Chemistry of 11-vertex rhodathiaboranes: Reactions with monodentate phosphines

Article abstract of DOI:10.1039/c1dt10355e

The reaction of [8,8-(PPh₃)₂-nido-8,7-RhSB ₉H₁₀] (1) with PR₃ in a 1:2 ratio affords mixtures that contain the mono-substituted bis-PR₃-ligated rhodathiaboranes [8,8-(PPh₃)(L)-nido-8,7-RhSB₉H ₁₀] [L = PMe₂Ph (5), PMe₃ (6)] and the corresponding tris-PR₃-ligated compounds [8,8,8-(L) ₃-nido-8,7-RhSB₉H₁₀] [L = PMe₂Ph (7), PMe₃ (8)]. These latter species are more conveniently prepared from the reaction of 1 with three equivalents of the monodentate phosphines, PMe₂Ph and PMe₃. Reaction between 1 and PMePh₂ in a 1:2 ratio yields the disubstituted rhodathiaborane [8,8-(PMePh ₂)₂-nido-8,7-RhSB₉H₁₀] (4), whereas the use of three equivalents of phosphine leads to the formation of B-ligated eleven-vertex [8,8,8-(PMePh₂)₂(H)-nido-8,7-RhSB ₉H₉-9-(PMePh₂)] (9). Compounds 4-9 have been characterized by NMR spectroscopy, and the structures of 8 and 9 confirmed by X-ray diffraction analyses. The characterization of the cluster compounds has been aided by the use of DFT calculations on some of the species. Variable-temperature NMR studies have demonstrated a lability of the PMePh ₂ ligands in compounds 4 and 9, providing mechanistic insights about the ligand substitutional chemistry in these eleven-vertex rhodathiaboranes.

Full text of DOI:10.1039/c1dt10355e

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PAPER
Chemistry of 11-vertex rhodathiaboranes: reactions with monodentate
phosphines†
Beatriz Calvo,a Martin Kess,a,c Ramo´n Mac´ıas,*a Carmen Cunchillos,b Fernando J. Lahoz,a,b John D. Kennedyd
and Luis A. Oroa,b,e
Received 2nd March 2011, Accepted 12th April 2011
DOI: 10.1039/c1dt10355e
The reaction of [8,8-(PPh3)2-nido-8,7-RhSB9H10] (1) with PR3 in a 1 : 2 ratio affords mixtures that
contain the mono-substituted bis-PR3-ligated rhodathiaboranes [8,8-(PPh3)(L)-nido-8,7-RhSB9H10]
[L = PMe2Ph (5), PMe3 (6)] and the corresponding tris-PR3-ligated compounds [8,8,8-(L)3-nido-8,7-
RhSB9H10] [L = PMe2Ph (7), PMe3 (8)]. These latter species are more conveniently prepared from the
reaction of 1 with three equivalents of the monodentate phosphines, PMe2Ph and PMe3. Reaction
between 1 and PMePh2 in a 1 : 2 ratio yields the disubstituted rhodathiaborane [8,8-(PMePh2)2-
nido-8,7-RhSB9H10] (4), whereas the use of three equivalents of phosphine leads to the formation of
B-ligated eleven-vertex [8,8,8-(PMePh2)2(H)-nido-8,7-RhSB9H9-9-(PMePh2)] (9). Compounds 4–9 have
been characterized by NMR spectroscopy, and the structures of 8 and 9 confirmed by X-ray diffraction
analyses. The characterization of the cluster compounds has been aided by the use of DFT calculations
on some of the species. Variable-temperature NMR studies have demonstrated a lability of the PMePh2
ligands in compounds 4 and 9, providing mechanistic insights about the ligand substitutional chemistry
in these eleven-vertex rhodathiaboranes.
alkanes, silanes, or boranes, could also be activated by modifying
the reaction conditions and/or by an appropriate tailoring of the
clusters.
The ease of preparation, the versatility, and the stability of
these eleven-vertex cluster compounds make them attractive can-
Introduction
Recent research has led to the discovery that the eleven-vertex
rhodathiaborane [8,8-(PPh3)2-nido-8,7-RhSB9H10] (1)1 reacts with
pyridine to form the B-pyridine-substituted hydridorhodathiab-
didates for homogeneous catalyst precursors.3,4 This consideration
prompted us to examine how we might modify the exo-polyhedral
phosphine-ligand sphere in order to explore the reactivity of
new phosphine-ligated derivatives of 2, the overall target being
the optimization of the catalytic activity of these polyhedral
compounds. In this regard, it has previously been demonstrated
that the rhodathiaborane 1 undergoes facile substitution of PPh3
by other monodentate, and also bidentate, phosphines;5,6 herein
we deal with reactions of 1 with the monodentate phosphines
PMePh2, PMe2Ph and PMe3.
orane [8,8,8-(PPh3)2(H)-nido-8,7-RhSB9H9-9-(NC5H5)] (2), which
undergoes thermal dehydrogenation to afford [1,1-(PPh3)2-closo-
1,2-RhSB9H8-3-(NC5H5)] (3).2 Compounds 2 and 3 have been
shown to be active catalyst precursors for the homogeneous hy-
drogenation and isomerization of alkenes.3 In addition, reactivity
studies of the reaction of alkynes with 2 have demonstrated that
its dehydrogenation can promote the oxidative addition of sp
C–H bonds,4 suggesting that other E–H bonds, in for example
aInstituto Universitario de Cata´lisis Homoge´nea, Universidad de Zaragoza,
Zaragoza, Spain. E-mail: rmacias@unizar.es; Fax: +34 976 762 453;
Tel: +34 976 761 146
bDepartamento de Qu´ımica Inorga´nica, Instituto de Ciencia de Materiales
de Arago´n, Universidad de Zaragoza-Consejo Superior de Investigaciones
Cient´ıficas, Zaragoza, Spain
Results and discussion
Reactions of [8,8-(PPh3)2-nido-8,7-RhSB9H10] (1) with PMePh2,
PMe2Ph and PMe3
cInstitute of Inorganic Chemistry, Lehrstuhl II, Am Hubland, 97074,
Wu¨rzburg
dSchool of Chemistry, University of Leeds, Leeds, UK, LS2 9JT
Treatment of the bis-(PPh3) compound 1 with two equivalents
of PMePh2 affords the disubstituted rhodathiaborane, [8,8-
(PMePh2)2-nido-8,7-RhSB9H10] (4). In contrast, the use of two
equivalents of PMe2Ph or PMe3 gives mixtures that contain both
the monosubstituted bis-(phosphine) species, [8,8-(PPh3)(L)-nido-
8,7-RhSB9H10], where L = PMe2Ph (5) or PMe3 (6), together with
the products of addition of a third phosphine at the metal, viz.
eKing Fahd University of Petroleum and Minerals, KFUPM Visiting
Professor, Dhahran 31261, Saudi Arabia
† Electronic supplementary information (ESI) available: Experimental and
DFT-calculated NMR data for 1, 6 and 8; NMR spectra of the reaction
of 1 with PPh2Me at different temperature, as well as 9 with PPh3; DFT-
calculated coordinates for 1, 6, 8 and 9. CCDC reference numbers 814662–
814663. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c1dt10355e
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Scheme 1
the tris-(phosphine) species [8,8,8-(L)3-nido-8,7-RhSB9H10], where
L = PMe2Ph (7) or PMe3 (8). These last two tris-(phosphine)
{Rh(PR3)3}-containing compounds are more conveniently pre-
pared from the reactions of 1 with three equivalents of phosphine.
In an interesting contrast, the same reaction, but now with PMePh2
in a 1 : 3 rhodathiaborane : phosphine molar ratio, yields the
boron-substituted hydridorhodathiaborane [8,8,8-(PMePh2)2(H)-
nido-8,7-RhSB9H9-9-(PMePh2)] (9). Scheme 1 illustrates these
results, and it can be used as a guide throughout the paper.
of phosphines, {(PPh3)(PMe3)} and {(PPh3)(PMe2Ph)} in the
1
11
monosubstituted counterparts 5 and 6. The H–{ B} spectra of
these new PR3-ligated rhodathiaboranes show that there is also a
shielding of the BHB bridging-hydrogen atom with respect to the
PPh3 derivative 1, with the higher shielding [at d(1H) -0.46 ppm]
being that of compound 4. It has been suggested that higher-
field shifts of the BHB proton resonances in nido eleven-vertex
rhodathiaboranes of this type correlate with a negative charge
build-up on the metal centers.7 Thus, the trend of high-field shifts
found in the series of rhodathiaboranes, 1, 4, 5 and 6 could be
related, at least in part, to an increase in the electron density
at the rhodium centre: {Rh(PPh3)2} < {Rh(PPh3)(PMe2Ph)} <
{Rh(PPh3)(PMe3)} < {Rh(PMePh2)2}.
1
The 160 MHz 11B–{ H} NMR spectra of the PMe3 and PMe2Ph
mixed bis-(phosphine) species 5 and 6 (see Table 1) show eight
peaks in a 1 : 1 : 2 : 1 : 1 : 1 : 1 : 1 intensity ratio, slightly shielded
[mean d(11B) ca. -6 ppm] with respect to compound 1 [mean d(11B)
ca. -4 ppm.]. The signal of relative intensity 2 results from the
accidental coincidence of resonances from two chemically different
boron sites: these are effectively distinguished individually by 1H–
1
The 31P-{ H} spectra of the PMe3 and PMe2Ph mixed-ligand
PPh3 compounds 5 and 6 at room temperature exhibit two
doublets of doublets, which conform to the asymmetric structure
of these cluster compounds. The signals around d(31P) ca. +25
ppm correspond to the PPh3 ligands, whereas the resonances of the
methyl-ligated phosphines are at higher field. In contrast, the bis-
(PMePh2)-ligated rhodathiaborane 4 exhibits a very broad signal
11
{ B(selective)} experiments. In contrast, the expected nine signals
are all resolved in the 11B spectra of the bis-(PMePh2) species 4 and
they exhibit the largest overall shielding in the series [mean d(11B)
ca. -10 ppm], which may reflect a different electron contribution
of the two PPh2Me ligands in 4 compared to the mixed pairs
1
in the 31P-{ H} spectrum at room temperature: this signal splits
into two doublets of doublets at lower temperatures, resembling
1
11
Table 1 11B and 1H–{ B} NMR data for the bis-phosphine compounds
the pattern of 5 and 6. The H NMR spectrum of 4 also shows
5 and 6 in CDCl3 at room temperature
a temperature-dependent behaviour that at high temperatures
renders equivalent the two methyl resonances of the PMePh2
ligands, and leads to the coalescence in pairs of some of the
BH terminal hydrogen resonances. Therefore, the NMR data for
4 at different temperatures are suggestive of an intramolecular
dynamic process, which resembles the fluxional behavior of
other analogous nido-structured eleven-vertex metallaheterobo-
ranes {MEB9} where M = Pt or Rh and E = S, N or C (see Table
2). The activation energies for these processes are readily estimated
from the temperatures at which peak-coalescences are observed in
the NMR spectra. This type of non-rigid behaviour has become
increasingly recognised in eleven-vertex metallaheteroboranes that
incorporate formally unsaturated pseudo-square-planar Rh(I) or
Pt(II) vertices; and recent DFT calculations have revealed that
isonido- and closo-shaped clusters are points along the reaction
coordinate of this inter-enantiomeric fluxional process.8
5
6
Assignmenta
d(11B)
d(1H)
d(11B)
d(1H)
3
9
11
6
4
5
1
10
+15.5
+10.6
+2.6b
+3.65
+3.20
+3.24
+2.37
+3.09
+2.29
+0.83
+1.14
+0.83
-1.30
+15.8
+10.3
+2.2b
+3.74
+3.10
+3.22
+2.29
+3.02
+2.39
+1.55
+0.91
+0.70
-1.33
-1.3
-1.3
-8.2
-23.2
-25.6
-27.7
-8.5
-18.5c
-24.4
-28.2d
2
m-(9,10)
a Assignments based on H–{ B} selective experiments and comparison
1
11
with 1. b Accidentally coincident 11B resonances for B(6) and B(11).
1J(11B–1H) ca. 150 Hz. d 1J(11B–1H) ca. 141 Hz.
c
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Table 2 Activation energies for the fluxional process of eleven-vertex
nido-metallaheteroboranes
this large high-field shift in the BHB bridging proton resonance
is diagnostic of formation of {RhL3} tris-ligated eleven-vertex
rhodathiaboranes versus their {RhL2} bis-ligated counterparts.
For 7 and 8, the presence of three phosphine ligands bound to
Compound
DG‡ (kJ mol-1)
Reference
1
the rhodium atom is clear in the 31P-{ H} NMR spectra that show
[(PPh3)2RhSB9H10] (1)
[(PMePh2)2RhSB9H10] (4)
[(dppp)RhSB9H10]
58
52
53
>64
46
62
>75
1
This work
three signals in a 1 : 1 : 1 relative intensity ratio. For compound 8,
the comparison of the spectrum at 223 K with that at 300 K is a
nice illustration of the “thermal decoupling” of the boron nuclei,11
5
5
8
8
9
[(dppp)RhSB9H9(OEt)]a
[(PPh3)2RhNB9H11]
2
which leads to a resolution of the coupling constants J(31P–31P)
[(PMe2Ph)2PtCB9H11]
[(PMe2Ph)2PtCB9H10(OEt)]
at low temperature that do not manifest at room temperature (see
Fig. S3 in the supporting information†).
a dppp = Ph2P(CH2)3PPh2
The characterization of the new B-ligated hydridorhodathiab-
orane 9 has also been carried out by X-ray diffraction analysis
1
(vide infra) and by NMR spectroscopy. The 11B–{ H} NMR
The tris-(PMe2Ph)-ligated nido-rhodathiaborane 7 has been
reported previous to this work,10 but for convenience of com-
parison, its spectroscopic data are described here together with
those of 8. The molecular structure of 7 was also determined
by single-crystal X-ray diffraction in the previous work.10 We
now report the crystallographic analysis of the new tris-(PMe3)-
spectrum shows eight peaks in a 2 : 1 : 1 : 1 : 1 : 1 : 1 : 1 intensity
ratio, with the peak of relative intensity 2 resulting from an
accidental overlap of two resonances (Table 4). These latter two
are effectively distinguished by 1H–{ B} selective decoupling
11
experiments as mentioned above. Diagnostic of the formation of
1
ligated analogue 8 (vide infra). The 11B and H NMR data for
the eleven-vertex hydrido-nido-rhodathiaborane are the 1H–{ B}
11
NMR signals at d(1H) -1.79 and -12.10 ppm, which correspond
both compounds are listed in Table 3. The proton spectra of
each cluster exhibits nine signals in the positive region that can
be assigned to the nine BH units of the cluster, and a singlet at
the highest-field that corresponds to the BHB hydrogen atom.
Due to the overlapping of some resonances, the 11B spectra do not
show the separated nine-peak pattern expected for the asymmetric
to the BHB bridging hydrogen atom and the Rh-H hydride ligand,
respectively. In the 31P-{ H} NMR spectrum, compound 9 exhibits
1
three resonances with the highest-field signal being significantly
broader than the other two, and, therefore, assigned to the PMePh2
ligand directly bound to the boron vertex B(9).
11
structure of these clusters; however, selective 1H–{ B} decoupling
experiments enabled the indirect resolution of the individual
components of the broad signals of relative intensity 2 and 3 in
the spectra of 7 and 8, respectively (Table 3).
Molecular structures of [(PMe3)3RhSB9H10] 8 and
[(PMePh2)2(H)RhSB9H9(PMePh2)] 9
The crystal and molecular structures of 8 and 9 were established
by single-crystal X-ray crystallography. A summary of selected
derived interatomic distances and angles for these two rhodathi-
aboranes are in Table 5, and drawings of these molecules are
in Fig. 1 and 2. The molecular structure of each compound is
based on an eleven-vertex nido-arrangement that has a pentagonal
face, formally obtained by the removal of one vertex from an
icosahedron. Both 8 and 9 are formally (11 + 2) skeletal-electron-
pair (sep) nido-rhodathiaborane analogues of the hypothetical
parent borane B11H15: three bridging hydrogen atoms and three BH
Although 7 and 8 each bear a third phosphine ligand on the
metal centre, there is no overall significant shift towards higher
fields [mean d(11B) -6 ppm] with respect to the monosubstituted
bis-(phosphine) parent compounds 5 and 6, suggesting that the
additional electron density donated by the third ligand is highly
localized at the rhodium centre, and that the electronic structures
within the clusters of 5, 6, 7 and 8 are all very similar. In this regard,
and according to the rationale described above, the shift to higher
field of more than 2.6 ppm in the 1H NMR resonance of the BHB
hydrogen atoms in 7 and 8 may reflect an increase in the electron
density at the rhodium centre; differential neighbouring-group
magnetic anisotropic effects arising from P-phenyl versus P-methyl
substituents on the phosphines may also play a role. In any event,
Table 3 11B and 1H NMR data for the tris-phosphine compounds 7 and 8
in CDCl3 at room temperature. In brackets are the DFT/GIAO-calculated
11B nuclear shielding values for 8
7
8
Assignment
d(11B)
d(1H)
d(11B) [DFT]
d(1H)
6
9
3
4
11
10
5
1
+13.5
+7.1
+7.1
+5.3
-1.1
-14.1
-15.5
-21.2
-27.0
+4.03
+3.79
+2.14
+3.55
+2.40
+2.40
+1.97
+1.93
+1.24
-3.98
+13.0 [+13.8]
+6.1 [+12.2]
+6.1 [+11.9]
+6.1 [+8.1]
+3.95
+3.29
+3.12
+1.93
+2.83
+1.84
+1.70
+1.70
+1.20
-3.75
-2.3 [+1.8]
-15.4 [-14.0]
-15.5 [-15.7]
-22.9 [-20.9]
-27.1 [-28.6]
Fig. 1 (a) ORTEP-type drawing of compound 8 (50% thermal ellipsoids
for non-hydrogen atoms); (b) detail of the {Rh(PMe3)3}-to-{SB9H10}
metal-to-cluster coordination sphere.
2
m-(9,10)
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Table 4 11B, 1H and 31P NMR data for compound 9 in CDCl3 at room temperature compared with the corresponding DFT-calculated chemical shifts
Cluster data
Assignmenta
d(11B) (ppm)
d(1H) (ppm)
1J(11B–1H) (Hz)
Measured
+4.2
Calculated
+8.3, +7. 7
+2.7
+0.5
-0.9
-11.3
-22.3, -22.8
-24.5
3, 6
4
11
9
+2.81, +3.58
+3.01
+3.11
[B-PPh2Me]
+1.86
+1.31
+1.58
-1.79
-12.10 [Rh-H]
122
127
-0.8
-2.8
-4.0
5
-7.5
122
132
2, 10
1
m (9, 10)
8
-21.2
-25.1
PPh2Me data in CDCl3 solutionb
Assignmentc
d(31P) (ppm)
1J(103Rh–31P) (Hz)
d(1H) (ppm)
2J(31P–1H) (Hz)
Measuredd
+15.3
Calculated
+14.7
+10.6
P(1)
P(2)
P(3)
85
128
—
+1.72
+1.64
+1.67
7.4
6.8
11.2
+9.6
-2.9e
-7.5
11
31
a Based on 1H–{ B(sel)}experiments and DFT calculations b Measured at 273 K. c Based on 1H–{ P(selective)} experiments and DFT-calculations.
d Broad signals: 2J(31P–31P) coupling constants not observed 1J(31P–11B) = 129 Hz e Broad pseudo-doublet, 1J(31P–11B) = 129 Hz
◦
˚
Table 5 Selected distances (A) and angles ( ) for 8 and 9 with standard
uncertainties (s.u) in parentheses
8
9
Rh(8)–S(7)
Rh(8)–P(1)
Rh(8)–P(2)
Rh(8)–P(3)
Rh(8)–B(3)
Rh(8)–B(4)
Rh(8)–B(9)
S(7)–B(2)
2.3736(7)
2.4016(4)
2.3619(4)
2.2869(4)
2.291(3)
2.2517(18)
2.2339(17)
2.0012(19)
2.047(3)
1.9140(19)
n/a
1.739(3)
1.885(4)
1.795(1)
96.256(14)
93.443(15)
174.29(5)
n/a
97.946(15)
148.61(8)
133.44(5)
n/a
86.400(15)
98.40(2)
163.57(2)
88.04(4)
n/a
2.4172(5)
2.3206(6)
2.3175(5)
n/a
2.238(2)
2.224(2)
2.218(2)
1.970(3)
2.062(2)
1.930(3)
1.943(2)
1.742(4)
1.907(4)
1.796(1)
100.810(19)
n/a
159.43(6)
80.3(12)
n/a
147.57(6)
144.25(6)
89.1(12)
99.077(19)
95.149(18)
n/a
S(7)–B(3)
S(7)–B(11)
P(3)–B(9)
B(6)–B(11) (shortest)
B(2)–B(3) (longest)
B–B (average)
P(1)–Rh(8)–P(2)
P(1)–Rh(8)–P(3)
P(1)–Rh(8)–B(9)
P(1)–Rh(8)–H(1)
P(2)–Rh(8)–P(3)
P(2)–Rh(8)–B(3)
P(2)–Rh(8)–B(4)
P(2)–Rh(8)–H(1)
S(7)–Rh(8)–P(1)
S(7)–Rh(8)–P(2)
S(7)–Rh(8)–P(3)
S(7)–Rh(8)–B(9)
Rh(8)–B(9)–P(3)
Fig.
2
(a) ORTEP-type drawing of compound
9
(50%
thermal ellipsoids for non-hydrogen atoms); (b) detail of the
{Rh(PMePh2)2H}-to-{SB9H9(PMePh2)} metal-to-cluster coordination
sphere.
The interboron distances are all within normal ranges for poly-
hedral boron-containing clusters, although the larger distances in
˚
excess of 1.9 A for the B(2)–B(3) vectors that flank the sulfur
atoms should perhaps be noted. The Rh–P distances across the
whole series of eleven-vertex rhodathiaboranes 1, 2, 7, 8 and 9
˚
vary from the shortest in compound 8 at 2.2868(5) A to the
88.79(5)
124.17(11)
10
˚
longest in 7 at 2.4539(8) A. In each of these compounds, the
longest Rh–P linkage is with the phosphine ligand that is trans
to a boron vertex: B(9) for compounds 2, 7, 8 and 9, and B(4)
for compound 1; the shortest is assigned to the PR3 ligand that is
trans to the S(7) vertex, whereas the intermediate Rh–P distances
correspond to the phosphine groups that occupy positions transoid
to the B(3)–B(4) edge of the cluster. This trend suggests that a
boron vertex has a stronger trans-effect than either a B–B edge
or a sulfur vertex in metallathiaboranes. Overall, there are no
groups on the pentagonal open face of B11H15 are formally replaced
by a sulfur atom, a {B(PMePh2)} vertex and the {RhH(PMePh2)2}
fragment to give 9; two BH groups and three of the four bridging
hydrogen atoms of B11H15 are formally replaced by a sulfur atom
and the {Rh(PPh2Me)3} fragment in 8.
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significant differences in the corresponding distances and angles
among the five {RhSB9H9L} clusters [where L = H (1), NC5H5
(2) or PMePh2 (9)], although it may be noted that the Rh(8)–S(7)
˚
length in the hydridorhodathiaboranes 2 and 9 is ca. 0.05 A longer
than in the other derivatives, suggesting a higher structural trans-
effect of the hydride ligand compared to the phosphines, PPh3,
PMePh2, PMe2Ph and PMe3.
Bonding considerations
In view of the relevance of this set of compounds to the catalytic
potential of the {RhSB9} system,2–4 the electronic constitution of
the clusters, particularly with regard to the rhodium centre, merits
consideration. The bis-(PR3)-ligated rhodathiaboranes, 1, 4, 5 and
6, could be regarded as formally ‘unsaturated’ clusters, with the
“unsaturation” arising from the tendencies of rhodium to adopt a
square-planar 16-electron metal configuration.8,12,13
Thus, these species could be described as Rh(I) complexes of
+
-
{RhL2} with {nido-SB9H10} fragments acting as bidentate lig-
ands in a tetrahapto fashion, with the rhodium centre contributing
two orbitals into the cluster bonding scheme. Bearing in mind
the basic synthesis of 1 from [RhCl(PPh3)3], it is reasonable to
consider the {SBH10} fragment in compound 1 as an LX ligand
that formally replaces both the PPh3 and the chlorine ligand in the
Wilkinson’s catalyst. Therefore, the attribution of the oxidation
state +1 to the rhodium centre in these bis-(PR3)-ligated clusters is
reasonable. In accord with this, the 11B NMR shielding pattern
in compound 1 and its bis-(ligand) congeners 4, 5 and 6 is
clearly related to that of nido-6-SB9H11 itself (Fig. 3, bottom
two traces).14 With three exo-polyhedral ligands on the metal
centre, the situation in the tris-(PR3)-ligated compounds 7 and
8 is potentially somewhat different. In principle, Wade’s rules15,16
would allocate to a {RhL3} fragment a contribution of three
orbitals and three electrons to the cluster framework. If this were
the case, then the stereochemistry around the metal centre could
be described as pseudo-octahedral, with three bonding vectors
mainly directed towards the sulfur vertex, the boron atom B(9),
and the B(3)–B(4) edge, and with the other three to the three exo-
polyhedral ligands [Fig. 1 (b)]. Therefore, a description of 7 and
Fig. 3 Stick diagrams representing the chemical shifts and relative
intensities in the 11B NMR spectra of nido-6-SB9H11 (lowest trace), the
bis-(ligand) species [8,8-(PPh3)2-nido-8,7-RhSB9H10] 1 (low centre trace),
the tris-(ligand) species [8,8,8-(PMe2Ph)3-nido-8,7-RhSB9H10] 7 (centre
5
trace), [8-(h -C5Me5)-nido-8,7-IrSB9H10] 10 (upper centre trace), and the
[arachno-6-SB9H12]- anion (uppermost trace). The values for 1, 7 and 10
are averaged across the pseudo-mirror-plane of the {SB9} fragment for
purposes of comparison. Hatched lines connect equivalent positions. Note
the inversion of cluster nuclear shielding characteristics between 7 and 10
(see text).
centre trace). It can be seen that, comparing compound 10 to
compounds 1, 4, 5, 6, 7 and 8, there is a fundamental change
in the cluster electronic structure, as manifested in the cluster
11B shielding characteristics: an essential inversion of the overall
shielding pattern. This inversion is characteristic of nido versus
arachno ten-vertex cluster species,18 and it can be seen (Fig. 3, top
trace) that the pattern of the cluster shielding of the {SB9} unit
of compound 10 clearly relates to that of the arachno [6-SB9H12]-
anion, rather than that of nido-6-SB9H11.
3+
8 as Rh(III) octahedral complexes of {RhL3} with a tridentate
3-
tetrahapto arachno-type {SB9H10} ligand, might at first sight
be reasonable from the electron-counting rules approach as well
as from a metal-to-ligand coordinative interaction. However, this
would imply that the clusters of these tris-(ligand) species would
have quite different electronic structures from the bis-(ligand)
species, which would not be consistent with the similarities (a) of
the cluster 11B NMR shielding patterns (Fig. 3, centre trace) and
(b) of the cluster dimensions among all the bis- and tris-ligated
compounds 1, 4, 5, 6, 7 and 8. These last two considerations
suggest closely related electronic structures, and thence imply
that 7 and 8 are best regarded as eighteen-electron rhodium(I)
species, with five implied bonding vectors, two directed into the
cluster as with compounds 1, 4, 5, and 6, with the other three
directed towards the three phosphine ligands. This conclusion is
supported by comparison of the 11B nuclear shielding patterns
with that of a cluster compound of the eleven-vertex {nido-
8,7-MSB9} configuration that does have a three-orbital cluster
The electronic structure within the cluster of the B-ligated
hydridorhodathiaborane
9 is closely related to those of
compounds 7 and 8. Wade’s rules16 could be taken to describe
the {RhH(PMePh2)2} fragment as a three-orbital two-electron
contributor to the cluster bonding, which would leave the rhodium
centre with a formal oxidation state of three. The stereochemistry
[Fig. 2 (b)] could thence be viewed in terms of a quasi-octahedral
Rh(III) complex of an arachno-type {SB9H9(PMePh2)}2- fragment
2+
with a {RhH(PMePh2)2} centre. Alternatively, however, the
{nido-SB9H9(PMePh2)} fragment in 9 could be regarded as
a ‘charge-compensated’ neutral ligand acting in a bidentate
tetrahapto fashion, resulting in a Rh(I) species. In view of the
arguments enunciated in the previous paragraph, we favour this
5
contribution from the metal centre, e.g. [8-(h -C5Me5)-nido-8,7-
IrSB9H10] (10)17 with an octahedral Ir(III) centre (Fig. 3, upper
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second interpretation. The 11B nuclear magnetic shielding pattern
is consistent with this.
+5.0 ppm that correspond to compound 4, (c) a singlet due to
free PPh3 and (d) traces of the starting material (see Fig. S4,
1
11
supplementary material†). The H–{ B} NMR spectrum at the
same temperature exhibits signals at d(1H) -1.66 and -1.95 ppm
in a 1 : 0.3 relative intensity ratio. The latter minor component
is assigned to the bis-(PMePh2)-substituted species 4, whereas the
major component is reasonably identified as the mono-substituted
intermediate [8,8-(PMePh2)(PPh3)-nido-8,7-RhSB9H10] (11) (Fig.
S4†). Upon addition of a second equivalent of PMePh2, there is a
clear decrease in the intensity of the 31P signals of 11, together
with a marked broadening of the signals corresponding to 4.
Interestingly, a new broad doublet at d(31P) ca. +20 ppm is formed,
and the signal of free PMePh2 at d(31P) ca. -20 ppm is also very
broad, suggesting that there is chemical exchange between the free
Ligand-exchange processes and mechanistic considerations
In order to obtain some mechanistic insights about the substi-
tutional chemistry of [8,8-(PPh3)2-nido-8,7-RhSB9H10] (1) with
monodentate phosphines, the reactions (small scale) were studied
at low temperatures by NMR spectroscopy. The reaction of two
equivalents of PMe3 with the rhodathiaborane 1 at 223 K immedi-
ately forms the bis-(ligand) monosubstituted derivative 6 and the
tris-(PMe3)-ligated cluster 8. Reaction of 1 with PMe2Ph similarly
gives mixed-ligand bis-(phosphine) 5 and tris-(ligand) 7. The
relative temperature-dependent broadening of the signals in the
1
31P-{ H} NMR spectra of 5 and 6, together with GIAO nuclear-
11
ligand and the rhodathiaborane clusters (Fig. S4†). The 1H–{ B}
shielding calculations (see supplementary material†), suggest that
the substitution in 1 takes place selectively by the displacement of
the PPh3 ligand that is trans to the S(7) vertex (Scheme 1).
The formation of {RhL3}-containing species indicates that
the addition of a third phosphine (PMe3 or PMe2Ph) to the
rhodium centre in the presumed [8,8-(PR3)2-nido-8,7-RhSB9H10]
intermediates (where PR3 is PMe3 or PMe2Ph) is faster than the
substitution reaction of PPh3 in 5 and 6. The results suggest that
the presence of more basic and less sterically demanding phosphine
ligands on the rhodium centre, facilitates the substitution of the
second PPh3 ligand in 5 and 6, and that the resulting supposed
di-substituted intermediates react faster with free PR3 than the
mono-substituted compounds (Scheme 2).
NMR spectrum confirms these observations, showing a decrease
in the intensities of the BHB signals at d(1H) -1.66 and -1.95 ppm
and the appearance of a new broad peak at d(1H) -3.42 ppm with
the highest relative intensity (Fig. S5†).
As the temperature is increased further, the 31P signals of 11
disappear and the spectrum shows a broad doublet at d(31P) ca.
+21 ppm and a very broad signal at ca. +5 ppm, overall resembling
the spectrum of compound 4. In addition, the signal from free
PMePh2 is virtually lost on the base line. The corresponding 1H–
11
{ B} NMR spectra show a significant broadening in the signals
at d(1H) -2.00 and -3.00 ppm, and a final coalescence to give
a single peak at -2.28 ppm. These observations suggest strongly
that, at low temperatures, the monosubstituted {(PPh3)(PMePh2)}
intermediate 11 and the bis-(PMePh2)-substituted species 4 are
in equilibrium with the free phosphines and a tris-(PR3)-ligated
cluster compound (Scheme 3). In this regard, as commented in
1
Under the same conditions, the 31P-{ H} spectrum of the
reaction mixture of 1 with one equivalent of PMePh2 at 223 K
shows (a) two sharp doublets of doublets at d(31P) +25.0 and
+20.0 ppm, (b) two minor broad signals at d(31P) +21.0 and
Scheme 2
Scheme 3
6560 | Dalton Trans., 2011, 40, 6555–6564
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Scheme 4
the previous sections, a shift to higher field of around 2.00 ppm
in the proton NMR signal of the BHB hydrogen atom in eleven-
vertex nido-rhodathiaboranes is diagnostic of formation of the
tris-(phosphine) {RhL3} species.
the rate of monosubstitution in 1 and the subsequent formation
of the tris-(phosphine) {Rh(PR3)3}-containing species appear to
be of the same order of magnitude. In contrast, the reaction
rate of the monosubstituted bis-(ligand) {Rh(PPh3)(PMePh2)}
intermediate 11 with more PMePh2 to give 4 is slower, and the
NMR data suggest that a possible step for the substitution of the
second PPh3 phosphine in 11 is the formation of a tris-(ligand)
rhodathiaborane species, viz. [(PPh3)(PMePh2)2-nido-RhSB9H10],
in equilibrium with compound 4 and free ligand (Scheme 3). This
conclusion implies that the most plausible mechanism for the
reactions of 1 with monodentate phosphines is associative, i.e.
addition of phosphine to the bis-(phosphine) species 1, followed
by loss of phosphine to give the observed bis-(ligand) products.
On the other hand, although the NMR data reveal an interesting
chemical lability for 9, it is not clear whether the formation of this
hydridorhodathiaborane is also driven by the formation of a Rh-
centred adduct that could promote the exchange of a Rh-bound
PMePh2 ligand with the terminal hydrogen atom at the boron B(9)
position; or whether, alternatively, the B–PMePh2 bond undergoes
incipient dissociation, partially opening coordination sites on the
cluster that could facilitate the exchange processes.
At room temperature, the equilibrium is completely shifted
towards the disubstituted species 4, but a lowering of the tem-
perature back to 188 K reproduces the spectrum discussed above,
demonstrating the reversibility of the system. However, at room
temperature overnight, the system evolves to a mixture of two
hydridorhodathiaboranes: one of them readily resembles 9 and
the other is also very similar, but with small differences in the
chemical shifts of the NMR spectra (Fig. S6†). This indicates
that the presence of free PPh3 in the media leads to formation of
the mixed-phosphine species [8,8,8-(PR3)2(H)-nido-8,7-RhSB9H9-
(PR3)], in which the phosphine ligands can be combinations of
PPh3 and PMePh2.
The
temperature-dependent
behaviour
of
B-ligated
[(PMePh2)2(H)RhSB9H9(PMePh2)] 9 in the presence of one
equivalent of PPh3 was also studied. As the temperature
1
decreases, the signals in the 31P-{ H} NMR spectrum broaden
significantly. At 223 K, this behaviour is accompanied with
formation of small amounts of free PMePh2 (Fig. S7†).
11
The corresponding 1H–{ B} spectrum shows concomitant
broadening of the signals that arise from the BHB bridging
hydrogen atom and the Rh–H hydride ligand. As the temperature
is decreased further to 188 K, some of the signals sharpen in the
Conclusions
The series of compounds 4–9 illustrates nicely that the reactivity
of the parent rhodathiaborane 1 with monodentate phosphines,
and the composition and configuration of the resulting deriva-
tives, are ultimately controlled by the nature of the phosphine.
Thus the treatment of 1 with PPh3 does not form tris-PPh3-
ligated species analogous to 7 and 8, and the cluster remains
formally unsaturated in the presence of triphenylphosphine.1,6
In contrast, the less bulky and more basic phosphines, PMePh2
and PMe3, afford {Rh(L)3}-vertices that confer one additional
electron pair to the resulting eleven-vertex clusters compared
with 1. Interestingly, PMePh2 exhibits an alternative behaviour:
too bulky to form a stable {Rh(PMePh2)3}-fragment in this
system, it has sufficient Lewis basicity to bind the boron B(9)
vertex and to form a stable hydridorhodathiaborane. Variable-
temperature experiments demonstrate the chemical lability of
these eleven-vertex rhodathiaboranes, suggesting strongly that the
most plausible mechanism for the substitution of the phosphine
ligands is associative, implying the binding of a third phosphine
at the rhodium centre. Likewise, the chemical lability exhibited
by the hydridorhodathiaborane 9 could take place by associative
processes at the {Rh(PMePh2)2(H)} metal centre, which could be
linked with an incipient partial dissociation of the B(9)-bound
phosphine.
1
31P-{ H} spectrum, and there is formation of new peaks; the 1H–
11
{ B} NMR spectrum reflects these changes, showing now two
BHB proton signals and three Rh–H hydride resonances with
different relative intensity ratios (Fig. S8†). At room temperature,
the NMR data show only the signals of compound 9.
These observations are consistent with chemical exchange
reactions involving free PPh3 and the PMePh2 ligands in the
hydridorhodathiaborane 9 (Scheme 4). The observations described
above permit a rudimentary reaction pathway to be described. In
the conditions used for this work, the reaction rates of 1 with
the more basic phosphines, PMePh2, PMe2Ph and PMe3, to give
the corresponding monosubstituted bis-(phosphine) derivatives,
4, 5 and 6 by the displacement of the PPh3 group transoid to
the sulfur atom, appear to be similar. However the subsequent
outcome of the reaction depends on the nature of the entering
monodentate phosphine. Thus, the reaction rates of PMe2Ph
and PMe3 with the supposed bis-substituted rhodathiaborane
intermediates, [8,8-(PR3)2-nido-8,7-RhSB9H10], where PR3 = PMe3
or PMe2Ph, are faster than with the monosubstituted precursors 5
and 6, leading to the formation of the tris-(PR3)-ligated species 7
and 8. Since at low temperature we have not observed any stepwise
formation of products that could indicate a rate-determining step,
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The treatment of 1 with monodentate phosphines is a convenient
synthetic route for the alteration of the exo-polyhedral ligand
sphere, confirming the tailorability of this eleven-vertex rhodathi-
borane system. Following the synthetic approach used for the
preparation of 2,2 the bis-PR3-ligated clusters, 4–6, can (a priori)
be easily modified by the treatment with pyridine, thus opening the
door to the synthesis of hydridorhodathiaboranes that may exhibit
an enhancement of the catalytic activity with respect to compound
2. In the case of PMePh2, the reaction with 1 affords directly a new
hydridorhodathiborane with an interesting ligand non-rigidity,
which augurs for a rich chemistry to complement and extend the
reported reactivity of the pyridine-ligated rhodathiaborane 2 with
unsaturated organic molecules.
perfluoropolyether. Data were measured in a single axis HUBER
diffractometer, equipped with Oxford 600 Cryosystem open-flow
nitrogen cryostat (100(1) K), using Silicon(111) monochromated
˚
synchrotron radiation (l = 0.73780 A). Intensities were integrated
with HKL2000 suite19 and corrected for absorption using a
multiscan method applied with the SORTAV program.20 The
structures were solved and refined using the programs SHELXS-
9721 and SHELXL-9722 respectively. Anisotropic displacement
parameters were used in both refinements for all non-disordered
atoms. In the case of 8, a minor disorder (0.917 & 0.083(2)
occupancy factors) affected the coordination of the sulfur atom
to the metal. In the case of 9, a clear situation of disorder
involved a phenyl ring of one of the metal-bonded phosphines;
this disorder was modelled on the base of two slightly displaced
phenyl rings with similar occupancy factors (0.422 & 0.578(9)).
Additionally, in the crystal structure of 9, the presence of a
highly disordered solvent molecule—interpreted as n-hexane—
was also identified. The electronic density in the solvent region was
estimated with PLATON/SQUEEZE program,23,24 and eventually
five residual peaks with complementary occupancies were included
in the refinement to compensate for the presence of half a hexane
molecule. For both structures, hydrogen atoms of the phosphine
ligands were included in calculated positions and allowed to refine
riding on their parent carbon atoms; the hydrogen atoms of the
borane cluster in both structures, and the hydride ligand in 9,
were included from the difference Fourier maps and refined as free
isotropic atoms.
Experimental
General procedures
Reactions were carried out under an argon atmosphere using stan-
dard Schlenk-line techniques. Solvents were obtained dried from
a Solvent Purification System marketed by Innovative Technology
Inc. The commercially available phosphines PMe3, PMe2Ph and
PMePh2 were used as received without further purification. The
eleven-vertex rhodathiaborane [8,8-(PPh3)2-nido-8,7-RhSB9H10]
(1) was prepared according to the literature method.6 Preparative
thin-layer chromatography (TLC) was carried out using 1 mm
layers of silica gel G (Fluka, type GF254) made from water slurries
on glass plates of dimensions 20 ¥ 20 cm and dried in air at
25 ◦C. Infrared spectra were recorded on a Perkin-Elmer Spectrum
100 spectrometer, using a Universal ATR Sampling Accessory.
NMR spectra were recorded on Bruker Avance 300-MHz AV,
Crystal data for 8: C9H37B9P3RhS, M = 450.56, crystal size
0.105 ¥ 0.056 ¥ 0.049 mm3, orthorhombic, Pbca, a = 16.3060(10),
3
˚
˚
b = 15.5790(10), c = 17.4840(10) A, V = 4441.5(5) A , Z = 8, Dc =
1.407 g cm-3, m = 1.088 mm-1, 74697 measured, 5464 independent
reflections (5211 observed), 261 parameters, Rint = 0.058, R1 =
1
400-MHz and 500 MHz spectrometers, using 31P-{ H}, [31P–
1
0.0252, wR(F2) = 0.0693. Crystal data for 9: C39H49B9P3RhS.
1
11
11
1H]-HMBC, 11B, 11B–{ H}, 1H, 1H–{ B} and 1H–{ B(selective)}
2
C6H14, M = 886.04, crystal size 0.10 ¥ 0.08 ¥ 0.02 mm3, triclinic,
1
techniques. H NMR chemical shifts were measured relative to
˚
P-1, a = 11.4960(10), b = 12.6030(10), c = 17.1240(10) A, a =
75.9350(10), b = 82.6070(10), g = 65.0700(10) , V = 2181.4(3) A ,
residual proton resonances in the deuterated solvents used, and
are reported in ppm relative to tetramethylsilane. 11B chemical
shifts are quoted relative to [BF3(OEt)2)] and 31P chemical shifts
are quoted relative to 85% aqueous H3PO4. Note that the
values for 1J(11B–1H) quoted are from the measured doublet
maximum-to-maximum peak separation at room temperature in
the solvents specified, and, for broader resonances in particular,
◦
3
˚
Z = 2, Dc = 1.349 g cm-3, m = 0.577 mm-1, 43398 measured,
11291 independent reflections (10602 observed), 576 parameters,
Rint = 0.0412, R1 = 0.0402, wR(F2) = 0.1105. Full crystallographic
details of both complexes have been deposited with the Cambridge
Crystallographic Data Centre with CCDC reference numbers
814662 (8) & 814663 (9).
1
will generally be smaller than the real coupling constant J(11B–
1H). Mass spectrometric data were recorded on a MICROFLEX
instrument operating in either positive or negative modes, using
matrix-assisted laser desorption/ionization (MALDI). A nitrogen
laser of 337 nm (photon energy of 3.68 eV) was used for the
ionization processes, and the molecules under study were pro-
tected with a matrix of trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-
propenylidene]malononitrile (DCTB).
Criteria for purity for the bis-(ligand) species 4, 5 and 6,
which are straightforward analogues of the well-characterised
species 1 were clean multinuclear NMR spectra allied with mass
spectrometric confirmation of the molecular ions.
Calculations
All calculations were performed using the Gaussian 03 package.25
Structures were initially optimized using standard methods with
the STO-3G* basis-sets for C, B, P, S, and H with the LANL2DZ
basis-set for the metal atom. The final optimizations, includ-
ing frequency analyses to confirm the true minima, together
with GIAO nuclear-shielding calculations, were performed using
B3LYP methodology, with the 6-31G* and LANL2DZ basis-sets.
The GIAO nuclear shielding calculations were performed on the
final optimized geometries, and computed 11B and 31P shielding
values were related to chemical shifts by comparison with the
computed value for B2H6 and H3PO4, which were taken to be
d(11B) +16.6 ppm relative to the BF3(OEt2) = 0.0 ppm standard,
and d(31P) = 0.0 ppm, respectively.
X-ray crystallography
X-ray diffraction data for compounds 8 and 9 were collected on the
BM16 CRG beam line at the ESRF. We mounted single crystals of
both complexes on micro Mount supports and were covered with
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Synthesis of [8,8-(PMePh2)2-nido-8,7-RhSB9H10] (4)
Synthesis of [8,8,8-(L)3-nido-8,7-RhSB9H10] [L = PMe2Ph (7),
PMe3 (8)]
Compound 4. A Schlenk tube equipped with a magnetic stirrer
was charged with [(PPh3)2RhSB9H10] (1, 180 mg, 0.24 mmol). The
rhodathiaborane was dissolved in CH2Cl2 (15 mL), and PMePh2
(89 mL, 0.48 mmol) was added (with a syringe) to the tube.
The resulting orange solution was stirred at room temperature
under an atmosphere of argon for 16 h. The reaction mixture was
concentrated to ca. 0.5 mL. Slow addition of hexane produced
an orange solid that was separated by decantation, washed with
hexane, and dried in vacuum (0.129 g, 84%). 11B NMR (160 MHz;
CDCl3; 298 K): d 10.6 (1B, br, BH), -1.8 (1B, br, BH), -9.1 (1B,
Compound 7. 58 mg of 1 (0.076 mmol) was dissolved in
15 mL of CH2Cl2 in a Schlenk tube. 3.37 mL (0.23 mmol) of
PMe2Ph was added to the orange solution of the rhodathiaborane,
and the reaction mixture was stirred at room temperature under
an atmosphere of argon for 4 h. The reaction mixture was
concentrated to ca. 0.5 mL. Slow addition of hexane produced
an orange solid that was separated by decantation, washed with
hexane, and dried in vacuum to give 0.0479 g of 6 (96%). Elemental
analysis: found: C, 42.5; H, 6.5; P, 14.2%. C24H43B9P3Rh1S1
requires C, 43.9; H, 6.6; P, 14.2%. nmax(ATR)/cm-1 3055 m, 2915 m,
2545w, 2345 m, 2117 w, 1992 w (BH), 1432 w, 1296 m, 1280 m, 1001
m, 933 w and 895 w. 1H NMR (500 MHz; CDCl3; 298 K): d 7.93–
6.84 (aromatic), 2.01 (9H, d, 2JPH = 3 Hz, P(CH3)3), 1.91 (9H, d,
2JPH = 3 Hz, P(CH3)3), 1.68 (9H, d, 2JPH = 5 Hz, P(CH3)3), 1.59 (9H,
d, 2JP,H = 10 Hz, P(CH3)3), 1.56 (9H, d, 2JPH = 7 Hz, P(CH3)3), 1.33
1
11
d, BH), -27.6 (1B, br, BH); H–{ B} NMR (300 MHz; CDCl3;
298 K): d 7.89–6.86 (20 H, m, Ph), 2.15 (1H, br s, BH), 1.81 (1H, br
1
s, BH), 1.78 (1H, br s, BH), -1.61 (1H, br s, BHB); 31P-{ H} NMR
1
(161 MHz; CDCl3; 202 K): d 21.8 (dd, JRhP = 149 Hz, PMePh2,
2JPP = 38 Hz), 5.8 (dd, 1JRhP = 137 Hz, 2JPP = 38 Hz, PMePh2), at
296 K these two doublet of doublets coalesce; 1H NMR (400 MHz;
CDCl3; 202 K): d 2.27 (3H, s, 2JPH = 9.7 Hz, PPh2Me), 1.38 (3H,
s, 2JPH = 7 Hz, PMePh2), at 279 K these two singlets coalesce; 1H
(9H, d, 2JPH = 8 Hz, P(CH3)3). 31P-{ H} NMR (202 MHz; CDCl3;
1
300 K): d +11.6 (1P, dt, 1JRhP = 145 Hz, 2JPP = 27 Hz), -11.8 (1P,
dt, 1JRhP = 145 Hz, 2JPP = 20 Hz), -24.2 (1P, d, br, 1JRh–P = 81 Hz);
2
NMR (400 MHz; CDCl3; 327 K): d 1.81 (6H, s, JPH = 7.5 Hz,
PMePh2); DG‡ = 52 3 kJ mol-1.
cluster 11B and H NMR data are summarized in Table 3. m/z
1
+
(MALDI) 381 {M–PMe2Ph} isotope envelope: P1C8H21Rh1S1B9
requires 381 and complete molecule P2C16H32Rh1S1B9 requires
519).
Synthesis of [8,8-(PPh3)(L)-nido-8,7-RhSB9H10] [L = PMe2Ph (5),
PMe3 (6)]
Compound 5. PMe2Ph (16.6 mL, 0.124 mmol) was added to
a red solution of 1 (0.0473 g, 0.062 mmol) in dichloromethane
at room temperature. The reaction mixture was stirred for 1 h
under an argon atmosphere, then reduced in volume and applied
to a preparative TLC plate. The chromatogram was developed
with a mixture of 4 : 1 CH2Cl2/hexane, leading to the isolation
an orange component of RF = 0.8 that was characterized as 5
(0.0201 g, 0.0313 mmol, 51%); and a yellow product of RF = 0.5
that was identified as 7 (0.0093 g, 0.014 mmol, 23%, see below).
Compound 8. Following the same procedure as with 6 above,
141.3 mg (0.18 mmol) of 1 were treated with 42.0 mg (0.55 mmol)
of PMe3. Yield: 70.6 mg, 0.15 mmol, 82%. Elemental analysis:
found: C, 22.9; H, 7.9; P, 19.6%. C9H37B9P3Rh1S1 requires C, 22.9;
H, 7.9; P, 19.7%. IR(ATR): nmax/cm-1 3422 w, 2927 w, 2909 w,
2551 m, 2503 w, 2474 m (BH), 1645 s, 1433 m, 1418 w, 1303 m,
1
1282 w, 1261 m, 1093 m, 1038 m, 957 s, 910 w, 722 m. 31P-{ H}
(CDCl3; 223 K) and 1H NMR (CDCl3; 300 K) ordered as d(31P)
(intensity, multiplicity, 1JRh–P, 2JP–P) [d(1H) of directly bound CH3
1
Compound 5: H NMR (500 MHz; CDCl3; 298 K): d 8.10–6.92
2
1
2
groups, JP–H]: +3.2 (1P, dt, JRh–P = 124 Hz, JPP = 25 Hz) [1.54,
(20 H, m, Ph), 1.94 (3H, d, 2JPH = 11 Hz, P(CH3)2Ph2), 1.29 (1H,
2
1
2
d, JP–H = 10 Hz], -25.3 (1P, ddd, JRhP = 127 Hz, JPP = 25 Hz,
2
1
s, BH), 1.17 (3H, d, JPH = 10 Hz, P(CH3)2Ph2). 31P-{ H} NMR
2JP–P = 33 Hz) [1.59, d, 2JPH = 10 Hz], -30.6 (1P, broad d, 1JRh–P
=
(161 MHz; CDCl3; 298 K): d 23.8 (1P, dd, 1JRhP = 132 Hz, 2JPP
=
87 Hz) [1.45, d, JPH = 7 Hz]; cluster 11B and H NMR data are
summarized in Table 3. m/z (MALDI+) 471 (M+ isotope envelope;
P3C9H37Rh1S1B9 requires 471).
2
1
45 Hz, PPh3), 10.7 (1P, dd, 1JRhP = 150 Hz, P(CH3)2Ph2). Cluster
11B and 1H NMR data are summarized in Table 1. m/z (MALDI+)
+
380 ({M-(PPh3+H)} isotope envelope; P1C8H21Rh1S1B9 requires
380).
Synthesis of [8,8,8-(PMePh2)2(H)-nido-8,7-RhSB9H9-9-
(PMePh2)] (9)
Compound 6. Following the same procedure as above, 0.037
mg (0.048 mmol) of 1 was treated with 36.8 mL (0.097 mmol),
and the resulting reaction mixture was separated by TLC, using a
3 : 2 ratio of CH2Cl2/hexane, The chromatogram gave two bands:
red, RF = 0.5, and yellow RF = 0.2. The red component was
characterized as compound 6 (0.0173 g, 0.030 mmol, 62%) and
the yellow product was identified as 8 (0.0106 g, 0.018 mmol,
38%). Compound 6: nmax(ATR)/cm-1 2918 m, 2849 m, 2517 s
(BH), 2159 s, 2030 s, 1996 s. 1H NMR (500 MHz; CDCl3; 298 K):
0.608 g (0.79 mmol) of 1 in 15 mL of CH2Cl2 were treated
with 0.44 mL (2.38 mmol) of PPh2Me in a Schlenk tube under
argon at room temperature. After two hours of vigorous stirring,
hexane was added and the resulting solid was separated by
decantation, washed with hexane, and dried in vacuum to give
0.615 g (0.73 mmol; 93%) of 9. Elemental analysis: found: C, 51.8;
H, 5.54; P, 11.0%. C39H49B9P3Rh1S1 requires C, 55.6; H, 5.9; P,
11.0%. IR(ATR): nmax/cm-1 3054 w, 2921 w, 2852 w, 2519 m (BH),
2050 sh (RhH), 1585 s, 1572 s, 1481 m, 1433 w, 1093 m, 1026 m,
2
d 7.67–6.79 (15 H, m, Ph), 1.30 (9H, d, JPH = 10 Hz, P(CH3)3).
1
31P-{ H} NMR (161 MHz; CDCl3; 223 K): d 25.1 (1P, dd, 1JRhP
=
=
2
1
2
1
11
141 Hz, JPP = 38 Hz, PPh3), 5.1 (1P, dd, JRhP = 107 Hz, JPP
999 m, 881 w, 738 w, 690 w. H–{ B} NMR (500 MHz; CDCl3;
298 K) d 7.69–6.90 (aromatic); other NMR data are summarized in
38 Hz, P(CH3)3). Cluster 11B and 1H NMR data are summarized in
+
+
Table 1. m/z (MALDI+) 319 ({M-(PPh3+H)} isotope envelope;
Table 3. m/z (MALDI+) 641 ({M-2HPMePh2} isotope envelope.
P1C3H19Rh1S1B9 requires 319).
P2C26H34Rh1S1B9 requires 641).
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The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 6555–6564 | 6563
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Variable-temperature NMR studies
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1
set to 188 K. The 1H–{ B} and 31P-{ H} NMR spectra was then
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1
{ H} spectra measured at this temperature. The sample tube was
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PPh3, and the resulting yellow solution was studied over a range of
11
1
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S7–S8†).
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Acknowledgements
51, 33.
21 G. M. Sheldrick, SHELXS-97, University of Go¨ttingen, Go¨ttingen,
Germany, 1997.
We acknowledge the Spanish Ministry of Science and Innovation
(CTQ2009-10132, CONSOLIDER INGENIO, CSD2009-00050,
MULTICAT and CSD2006-0015, Crystallization Factory) for
support of this work. B.C. thanks the ‘Diputacio´n General
de Arago´n’ for a pre-doctoral scholarship. M.K. thanks the
Universities of Wu¨rzburg (Germany) and Zaragoza (Spain) for
support through the Erasmus Exchange Program. We also thank
BM16 staff, specially to A. Labrador, for their valuable assistance
during X-ray diffraction measurements.
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6564 | Dalton Trans., 2011, 40, 6555–6564
This journal is
The Royal Society of Chemistry 2011
©