Polyhedral platinaborane chemistry. Interaction of PMe2Ph with [(PMe2Ph)2PtB10H12]

Article abstract of DOI:10.1021/om200940h

In solution, [(PMe₂Ph)₂PtB₁₀H ₁₂] (1) and PMe₂Ph exist in dynamic equilibrium with [(PMe₂Ph)₃PtB₁₀H₁₂] (2). The activation energy for the dynamic process, δG^≠, is ca. 63 kJ mol^-1 at +17 °C, with δS being ca. 335 J mol^-1 deg^-1 and δH ca. 105 kJ mol^-1 for the equilibrium. At low temperatures a rocking fluxionality of the {Pt(PMe₂Ph) ₃} unit versus the {η⁴-B₁₀H₁₂} unit in 2 becomes apparent, with an activation energy δG^{++ ≠} of ca. 28 kJ mol^-1 at ca. -105 °C. Compound 2 is characterized by NMR spectroscopy and by a single-crystal X-ray diffraction analysis, for which the results suggest that, in contrast to the common view, the extra electron pair gained by the metal-atom center in going from 1 to 2 does not disrupt the cluster electron count proper nor the observed nido electronic structure and geometry.

Full text of DOI:10.1021/om200940h

Article
pubs.acs.org/Organometallics
Polyhedral Platinaborane Chemistry. Interaction of PMe2Ph with
[(PMe2Ph)2PtB10H12]
Jonathan Bould,*,† Ivana Císarova,§ and John D. Kennedy*,†,‡
̌
́
†
̌
Institute of Inorganic Chemistry of the Academy of Sciences of the Czech Republic, 250 68 Husinec-Rez CP 1001, Czech Republic
̌
‡The School of Chemistry of the University of Leeds, Leeds LS2 9JT, England
§The Faculty of Natural Sciences of Charles’ University, Hlavova 2030, 12842 Prague 2, Czech Republic
S
* Supporting Information
ABSTRACT: In solution, [(PMe2Ph)2PtB10H12] (1) and PMe2Ph exist in dynamic
equilibrium with [(PMe2Ph)3PtB10H12] (2). The activation energy for the dynamic process,
ΔG⧧, is ca. 63 kJ mol−1 at +17 °C, with ΔS being ca. 335 J mol−1 deg−1 and ΔH ca. 105 kJ
mol−1 for the equilibrium. At low temperatures a rocking fluxionality of the {Pt(PMe2Ph)3}
unit versus the {η4-B10H12} unit in 2 becomes apparent, with an activation energy ΔG⧧ of
ca. 28 kJ mol−1 at ca. −105 °C. Compound 2 is characterized by NMR spectroscopy and by
a single-crystal X-ray diffraction analysis, for which the results suggest that, in contrast to
the common view, the extra electron pair gained by the metal-atom center in going from 1
to 2 does not disrupt the cluster electron count proper nor the observed nido electronic
structure and geometry.
ligand to the metal coordination sphere of compound 1. This
INTRODUCTION
■
would have the effect of conferring a formal nido electron count
to the cluster and also be of relevance to understanding the addi-
tion of small molecules to dimetallic species such as [(PMe2Ph)4-
Pt2B10H10] (3) mentioned above, as well as being germane to the
catalytic propensities of related 11-vertex formally nido com-
pounds such as [8,8-(PR3)2-nido-8,7-RhSB9H10],22 for which the
mechanism is proposed to involve reversible ligand addition at
the metal center and, for several of which, addition of a third ligand
at the metal center has indeed recently been categorized.22b,23
Gordon Stone and his co-workers at the University of Bristol pio-
neered an extensive and interesting chemistry of platinacarbaboranes,
reporting families of new compounds such as [(PR3)2PtC2R2B4-
H4],1 [(PR3)2PtC2R2B5H5],2 [(PR3)2PtC2R2B6H6],3 [(PR3)2-
PtC2R2B7H9],4 [(PR3)2PtC2B8H10],3 [(PR3)HPtC2B8H11],3
[(PR3)2PtC2R2B9H9],5 [(PR3)2PtB10H10C(NMe3)],6 and [(PPh3)-
HPtCB10H11].7 This interesting new chemistry acted as inspiration
and encouragement for many groups throughout the world. For
our part in Leeds, this work formed part of a stimulus to examine
platinum compounds based initially on borane residues, rather than
on carbaborane residues, and these have included species such as
[(PR3)2ClPtB5H8],8 [(PR3)2Pt2B12H18],9 [(PR3)2PtB8H12],10
[(PR3)4Pt2B8H10],10 [(PMe2Ph)2PtB9H11(PMe2Ph)],11
[(PMe2Ph)2PtB10H12],12 [(PR3)2PtB18H20],13 [(PMe2Ph)-
PtB16H18(PMe2Ph)],14 [(PR3)2Pt2B8H14],15 [(PR3)4Pt3B14H18],14
[(PMe2Ph)2Pt2B16H15(C6H4Me)(PMe2Ph)],16 and [(PMe2Ph)2-
PtB26H26(PMe2Ph)].17 Of these, [(PMe2Ph)2PtB10H12] (1) is a
particularly useful and interesting species. First, it can be used as
a starting substrate for dimetallaborane syntheses,18 and in this
area, the behavior of [(PMe2Ph)4Pt2B10H10] (3) and related
species can be singled out for their remarkable propensity to
reversibly sequester small molecules such as O2 and CO at the
dimetal site.18,19 Second, the molecule 1 exhibits a particular
contrarotational fluxionality of the {Pt(PMe2Ph)2} unit relative
to the {η4-B10H12} cluster unit.12 Third, because of the 16-
electron transition-element coordination-sphere characteristics of
the platinum center,20 the cluster structure, though geometrically
11-vertex nido, has a formal 11-vertex closo cluster electron
count.21 In view of this formal unsaturation of the metal center, it
was of interest to examine for the addition of a third 2-electron
RESULTS AND DISCUSSION
■
The addition of 1 equiv of PMe2Ph to a solution of yellow
[(PMe2Ph)2PtB10H12] (1) in CH2Cl2 gave a color change from
yellow to pale yellow, and fine very pale yellow crystals slowly
dropped out of solution. Larger crystals were formed from a
saturated solution of the compound in CH2Cl2/hexane and
were identified by a single-crystal X-ray diffraction analysis as
[(PMe2Ph)3PtB10H12] (2) (Figure 1).
Monitoring a solution of 2 by NMR spectroscopy over the
ranges +70 to −50 °C (CDCl3) and +30 to −100 °C (CD2Cl2)
(Figure 2; numerical data in Table S1, Supporting Information)
established that 2 was in equilibrium with 1 and PMe2Ph
1
(eq 1), with the 11B and H cluster spectra each showing reso-
nances corresponding to a weighted mean of the amounts of
the two platinaboranes present within the system, rather than
Special Issue: F. Gordon A. Stone Commemorative Issue
Received: October 6, 2011
Published: November 29, 2011
© 2011 American Chemical Society
2691
dx.doi.org/10.1021/om200940h | Organometallics 2012, 31, 2691−2696

Organometallics
Article
Table 1. Measured NMR Parameters for
[(PMe2Ph)3PtB10H12] (2) Together with Selected Values for
[(PMe2Ph)2PtB10H12] (1) for Comparison
a
bc
,
compd 1
δ(11B)
compd 2
d
assignt
δ(1H)
δ(11B)
δ(1H)
1
+14.0
+14.5
+8.8
+3.52
+3.52
+2.98
+2.44
+2.39
+1.12
−2.12
+19.4
+9.5
+4.33
+3.67
+1.94
+1.74
2, 3
8, 11
9, 10
5
+0.7
−4.0
+1.5
−11.6
ca. −12
−27.7
ca. +1.75
+0.95
4, 6
μ-H
−27.6
−4.54
a
Conditions: 233 K/CDCl3. Also δ(1H) +1.62 (N(31P−1H) = 10.5 Hz,
PMe2), +1.86 (N(31P−1H) = 9.5 Hz, PMe2), δ(1H) +7.1−7.7 (PPh);
b
δ(31P) +1.3 (1J(195Pt−31P) = 2535 Hz). Conditions: 233 K/CD2Cl2/
c
excess PMe2Ph. δ(31P) (CD2Cl2/233 K) −31.8 (2P, J(195Pt−31P) =
2291 Hz) and −31.4 (1P, 1J(195Pt−31P) = 1684 Hz). At lower tempera-
tures the 2P resonance broadens and splits into two: δ(31P) (CD2Cl2/
168 K) −23.3 (1P), −31.4 (1P) and −38.2 (1P) (195Pt satellites not
1
d
apparent). δ(1H) (CD2Cl2/233 K) +1.47 (N(31P−1H) = ca. 8 Hz,
PMe2), +1.69 (N(31P−1H) = ca. 9 Hz, PMe2), +2.07 (3J(31P−1H) = ca.
8 Hz, PMe2), +7.1−7.7 (PPh).
two separate sets of resonances, indicating a dynamic exchange
process. NMR spectroscopy on a solution of 2 containing
excess PMe2Ph gave the NMR parameters for fully associated 2
(Table 1).
Figure 1. Crystallographically determined molecular structures of
(top) [(PMe2Ph)2PtB10H12] (1) (data from ref 12b) and of (bottom)
[(PMe2Ph)3PtB10H12] (2). Selected distances (in Å) for 2 (with
equivalent distances for 1 in brackets) are as follows: Pt(7)−P(1) =
2.4132(5) [2.309(1)], Pt(7)−P(2) = 2.3988(5) [2.337(1)], Pt(7)−
P(3) = 2.3905(5), Pt(7)−B(2) = 2.270(2) [2.214(5)], Pt(7)−B(3) =
2.252(2) [2.225(5)], Pt(7)−B(8) = 2.245(2) [2.279(6)], Pt(7)−
B(11) = 2.269(2) [2.301(6)]. Selected angles (deg): P(1)−Pt(7)−
P(2) = 95.542(19) [95.4(1)], P(3)−Pt(7)−B(8) = 97.60(6), P(3)−
Pt(7)−B(11) = 84.97(7), P(3)−Pt(7)−P(1) = 96.767(13), P(3)−
Pt(7)−P(1) = 96.296(12).
[(PMe Ph) PtB H ] + PMe Ph
2
2
1
10 12
2
⇌ [(PMe Ph) PtB H ]
2
3
10 12
(1)
2
The equilibration was best monitored by measurement of the
1
bridging hydrogen atom H resonance position (Figure 2),
which changed by some 2.5 ppm between extremes. Here it can
be seen from Figure 1 that, in contrast to the nonshielded
positioning in compound 1, the bridging hydrogen atom
position is in the shielding cone of the phenyl group on P(3) in
compound 2, hence accounting for the large shielding change.
For ca. 1 M solutions, the equilibrium of eq 1 was entirely to
the left-hand side at +70 °C (CDCl3 solution) and entirely over
to the right-hand side at −50 °C (CDCl3 and CD2Cl2
solutions).
An analysis of the curve of Figure 2 gives values for ΔH of
104 8 kJ mol−1 and for ΔS of 335 24 J mol−1 deg−1 for the
equilibrium (see the Supporting Information for details of the
calculations). The somewhat high ΔS value reflects the specific
relative and internal orientations of both the receiving
{Pt(PMe2Ph)2} ligand-sphere rotamers and of the attacking
PMe2Ph molecule that are required for a close mutual binding
approach. The equilibrium is dynamic: the activation energy
ΔG⧧ for the exchange process is best estimated by 31P NMR
spectroscopy, in which the frequency differences of relevant
resonances are far greater than any differences in the 1H
spectra. In the 162 MHz 31P{1H} spectrum, uncomplexed 1
shows a single 31P chemical shift with associated 195Pt satellites,
whereas completely complexed 2 (low temperature) exhibits
two chemical shifts, as expected from the two types of chemical
environments apparent from Figure 2, viz. equatorial (2P)
and endo (1P), each also with 195Pt satellites (Table 1). At
intermediate equilibrium, ca. −10 °C, the resonances from
both compounds are apparent, as is a resonance from excess
1
Figure 2. Variation with temperature of the μ-H H NMR resonance
of the {B10H12} residue for a 0.88 M sample of compound 2 in CDCl3
(●), a 1.27 M sample of compound 2 in CD2Cl2 (×), and a sample of
compound 1 in CDCl3 (■). There is an approximately linear decrease
in shielding with decreasing temperature for fully associated
compound 2 (left-hand side of the CD2Cl2 data plot) and for fully
dissociated compound 1 (uppermost data plot).
2692
dx.doi.org/10.1021/om200940h | Organometallics 2012, 31, 2691−2696

Organometallics
Article
phosphine (eq 1). At higher temperatures broadening and
coalescence occurs, with a coalescence temperature of ca.
+17 °C, giving a value for ΔG⧧ of ca. 63 kJ mol−1 for the
exchange process at that temperature.
Scheme 1. Representation of the Proposed Rocking
Fluxionality of the {Pt(PMe2Ph)3} Moiety Observed in
Compound 2 (IA/IB) Compared to the Complete Rotation
a
of the {Pt(PMe2Ph)2} Unit Seen in Compound 1 (IIA/IIB)
A further interesting dynamic phenomenon in 2 is apparent
1
from the 400 MHz H and 128 MHz 31P NMR spectra at the
lowest temperatures at which we were able to conduct NMR
experiments. Whereas the shift of the phosphorus resonance of
intensity 1P remains essentially unchanged at these lowest
temperatures, the resonance of intensity 2P broadens and
ultimately splits into two resonances, at δ(31P) −23.3 and
−31.4 ppm at −105 °C; this was the lowest temperature
available before solvent solidification, and the 195Pt satellites
were not quite apparent. It may be that CSA relaxation of 195Pt
is becoming significant at these lowest temperatures. Never-
theless, a coalescence temperature for the main signals was
apparent, at ca. −105 °C, which, together with their frequency
separation of 2440 Hz at the particular polarizing field used,
gives a value for ΔG⧧ of ca. 28 kJ mol−1 for the process at
that temperature. At the same time one of the two PMe2Ph
1H resonances arising from the prochiral nature of the two
equatorial phosphines severely starts to broaden, but the
relevant frequency separations in the 1H spectrum are less, and
so separation into two resonances and assessment of a
a
For clarity the Me2 and Ph designators are omitted from each
1
coalescence in the H spectrum was not feasible. There also
phosphorus ligand. In all the cluster schematics in this paper, unlabeled
cluster vertices are {BH(exo)} units.
appears to be incipient differentiation into two components of
1
each of the cluster H resonances of relative intensity 2H at
these extreme lowest temperatures, but these could not be
definitively delineated before solidification ensued. Any
corresponding differentiation in the 11B spectra would not be
apparent because of the high 11B line widths arising from
relaxation broadening at these lowest temperatures. The
chemical shift difference between the two new resonances in
the 31P spectrum is nontrivial, implying two reasonably different
environments. This, coupled with the perturbation at the same
time of mainly just one of the two signals from the prochiral
methyl protons of the equatorial PMe2Ph ligands, implies a
rocking−twisting reciprocating motion of the {Pt(PMe2Ph)3}
unit relative to the {η4-B10H12} cluster unit about an axis
through the platinum atom (structures I, Scheme 1). Inspection
of Figure 1 suggests that this involves ligand rotamers such that
just one of the two PMe2 methyl groups on both P(1) and
P(2) experiences large 1H shielding changes. It can be seen that
the hydrogen atoms on the P(2) methyl carbon atom C(23)
are within the shielding cone of the phenyl group on P(1), with
one of the methyl hydrogen atoms on C(23) directed toward
the centroid of the phenyl ring on P(1) at a distance of 2.58 Å.
In the fluxional process, these methyl-group hydrogen atoms
will exchange to a less shielded position equivalent to the
C(12) methyl group on P(1), which in turn will become more
shielded by the phenyl group on P(2) as it adopts a position
equivalent to that of C(23). The other methyl group on P(2)
does not experience a similar large change in shielding
environment as it changes its position. This rocking fluxionality
of compound 2 is in contrast with the fluxionality of the
bis(phosphine) species 1,12 in which there is a complete
contrarotation of the {Pt(PMe2Ph)2} assembly relative to the
{PtB10H12} cluster assembly, though again about an axis
through the platinum atom (structures II, Scheme 1).
P(1)−Pt(7)−P(2) plane bends away from its position in 1,
which is essentially transoid to the midpoints of the B(2)−
B(11) and B(3)−B(8) cluster vectors, and in 2 it becomes
essentially coplanar with the five-membered nido 11-vertex open
face, albeit with a slight tilt that corresponds to one extreme of
the rocking fluxionality discussed in the previous paragraph. The
essentially orthogonal angle between the Pt(7)−P(1) and
Pt(7)−P(2) vectors remains essentially unchanged at ca. 94.5°,
with the new ligand positioned in turn approximately
orthogonal to those, with P(3)−Pt(7)−P(1) and P(3)−
Pt(7)−P(2) angles of 96.767(13) and 96.296(12)°, respectively.
In comparison to 1, the distances in 2 from Pt(7) to P(1)
and P(2) increase by ca. 0.06−0.1 Å, with all three platinum−
phosphorus distances now being very similar, although it is not
clear whether the increases arise from electronic effects or from
steric crowding. Within the cluster, there is a slight increase in
the distances from Pt(7) to B(2) and B(3) that are transoid to
the new PMe2Ph ligand and a concomitant reduction of similar
magnitude in the distances from Pt(7) to B(8) and B(11) on
the nido 11-vertex five-membered open face. These intracluster
changes are relatively small, implying no large changes within
the cluster bonding, including the metal-to-boron interactions,
when compounds 1 and 2 are compared. This conclusion is
supported by the cluster 11B NMR shielding considerations
discussed below.
In terms of the overall molecular structure (Figure 1), there
is an obvious change in the {(PMe2Ph)2} ligand disposition
exhibited by 1 as the third PMe2Ph ligand comes in to occupy
the endo position it displays in 2 (structures III and IV). The
2693
dx.doi.org/10.1021/om200940h | Organometallics 2012, 31, 2691−2696

Organometallics
Article
The measured 31P and 1H NMR properties of the phosphine
ligand sphere are summarized in Table 1. As discussed above, at
intermediate temperatures there is a 2:1 relative intensity
pattern in the 31P spectrum, as expected from the unique
position of P(3), and what is at intermediate temperatures a
time-averaged equivalency of P(1) and P(2). The coupling
1
constant J(195Pt−31P) for the time-averaged P(1)P(2) pair
at ca. 2290 Hz is significantly larger than that for P(3) at
ca. 1685 Hz, perhaps implying more s character in the equa-
torial bonding to P(1) and P(2), and correspondingly less in
the endo bonding to P(3). In borane chemistry in general,
1
differences in couplings J(11B−1H) for axial and endo B−H
linkages differ for similar reasons. The corresponding coupling
constant to P(1) and P(2) in the bis(phosphine) compound 1
is 2534 Hz. At extremely low temperatures P(1) and P(2)
become chemically inequivalent; in the crystallographically
determined molecular structure (Figure 1) they are different,
both in terms of the Pt−P and the other bond rotamers and
also in terms of their relative tilt with respect to the open face
of the cluster. There are corresponding features in the 1H NMR
spectrum. The phosphorus atoms P(1) and P(2) are prochiral
centers, and so on each of these the methyl groups are inequi-
valent. Each of the overall 1H resonance patterns for these two
different environments shows a pattern that approximates to
that of an [AX3]2 spin system, where A is 31P and X is 1H; there
3
is associated satellite structure arising from J(195Pt−1H). At
the lowest temperatures, the inequivalence of all six methyl
groups is expected as the molecular structure settles into that of
Figure 1 and its enantiomer. At the lowest temperatures attain-
able, these differences were not definitively resolvable because
of interference by the exchange process which was not suffi-
ciently slowed. Nevertheless, it was apparent that two pairs of
these do not differ significantly in their chemical environments,
as there is little apparent broadening at the lowest temper-
atures. In contrast, the other pair broadened significantly,
implying incipient de-coalescence into significantly differently
shielded resonances. As discussed above, these must correspond
to the C(13)/C(23) pair, one of which will be in the anisotropic
shielding cone of the phenyl unit on the other phosphine ligand.
Figure 3. Representation of the chemical shifts and relative intensities
in the 11B NMR spectra of (from top to bottom) nido-B10H14 (data
from ref 24), the [Me2TlB10H12]− anion (data from ref 25), the
[Pt(B10H12)2]2− anion (data from ref 26), [(PMe2Ph)2PtB10H12] 1
(data from ref 12a), [(PMe2Ph)3PtB10H12] 2 (this work), and the
[arachno-B10H14]2− anion (data from ref 27). Hatched lines connect
equivalent sites. Note that 10-vertex nido-B10H14 and [arachno-
B10H14]2− have a numbering convention different from that of the
11-vertex nido-type clusters of the other compounds.
an overall four-electron contribution to the cluster bonding
proper, is not apparent. This argues for an effective 18-electron
platinum(II) transition-element center for 2, as compared to
16-electron platinum(II) for 1. Thus, it can be argued, although
the third phosphine ligand ostensibly increases the formal
cluster count by two electrons, as required for the Wadian28
nido electron count, nevertheless, the intracluster bonding
retains a character very similar to that of the bis(phosphine)
compound 1, which has a closo formal electron count; in these
terms, the cluster proper of the tris(phosphine) species 2 also
has essentially an unchanged cluster-electron count.
In the more general case, this electron-undercounting syn-
drome applies also to 11-vertex metallaheteroboranes such as
[8,8-(PPh3)2-nido-8,7-RhSB9H10], which have attracted much
recent interest because of their particular catalytic properties
which involve the redox properties of the metallaheteroborane
cluster as a unit, as distinct from those of the metal center
alone.22,23 A view has been proposed, for this type of com-
pound, in which the nido electron count that is axiomatically
assumed to be required to provide the observed nido structure
is attained via two ortho-CH···Rh quasi-agostic interactions
from the adjacent phosphine-bound phenyl rings.29 However,
DFT calculations on models of [8,8-(PPh3)2-nido-8,7-
RhSB9H10] and [8,8-(PPh3)2-nido-8,7-RhNB9H10] have indi-
cated that the (stable) nido cluster structure does not require
1
The cluster 11B and H NMR chemical shifts for the tris-
(phosphine) compound 2 are given in Table 1, together with
those of the bis(phosphine) species 1. The 11B shielding
patterns are represented schematically in Figure 3, together
with those of comparison compounds. Nuclear magnetic
shielding depends intimately on the ground- and excited-state
molecular orbital structure. Significant changes in cluster
bonding will therefore be reflected in significant changes in
cluster nuclear shieldings, and it is useful to view the 11B NMR
chemical shifts represented in Figure 3 from this perspective.
Relative to nido-B10H14, visualizing a simple replacement of
two 2-electron bonds to two bridging hydrogen atoms with two
2-electron bonding vectors to the metal (see cluster structures
VI and VII), the effect of platinum in the [Pt(B10H12)2]2−
anion is similar to effect of the main-group metal thallium in
the [Me2TlB10H12]− anion, and the similarities in overall cluster
shielding to nido-B10H14 itself are clear. Progression down the
sequence of spectra represented in Figure 3 shows that the
shielding pattern of 1 deviates somewhat further from that of
nido-B10H14 and that of 2 somewhat more so, but neverthe-
less the shielding pattern is still directly traceable to that of
nido-B10H14, and a crossover to arachno 10-vertex character,
typified here by the [B10H14]2− anion, which might be expected
for a more oxidative further insertion by the Pt center, to give
2694
dx.doi.org/10.1021/om200940h | Organometallics 2012, 31, 2691−2696

Organometallics
Article
site that is filled by the PMe2Ph ligand in the monometallic
compound 2 (structures IX and X; note, for ease of com-
parison, X repeats IV above). However, on dioxygen uptake, for
example, there is a reordering of the bonding within the cluster
bonding associated with the two metal atoms: the two platinum
centers share a pair of electrons from the incoming O2 moiety
to generate a peroxy linkage, and the Pt−Pt distance decreases
to a normal metal−metal cluster bonding distance of ca. 2.7 Å.
The bimetallic motif in 3 thus affords a combined stronger
affinity toward the bound molecules than in its monometallic
precursor 1, permitting an effective oxidative addition of the O2
molecule to the Pt−Pt linkage in the cluster. In contrast, for the
monometallic compound 1, the 11B NMR spectrum does not
show any change up to a pressure of ca. 4 bar, even with CO,
which binds to 3 more strongly than O2.
CONCLUSION
■
The evidence presented here shows that the take-up of
PMe2Ph, and therefore the acquisition of its associated electron
pair, by the metal vertex in the nido cluster compound
[(PMe2Ph)2PtB10H12] (1) to give [(PMe2Ph)2PtB10H12] (2)
does not significantly alter the overall electronic character of
the metallaborane cluster. This is revealed by the nuclear
shielding characteristics in its 11B NMR spectrum of 2, which
retain the basic pattern of 1 that is characteristic of nido
11-vertex metallaborane clusters (Figure 3). These results are of
significance in countering suppositions that, because nido
compounds such as 1 formally have Wadian closo cluster-
electron counts, then their structures are “anomalous”, rather
than expected, and/or that additional interactions involving the
metal atom must therefore be invoked to bring the formal
cluster electron count up from closo to nido; Wade’s rules,
although very useful, do not constitute an inviolable scientific
law. The behavior of [(PMe2Ph)2PtB10H12] (1) with PMe2Ph
to give 2 is also a useful model for the assessment of the approach
of small molecules such as O2, CO, and SO2 to the metal centers
of the diplatinum derivative [(PMe2Ph)4Pt2B10H10] (3), to which
they readily add across the Pt−Pt vector.
extra electrons donated from agostic interactions.30 The
crystallographic and comparative nuclear shielding evidence
reported here provides further evidence in this area of nido-
metallaundecaborane species and directly shows that the extra
electron pair from a third phosphine does not substantially alter
the structure or the electronic character of the 11-vertex
clusterthe effect of the incoming ligand remains localized on
the metal. Thus, even allowing for supposed electron-pair
“agostic” donations29 in [(PPh3)2-nido-RhSB9H10], it may not
be assumed that these would be required to account for its
observed structure.
Finally, it is pertinent to note that the behavior of
[(PMe2Ph)2PtB10H12] (1) may be taken as a complementary
probe in the assessment of the properties of its diplatinum
derivative [(PMe2Ph)4Pt2B10H10] (3), which readily adds small
molecules such as O2, CO, and SO2 across the Pt−Pt vector.18
The last compound features a 12-vertex {Pt2B10} cluster core
that has two adjacent {Pt(PMe2Ph)2} moieties separated by an
essentially nonbonding distance of ca. 3 Å (structure V). Simi-
larly to 2, the 11B NMR shielding characteristics of the cluster
in 3 are readily traceable to the [(PMe2Ph)2PtB10H12] and
B10H14 precursors.18a The {Pt(PMe2Ph)2} moiety in 1 can be
regarded as replacing a pair of B10H14 cluster bridging hydrogen
atoms (structures VI and VII). Correspondingly, in 3, each of
the two {Pt(PMe2Ph)2} moieties replaces a pair of B10H14
cluster bridging hydrogen atoms (structure VIII); there is a
weak-to-nonbonding link between the metal atoms, and the
metal moieties thereby retain the essentially empty coordina-
tion sites on the platinum to the extent that they are able to
accept the incoming small molecules such as O2 and CO. The
incoming molecule effectively occupies the vacant coordination
EXPERIMENTAL SECTION
■
General Procedures. [(PMe2Ph)2PtB10H12] (1) was prepared as
described previously.18b Solvents were stored over calcium hydride in
evacuated glass bulbs. PMe2Ph was used as received. Manipulations
were in vacuo or under dry dinitrogen; compound 1 and the final
product 2 are air-stable in the solid state. NMR spectroscopy was
performed at ca. 5.9 and 9.4 T (fields corresponding to 250 and
1
400 MHz H frequencies, respectively) using commercially available
instrumentation and using techniques and procedures as adequately
described and enunciated elsewhere.8b,24,27,31 Chemical shifts δ are
given in ppm relative to Ξ = 100 MHz for δ(1H) ( 0.05 ppm)
(nominally TMS), Ξ = 32.083 972 MHz for δ(11B) ( 0.5 ppm)
(nominally [BF3(OEt2)] in CDCl3),24 and Ξ = 40.480730 MHz for
δ(31P) ( 0.5 ppm) (nominally 85% aqueous H3PO4). Ξ is as defined
by McFarlane.32
Preparation of [(PMe2Ph)3PtB10H12] (2). Dichloromethane
(ca. 10 mL) was condensed into a Schlenk tube containing
[(PMe2Ph)2PtB10H12] (1; 147 mg, 248 μmol) and a magnetic fol-
lower. The tube was closed and warmed to ambient temperature with
stirring. One equivalent of PMe2Ph (35 μL) was added by syringe
against a flow of nitrogen, the Schlenk tube closed, and the stirring
continued. After 5 min the stirring was halted and the (now paler
yellow) solution allowed to stand for 2 h, during which time fine
crystals formed. The vessel was then stored at ca. −20 °C overnight.
The next day, filtration and washing with 50/50 CH2Cl2/hexane
afforded 0.11 g (151 μmol, 61%) of crystalline product identified as
2695
dx.doi.org/10.1021/om200940h | Organometallics 2012, 31, 2691−2696

Organometallics
Article
[(PMe2Ph)3PtB10H12] (2). Anal. Found (calcd): C, 39.34 (39.50); H,
6.23 (6.22). Crystals suitable for single-crystal X-ray diffraction analysis
were obtained from saturated CH2Cl2/hexane solution.
(6) Carroll, W. E.; Green, M.; Stone, F. G. A.; Welch, A. J. J. Chem.
Soc., Dalton Trans. 1975, 2263.
(7) Batten, S. A.; Jeffery, J. C.; Jones, P. L.; Mullica, D. F.; Rudd, M. D.;
Sappenfield, E. L.; Stone, F. G. A.; Wolf, A. Inorg. Chem. 1997, 36, 2570.
(8) (a) Greenwood, N. N.; Kennedy, J. D.; Staves, J. J. Chem. Soc.,
Dalton Trans. 1978, 1146. (b) Kennedy, J. D.; Staves, J. Z. Naturforsch.,
B 1979, 34B, 808.
Single-Crystal X-ray Diffraction Analysis. Crystal data for 2:
C24H45B10P3Pt, M = 729.70, yellow prism, 0.38 × 0.26 × 0.13 mm3,
triclinic, space group P1 (No. 2), a = 9.5125(2) Å, b = 10.2874(3) Å,
̅
c = 16.9338(5) Å, α = 87.8030(10)°, β = 85.9790(10)°, γ =
76.2210(10)°, V = 1605.07(7) Å3, Z = 2, Dc = 1.510 g/cm3, F000 = 724,
Bruker APEX-II CCD, Mo Kα radiation, λ = 0.710 73 Å, T = 150(2)
K, 2θmax = 55.0°, 54 835 reflections collected, 7369 unique reflections
(Rint = 0.0187), final GOF = 1.107, R1 = 0.0103, wR2 = 0.0237,
R indices based on 7083 reflections with I > 2(I) (refinement on F2),
350 parameters, 0 restraints, Lp and absorption corrections applied,
μ = 4.536 mm−1. The structure was solved by direct methods and was
refined by full-matrix least squares (against all the unique F2 data)
using SHELXL-97.33 All non-hydrogen atoms were refined with
anisotropic displacement parameters. The drawings of the molecular
structure in Figure 1 were made using the ORTEP-3 program.34
CCDC 846176 contains the supplementary crystallographic data for
this paper. These data can be obtained free of charge via http://www.
ccdc.cam.ac.uk/conts/retrieving.html or from the Cambridge Crys-
tallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, U.K.
(fax (+44) 1223-336-033; e-mail deposit@ccdc.cam.ac.uk).
(9) Greenwood, N. N.; Hails, M. J.; Kennedy, J. D.; McDonald, W. S.
J. Chem. Soc., Dalton Trans. 1985, 953.
(10) Boocock, S. K.; Greenwood, N. N.; Hails, M. J.; Kennedy, J. D.;
McDonald, W. S. J. Chem. Soc., Dalton Trans. 1981, 1415.
(11) Kim, Y. H.; Brownless, A.; Cooke, P. A.; Greatrex, R.; Kennedy,
J. D.; Thornton-Pett, M. Inorg. Chem. Commun. 1998, 1, 19.
(12) (a) Boocock, S. K.; Greenwood, N. N.; Kennedy, J. D. J. Chem.
Soc., Chem. Commun. 1980, 305. (b) Boocock, S. K.; Greenwood,
N. N.; Kennedy, J. D.; McDonald, W. S.; Staves, J. J. Chem. Soc., Dalton
Trans. 1981, 2573.
(13) Cheek, Y. M.; Greenwood, N. N.; Kennedy, J. D.; McDonald,
W. S. J. Chem. Soc., Chem. Commun. 1982, 80.
(14) Beckett, M. A.; Crook, J. E.; Greenwood, N. N.; Kennedy, J. D.
J. Chem. Soc., Dalton Trans. 1986, 1879.
(15) Ahmad, R.; Crook, J. E.; Greenwood, N. N.; Kennedy, J. D.
J. Chem. Soc., Dalton Trans. 1986, 2433.
(16) Bould, J.; Clegg, W.; Kennedy, J. D.; Teat, S. J. J. Chem. Soc.,
ASSOCIATED CONTENT
Dalton Trans. 1998, 2777.
■
(17) (a) Bould, J.; Clegg, W.; Teat, S. J.; Barton, L.; Rath, N. P.;
Thornton-Pett, M.; Kennedy, J. D. Inorg. Chim. Acta 1999, 289, 95.
(b) Bould, J.; Barrett, S. A.; Barton, L.; Rath, N. P.; Kennedy, J. D.
Inorg. Chem. Commun. 1998, 1, 365.
S
* Supporting Information
Tables, figures, and a CIF file giving variable-temperature NMR
data, details of the calculation of the thermodynamic
parameters, and crystal data and selected geometrical
dimensions for 2. This material is available free of charge via
the Internet at http://pubs.acs.org.
(18) (a) Bould, J.; Kilner, C. A.; Kennedy, J. D. Dalton Trans. 2005,
̌
1574. (b) Bould, J.; Base, T.; Londesborough, M. G. S.; Oro, L. A.;
́
Macıas, R.; Kennedy, J. D.; Kubat, P.; Fuciman, M.; Polivka, T.; Lang,
K. Inorg. Chem. 2011, 50, 7511.
(19) Bould, J.; McInnes, Y. M.; Carr, M. J.; Kennedy, J. D. Chem.
Commun. 2004, 2380.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: jbould@gmail.com (J.B.); J.D.Kennedy@leeds.ac.uk
(J.D.K.).
(20) (a) Kennedy, J. D. Main Group Met. Chem. 1989, 12, 149.
(b) Spalding, T. R.; Murphy, M. P.; Cowey, C.; Kennedy, J. D.;
Thornton-Pett, M.; Holub, J. J. Organomet. Chem. 1998, 550, 151.
(c) Kennedy, J. D.; Bould, J.; Cooke, P. A.; Dorfler, U.; Barton, L.;
Rath, N. P.; Thornton-Pett, M. Inorg. Chim. Acta 1999, 285, 290.
(21) Kennedy, J. D. In The Borane-Carborane-Carbocation Continuum;
Casanova, J., Ed.; Wiley: New York, 1998.
(22) (a) Macías, R.; Rath, N. P.; Barton, L. Organometallics 1999, 18,
3637. (b) Macías, R.; Calvo, B.; Kess, M.; Cunchillos, C.; Lahoz, F. J.;
Kennedy, J. D.; Oro, L. A. Dalton Trans. 2011, 40, 6555.
(23) Calvo, B.; Macías, R.; Cunchillos, C.; Lahoz, F. J.; Oro, L. A.
Organometallics 2011, DOI: 10.1021/om200707m.
(24) Kennedy, J. D. In Multinuclear NMR; Mason, J., Ed.; Plenum:
New York, London, 1987.
(25) Beckett, M. A.; Kennedy, J. D.; Howarth, O. W. J. Chem. Soc.,
Chem. Commun. 1985, 855.
(26) Klanberg, F.; Wegner, P. A.; Parshall, G. W.; Muetterties, E. L.
Inorg. Chem. 1968, 7, 2072.
ACKNOWLEDGMENTS
■
̌
This is contribution No. 122 from the Rez-Leeds Anglo-Czech
Polyhedral Collaboration (ACPC). We thank Yvonne McInnes
for assistance in the early stages of this work and Dr. Ramon
̌
́
́
Macıas for stimulating discussions and logistical support. This
work was supported by the Grant Agency of the Czech
Republic (Grant Nos. P207/11/1577 and P207/11/0705).
DEDICATION
■
Dedicated to the memory of Gordon Stone; a good man and a
good chemist.
̌ ́
(27) Hermanek, S. Chem. Rev. 1992, 92, 325.
REFERENCES
■
(28) Wade, K. Adv. Inorg. Chem. Radiochem. 1976, 18, 1.
(29) Welch, A. J.; Adams, K. J.; McGrath, T. D.; Rosair, G. M.;
Weller, A. S. J. Organomet. Chem. 1998, 550, 315.
(1) Barker, G. K.; Green, M.; Stone, F. G. A.; Welch, A. J. J. Chem.
Soc., Dalton Trans. 1980, 1186.
(2) Barker, G. K.; Garcia, M. P.; Green, M.; Stone, F. G. A.; Welch,
A. J. J. Chem. Soc., Chem. Commun. 1982, 46.
(3) (a) Barker, G. K.; Green, M.; Spencer, J. L.; Stone, F. G. A.;
Taylor, B. F.; Welch, A. J. J. Chem. Soc., Chem. Commun. 1975, 804.
(b) Green, M.; Spencer, J. L.; Stone, F. G. A. J. Chem. Soc., Dalton
Trans. 1979, 1679. (c) Barker, G. K.; Green, M.; Stone, F. G. A.;
Welch, A. J.; Wolsey, W. C. J. Chem. Soc., Chem. Commun. 1980, 627.
(d) Barker, G. K.; Green, M.; Stone, F. G. A.; Wolsey, W. C.; Welch,
A. J. J. Chem. Soc., Dalton Trans. 1983, 2063.
̌
(30) Macías, R.; Bould, J.; Holub, J.; Kennedy, J. D.; Stíbr, B.;
Thornton-Pett, M. Dalton Trans. 2007, 2885.
̌ ́
(31) (a) Reed, D. Chem. Soc. Rev. 1993, 22, 109. (b) Hermanek, S.
Inorg. Chim. Acta 1999, 289, 20. (c) Ferguson, G.; Kennedy, J. D.;
Fontaine, X. L. R.; Faridoon.; Spalding, T. R. J. Chem. Soc., Dalton
Trans. 1988, 2555.
(32) McFarlane, W. Proc. R. Soc. London, Ser. A 1968, 306, 185.
(33) Sheldrick, G. Acta Crystallogr., Sect. A 2008, 64, 112.
(34) Farrugia, L. J. J. Appl. Crystallogr. 1997, 30, 565.
(4) Green, M.; Howard, J. A. K.; Spencer, J. L.; Stone, F. G. A.
J. Chem. Soc., Dalton Trans. 1975, 2274.
(5) Green, M.; Spencer, J. L.; Stone, F. G. A.; Welch, A. J. J. Chem.
Soc., Dalton Trans. 1975, 179.
2696
dx.doi.org/10.1021/om200940h | Organometallics 2012, 31, 2691−2696