geometries based on the icosahedron,
distorted from the regular as exhibited by the [B12H12]2− anion by
the accommodation of the disparately sized heteroatoms. Both
compounds appear to exhibit pentahapto gold-to-heteroborane
coordination and therefore they are the first gold pentahapto-
heteroborane species to be reported. Previously reported gold
carbaborane compounds had gold-to-heteroborane dihapto or tri-
hapto bonding interactions. Therefore, by the selection of “softer”
ligands, namely the {As2B9H9} and {TeB10H10} fragments, the
possibility of synthesising gold heteroborane compounds with
gold hapticity greater than three has been realised. It could perhaps
be argued that the disorder in the crystals of 5 and 7 makes the
evidence for the confirmation pentahapto coordination of the
gold atom somewhat ambiguous. In this context, however, it is
noteworthy that, in the crystals of the unambiguously pentahapto-
configured closo platinacarbaborane, [3,3-(PhMe2P)2-closo-3,1,2-
PtC2B9H11], there were two distinctly different conformers in the
asymmetric unit, and also that, in the diarsenic analogue [3,3-
(Ph3P)2-closo-3,1,2-PtAs2B9H9], which is also described unam-
biguously as pentahapto, one of the arsenic atoms was disordered
over two sites.
Synthesis of [Me4N][3-(Cy3P)-closo-3,1,2-AuAs2B9H9] 6. To a
suspension of [Me4N][nido-7,8-As2B9H10] 9, (0.11 g, 0.332 mmol)
in
a 1 : 1 EtOH–acetone mixture (70 ml) was added
[(Cy3P)AuC≡CC(Me)(OH)Et] 2, (0.191 g, 0.332 mmol). The
mixture was heated at reflux for 2.5 h during which time a pale
yellow colour developed. The reaction solution was filtered and the
solvent removed from the filtrate under reduced pressure (50 ◦C).
Crystallisation from CH2Cl2–hexane afforded pale yellow micro-
crystals of [Me4N][3-(Cy3P)-closo-3,1,2-AuAs2B9H9] 6 (0.094 g,
35.2%). C22H54As2AuB9NP requires C, 32.7; H, 6.7; N, 1.7; found:
C, 33.4; H, 4.6; N, 1.6%. IR: mmax (KBr) 2561(w), 2547(sh),
2521(vs), 2513(vs), 2487(s), 2434(sh) (BH) cm−1. Cluster NMR
data, ordered as d(11B)/assignment/[d(1H) of directly attached
H(exo) atom]: −4.1(9,12) [+1.51]; −4.1, B(4,7) [+2.64]; −8.0, B(6)
[+1.88]; −11.2, B(8) [+3.38]; −17.9, B(10) [+4.04]; −18.7, B(5,11)
[+2.19]. 31P NMR (CD3CN, 243 K): d +58.6.
Synthesis of [Et4N][2-(Ph3P)-closo-2,1-AuTeB10H10] 7. To a
suspension of [Et4N][nido-7,8-TeB10H11] 10 (0.094 g, 0.25 mmol)
in
a 1 : 1 EtOH–acetone mixture (70 ml) was added
[(Ph3P)AuC≡CC(Me)(OH)Et] 1 (0.139 g, 0.25 mmol). The mix-
ture was heated at reflux for 4 h during which time a pale
yellow colour developed. The reaction solution was cooled to
room temperature, filtered and the solvent removed from the
filtrate (rotary evaporator, 50 ◦C). The residue was crystallised
from CH2Cl2 to yield yellow-green crystals of [Et4N][2-(Ph3P)-
closo-2,1-AuTeB10H10] 7 (0.042 g, 20%). Found: C, 37.7; H,
5.5; N, 2.15; C26H44TeAuB10PN requires C, 37.4; H, 5.4; N,
1.7%. IR: mmax (KBr) 2549(s), 2541(s), 2526(s), 2501(vs), 2488(vs),
2478(vs), 2413(s) (BH) cm−1. Cluster NMR data, ordered as
d(11B)/assignment/[d(1H) of directly attached H(exo) atom]: +2.2,
B(12) [+4.10]; −7.7, B(4,5) [+2.24]; −11.2, B(3,6) [+1.25]; −17.9,
B(7,11) [+2.82]; −18.2, B(8,10) [+1.69]; −18.7, B(9) [+3.76]. 31P
NMR (CD3CN, 243 K): d +38.7.
Experimental
General
The compounds [Me4N][nido-7,8-As2B9H10] 9 and [Et4N][nido-7,8-
TeB10H11] 10 were prepared according to literature methods.29,30
The acetylene, HC≡CC(Me)(OH)Et, and phosphines, Ph3P and
Cy3P, were used as purchased from the Aldrich Chemical Co.
Ltd. The gold reagent H[AuCl4]·2H2O was generously supplied
by Johnson Matthey Plc. The phosphine gold acetylides were
prepared as described previously.16 NMR spectroscopy was per-
formed at ca. 5.9 T and 9.4 T (fields corresponding to nominal 250
1
and 400 MHz H frequencies respectively) using commercially
available instrumentation and using techniques and procedures
as adequately described and enunciated elsewhere.31–33 Spectra
were recorded for solutions in CD3CN, and at 294–297 K unless
otherwise indicated. Chemical 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 Et2OBF3
in CDCl3)31 and N = 40.480730 MHz for dm(31P) ( 0.5 ppm)
(nominally 85% aqueous H3PO4). N is as defined by McFarlane.34
Synthesis of [Et4N][2-(Cy3P)-closo-2,1-AuTeB10H10] 8. To a
suspension of [Et4N][nido-7,8-TeB10H11] 10 (0.106 g, 0.28 mmol)
in
a 1 : 1 EtOH–acetone mixture (70 ml) was added
[(Cy3P)AuC≡CC(Me)(OH)Et] 2 (0.1535 g, 0.27 mmol). The
mixture was heated at reflux for 4 h during which time a pale
yellow colour developed. The reaction solution was cooled to room
temperature, filtered and the solvent removed from the filtrate
(rotary evaporator, 50 ◦C). The residue was crystallised from
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Dalton Trans., 2006, 2133–2139 | 2137
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CH2Cl2–hexane to yield yellow-green crystals of [Et4N][2-(Cy3P)-
closo-2,1-AuTeB10H10] 8, (0.037 g, 37.3%). C26H62TeAuB10PN
requires C, 36.55; H, 7.55; N, 1.6; found: C, 36.4; H, 7.6; N, 1.6%.
IR: mmax (KBr) 2559(s), 2519(s), 2503(vs), 2497(vs) (BH) cm−1.
[Et4N]+ cations in the asymmetric unit. It soon became apparent
from electron-density peak heights and numerous subsequent
refinement trials that in each case there was some B/Te interchange
at the {TeB4} face bonded to the gold atom. In one anion the
tellurium atom was disordered unequally over three sites; in the
other it was disordered unequally over two sites. For the final
refinement with the SHELXL97 programme,35 each disordered site
was allowed for as a Te/B mixture with the occupancies restrained
so that the sum of the tellurium occupancies added to 1. At each
disordered site the boron occupancy was set at {1 − (occupancy
of Te)}. Because of the disorder, all the B atoms were only refined
isotropically with an overall Uiso value. The disorder may very well
affect the crystal packing and several phenyl C atoms have prolate
ellipsoids. H-atom coordinates were generated geometrically and
X-Ray crystallographic studies
X-Ray analysis of [(Cy3P)AuC≡CC(Me)(OH)Et] 2.
¯
Crystal Data. C24H42AuPO, M = 574.51, triclinic, P1, a =
˚
9.8012(11), b = 11.2980(18), c = 12.587(2) A, a = 110.808(16),
b = 106.498(15), c = 92.064(15)◦, Z = 2, Dx 1.546 g cm−3,
3
−1
˚
V = 1234.6(4) A , F(000) = 576, l = 6.033 mm , k (Mo-Ka) =
˚
0.71073 A. Total number of reflections = 4371, RF = 0.0524 based
on 2805 data with [F2 > 2r(F2)], Rw = R[wR(F2)] = 0.1293 for
4371 data.
˚
treated as riding atoms (C–H 0.93 to 0.97, B–H 1.10 A).
Crystals suitable for investigation were grown by slow evapora-
tion of an acetone solution of [(Cy3P)AuC≡CC(Me)(OH)Et] 2. It
became clear during the structural analysis that several atoms were
disordered with one of the cyclohexyl rings disordered over two
unequal sites and the five C atoms of the 3-hydroxy-3-methyl-1-
pentyne moiety disordered over two sites. However, it was possible
to model the disorder successfully using DFIX restraints with
SHELXL97.35
CCDC reference numbers 270401, 270402 and 270403 for
compounds 2, 5 and 7 respectively.
For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b515165a
Acknowledgements
J. D. K thanks the UK EPSRC and G. F. thanks the NSERC
Canada for financial support for instrumentation, A.-M. K.
thanks Johnson Matthey Plc. for support.
X-Ray analysis of [Me4N][3-(Ph3P)-closo-3,1,2-AuAs2B9H9] 5.
¯
Crystal Data. C23H38As2AuB9Cl2NP, M = 874.58, triclinic, P1,
˚
a = 9.564(3), b = 9.882(3), c = 18.937(3) A, a = 102.84(2), b =
93.69(2), c = 101.08(2)◦, Z = 2, Dx 1.707 g cm−3, V = 1701.8(7)
References
3
−1
˚
˚
A , F(000) = 844.1, l = 6.474 mm , k (Mo-Ka) = 0.71073 A.
Total number of reflections = 7389, RF = 0.0464 based on 4502
data with [F2 > 2r(F2)], Rw = R[wR(F2)] = 0.1037 for 7389 data.
Crystals suitable for study were grown by diffusion of hexane
into CH2Cl2 solutions of 5. The structure analysis showed that
the two arsenic atoms of the {AuAs2B9H9} cage were disordered
unequally over all five sites of the {As2B3} face bonded to the
gold atom. There were two major arsenic sites and three minor
ones. For the final refinement with the SHELXL97 programme,35
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occupancies restrained so that the sum of the arsenic occupancies
added to 2 and the boron atoms to 3. At each site the boron
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Fig. 2) are: (1) 0.825(3), (2) 0.706(3), (4) 0.263(3), (7) 0.135(3), (8)
0.072(3). In Fig. 2 the atom labels of these disordered atoms are
given as As(1), As(2), B(4), B(7) and B(8) but in the figure legend
the atoms are given as As/B labels with the predominant atom
type first.
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2138 | Dalton Trans., 2006, 2133–2139
This journal is
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This journal is
The Royal Society of Chemistry 2006
Dalton Trans., 2006, 2133–2139 | 2139
©