Polyhedral metallaheteroborane chemistry. Synthesis, spectroscopy, structure and dynamics of eleven-vertex {RhNB9} and {PtCB 9} metallaheteroboranes

Article abstract of DOI:10.1039/b702767b

Reaction between [RhCl(PPh₃)₃] and the [nido-6-NB₉H₁₁]^- anion in CH₂Cl ₂ yields orange eleven-vertex [8,8-(PPh₃) ₂-nido-8,7-RhNB₉H₁₁] (1). Reaction of the [nido-6-CB₉H₁₂]^- anion with [cis-PtCl ₂(PMe₂Ph)₂] in methanol affords yellow eleven-vertex [9-(OMe)-8,8-(PMe₂Ph)₂-nido-8,7-PtCB ₉H₁₀] (2), which is also formed from the reaction of MeOH with [8,8-(PPh₃)₂-nido-8,7-PtCB₉H₁₀] (3). Both compounds have been characterised by single-crystal X-ray diffraction analysis and examined by NMR spectroscopy and have structures based on eleven-vertex nido-type geometries, with the metal centre and the heteroatoms in the adjacent (8)- and (7)-positions on the pentagonal open face. The metal-to-heteroborane bonding sphere of 1 is fluxional, with a ΔG ^? value of 48.4 kJ mol^-1. DFT calculations on the model compounds [8,8-(PH₃)₂-nido-8,7-RhNB ₉H₁₁] (1a) and [8,8-(PH₃)₂-nido-8,7- RhSB₉H₁₀] (4a) have been carried out to define the fluxional process and the intermediates involved. The Royal Society of Chemistry.

Full text of DOI:10.1039/b702767b

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Polyhedral metallaheteroborane chemistry. Synthesis, spectroscopy,
structure and dynamics of eleven-vertex {RhNB9} and {PtCB9}
metallaheteroboranes.†
a
c
Ramo´n Mac´ıas,*a,b Jonathan Bould,a Josef Holub,a,c John D. Kennedy, Bohumil St´ıbr and
ˇ
Mark Thornton-Petta
Received 22nd February 2007, Accepted 23rd March 2007
First published as an Advance Article on the web 1st May 2007
DOI: 10.1039/b702767b
Reaction between [RhCl(PPh3)3] and the [nido-6-NB9H11]− anion in CH2Cl2 yields orange eleven-vertex
[8,8-(PPh3)2-nido-8,7-RhNB9H11] (1). Reaction of the [nido-6-CB9H12]− anion with [cis-PtCl2(PMe2Ph)2]
in methanol affords yellow eleven-vertex [9-(OMe)-8,8-(PMe2Ph)2-nido-8,7-PtCB9H10] (2), which is also
formed from the reaction of MeOH with [8,8-(PPh3)2-nido-8,7-PtCB9H10] (3). Both compounds have
been characterised by single-crystal X-ray diffraction analysis and examined by NMR spectroscopy and
have structures based on eleven-vertex nido-type geometries, with the metal centre and the heteroatoms
in the adjacent (8)- and (7)-positions on the pentagonal open face. The metal-to-heteroborane bonding
sphere of 1 is fluxional, with a DG‡ value of 48.4 kJ mol−1. DFT calculations on the model compounds
[8,8-(PH3)2-nido-8,7-RhNB9H11] (1a) and [8,8-(PH3)2-nido-8,7-RhSB9H10] (4a) have been carried out to
define the fluxional process and the intermediates involved.
cluster. The clusters could therefore perhaps be regarded as being
Introduction
formally unsaturated, albeit with the unsaturation localised on the
sixteen-electron metal centre. This unsaturation implies enhanced
reactivity, and, in accord with this, the rhodathiaborane cluster
can add a Lewis base without change of the nido geometry, it
can undergo facile substitution of the PPh3 ligands, and it can
undergo a nido to closo transformation, as well as exhibiting other
interesting reactivity.1,4
The nature of the apparent electronic disobedience of these
metallathiaboranes has been recently reviewed in Comprehensive
Organometallic Chemistry, third edition (COMC-III),5 where it
is pointed out that [8,8-(PPh3)2-nido-7,8-RhSB9H10] 4 and [8,8-
(dppe)-nido-7,8-RhSB9H10] 5,6 were proposed to exhibit two
‘agostic’ ortho-CH-to-Rh interactions that endow the metal centre
with extra electrons, and hence furnish the overall cluster with the
extra electron pair required to agree with Wade’s electron counting
rules.3 However, it is not clear how two such interactions, involving
a total of four electrons, should be necessary to provide only two
extra cluster electrons. As an alternative rationale, it has been
pointed out,7 though not so reviewed in COMC-III, that there
is no need to invoke such agostic interaction if the transition-
element centres concerned are comfortable with a sixteen-electron
configuration. An interpretative problem often arises because
simplistic applications of electron-counting rules often presume
eighteen-electron transition-element behaviour.
In 1990 the eleven-vertex rhodathiaborane, [8,8-(PPh3)2-nido-7,8-
RhSB9H10] 4 was reported.1 This compound exhibits a fluxional
behaviour in solution, which was described as the transfer of the
{Rh(PPh3)2} unit from one side to the other of the nido-shaped
ten-vertex {SB9H10} fragment. Subsequently, the isoelectronic
platinacarborane, [8,8-(PMe2Ph)2-nido-8,7-PtCB9H11] 3, was also
described.2 Compound 3 also exhibits dynamic behaviour related
to that of the rhodathiaborane, but, for this species, the presence of
the phosphine PMe2Ph generated prochiral centres that allowed
a more complete characterization of the fluxional process. This
process was described as a shift of the platinum atom across the
open face of the {CB9H10} cluster, linked with a half-rotation of
the {PMe2Ph}2 ligand sphere.
These two eleven-vertex metallaheteroboranes are examples
of borane-based clusters that contain square-planar sixteen-
electron transition-element centres, and which could be taken
to appear to deviate from the classical Wade–Williams cluster-
geometry electron-counting relationship.3 According to a common
interpretation of this relationship, the structural architecture,
geometrically classed as nido, would formally require 13 skeletal
electron pairs (sep) to attain a nido structure. However, an electron
count of the constituent vertices gives only 12 sep for either
aSchool of Chemistry, University of Leeds, Leeds, UK LS2 9JT
The principal synthetic route to MEB10-types of metalla-
heteroboranes (where E is C, N or S, and M is a transition-
element centre) has been that of metal-centre insertion into the
open face of the carbaborane precursor [arachno-6-CB9H14]−,8 the
isoelectronic thiaborane [arachno-6-SB9H12]−,9 and the azaborane
precursors [arachno-6-NB9H13]− and nido-6-NB9H12.10 However,
stable heteroborane substrates are often not robust and their
preparation is often complicated. These two factors have inhibited
bInstituto Universitario de Cata´lisis Homoge´nea, Universidad de Zaragoza,
Pedro Cerbuna 12, 50009, Zaragoza, Spain. E-mail: rmacias@
unizar.es
cInstitute of Inorganic Chemistry, The Academy of Sciences of the Czech
Republic, 24068, Rez near Prague, The Czech Republic
† Electronic supplementary information (ESI) available: X-ray diffraction
details, atomic coordinates, bond lengths and angles, isotropic and
anisotropic displacement parameters. DFT calculated coordinates for 1a,
4b (ground and transition states). See DOI: 10.1039/b702767b
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an extensive development of the corresponding metallated cluster
chemistry. This is particularly manifest in the metallaazaborane
area, which, compared to metallamonocarboranes and metalla-
thiaboranes, is sparsely examined. We have now found that a
convenient approach to eleven-vertex azametallaborane chemistry
can derive from the use of the salt [tmndH][NB9H11] (where
tmnd is N,N,Nꢀ,Nꢀ-tetramethylnaphthalene-1,8-diamine), which
by contrast with its air-unstable oily neutral precursor, nido-6-
NB9H12, is a yellow air-stable solid. Here we report that the
reaction of this salt with [RhCl(PPh3)3] affords orange [8,8-
(PPh3)2-nido-8,7-RhNB9H11] (compound 1). In related work, the
reaction of [Me4N][nido-6-CB9H12] with [cis-PtCl2(PMe2Ph)2] in
the presence of methanol yields yellow [9-(OMe)-8,8-(PMe2Ph)2-
nido-8,7-PtCB9H10] 2, the B(9)-methoxy derivative of previously
reported [8,8-(PMe2Ph)2-nido-8,7-PtCB9H11] 3.2
Metallaheteroboranes 1 and 2 are new examples of polyhedral
boron-containing compounds that are similarly anomalous in
terms of a simplistic approach to cluster electron counting, and
may therefore shed additional light on the interpretive problems
often associated with compounds such as 4 and 5 as mentioned
above. The rhodaazaborane 1 is fluxional with a mechanism that
mirrors the one proposed for the rhodathiaborane 4 and the
platinacarborane 3 mentioned above. In contrast, the methoxy-
substituted platinacarborane 2 does not exhibit dynamic behavior
up to 398 K, suggesting that the presence of the methoxy-
substituent increases the activation energy of the process. As
part of this work, DFT calculations on the models [8,8-(PH3)2-
nido-8,7-RhNB9H11] (1a) and [8,8-(PH3)2-nido-8,7-RhSB9H10] (4a)
provide further insights regarding the relative stabilities of the nido
versus the closo geometrical configurations in these types of eleven-
vertex systems, and also regarding the intermediates that play a
role in the fluxional process. The calculations demonstrate that
‘agostic’ types of interactions between the exo-polyhedral ligands
and the rhodium atom are not required to attain the observed nido
geometrical structures.
products respectively.2 We now find that 3, in lower yield, also
results from use of the [nido-6-CB9H12]− anion rather than the
corresponding arachno anion as reagent in the reaction with the
platinum complex [eqn (2)].
[CB9H12]− + [PtCl2(PMe2Ph)2] → [(PMe2Ph)2PtCB9H11]
+ HCl + Cl−
(2)
We have also found that the stirring of a solution of nido-8,7-
platinamonocarbaborane 3 with methanol at room temperature
results in the formation of the B-methoxy derivative [9-(OMe)-8,8-
(PMe2Ph)2-nido-8,7-PtCB9H10] (2) in quantitative yield [eqn (3)].
[(PMePh)2PtCB9H11] + CH3OH
→ [(PMe2Ph)2PtCB9H10(OCH3)] + H2
(3)
The methoxy compound 2 can be directly prepared in a slightly
more favorable overall yield (20% versus 12%) if the reaction
between the [Me4N][CB9H12] salt and the platinum complex is
carried out in dry methanol solvent.
In these syntheses [eqn (1) and (2)], the complete mechanism
is unclear, but one possibility could be via an initial coordi-
nation of the nido-6-monoheteroborane anion to the sixteen-
electron transition-element complex through hydridic B–H sites,
generating intermediates with exo-polyhedral M–H–B bonds that
can undergo cluster insertion and elimination of chlorine [eqn (1)
and (2)]. Interestingly, the reaction of the [nido-6-NB9H11]− anion
with the rhodium(I) complex, [RhCl(PPh3)3], to give nido species
1 does not appear to proceed with an oxidative addition of the
bridging hydrogen atom to the rhodium centre. This would lead
to the formation of the rhodium(III) isomer [1,1-(PPh3)2-1-H-
closo-1,2-RhNB9H10] (schematic II) rather than the nido isomer
(schematic I) as observed. This contrasts to isoelectronic [9,9-
(PEt3)2-nido-9,7,8-RhC2B8H11] that exists as its closo isomer III
in the solid state, although the nido form IV is observed to be in
equilibrium with the closo species in solution over a wide range of
temperatures.11
In this context, the dynamic behaviour of the rhodaazaborane
compound 1 (see below near Scheme 1) may well proceed through
a reversible internal redox reaction, involving the conversion of
the B(9)–H–B(10) bridging hydrogen atom to RhH terminal in a
nido–closo–nido sequence. In contrast to the rhodadicarbaborane
system (III ꢀ IV), the nido configuration is the only one detected
in the rhodaazaborane system. The energetic reasons for this
difference in behaviour will be quite subtle, and involve the
electronic properties of the heteroborane ligands (there will be a
greater localization of charge on the one N heteroatom compared
to two C heteroatoms) and the differential properties of PEt3 versus
PPh3 ligands. In either case, it is clear that the nido versus closo
energetic differences can be small in these systems. An interesting
effect of this is that the observed nido configuration of compound
1 apparently conflicts with its formal closo electron count. This
Results and discussion
Reaction of [tmndH][nido-6-NB9H11] with [RhCl(PPh3)3] in
dichloromethane solution at room temperature leads to the
formation of orange [8,8-(PPh3)-nido-8,7-RhNB9H11] 1 in 24%
yield [eqn (1)].
[NB9H11]− + [RhCl(PPh3)3] → [(PPh3)2RhNB9H11]
+ Cl− + PPh3
(1)
It has been previously reported that the reaction be-
tween the [arachno-6-CB9H14]− anion and cis-[PtCl2(PMe2Ph)2]
in dichloromethane at room temperature affords yellow [8,8-
(PMe2Ph)2-nido-8,7-PtCB9H11] (3, 46–64%) and pale yellow [9,9-
(PMe2Ph)2-arachno-9,6-PtCB8H12] (0–3%), as major and minor
2886 | Dalton Trans., 2007, 2885–2897
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Fig. 1 ORTEP-type of drawings of (left) [8,8-(PPh3)2-nido-8,7-RhNB9H11] 1 and (right) one member of the enantiomeric pair in the unit cell of
[9-(OMe)-8,8-(PMe2Ph)2-closo-8,7-PtCB9H10] 2.
˚
Table 1 Selected interatomic distances (A) for 1 and 2 with estimated standard uncertainties (s.u) in parentheses. Corresponding calculated distances
for the model compound [8,8-(PH3)2-nido-8,7-RhNB9H11] 1a are given in square brackets.
(a) From the metal atom
Compound 1
P(1)–Rh(8)
P(2)–Rh(8)
B(3)–Rh(8)
B(4)–Rh(8)
B(9)–Rh(8)
Compound 2: Molecule 1
P(1)–Pt(8)
P(2)–Pt(8)
B(3)–Pt(8)
B(4)–Pt(8)
B(9)–Pt(8)
C(7)–Pt(8)
2.2332(8) [2.246]
2.4163(8) [2.399]
2.208(3) [2.296]
2.230(3) [2.269]
2.205(3) [2.259]
2.231(2) [2.256]
2.271(3)
2.351(3)
2.305(7)
2.252(7)
2.312(6)
2.160(7)
N(7)–Rh(8)
(b) Heteroatom–boron
Compound 1
N(7)–B(2)
N(7)–B(3)
N(7)–B(11)
Compound 2: Molecule 1
C(7)–B(2)
C(7)–B(3)
1.663(4) [1.673]
1.596(4) [1.592]
1.545(4) [1.548]
1.673(9)
1.646(9)
1.590(9)
C(7)–B(11)
(c) Boron–boron
Compound 1
B(2)–B(1)
B(6)–B(1)
B(5)–B(1)
B(4)–B(3)
B(11)–B(10) (longest)
B(9)–B(4)
B(10)–B(9)
B(6)–B(2) (shortest)
(d) Others
Compound 2: Molecule 1
B(2)–B(1)
B(6)–B(1)
B(5)–B(1)
B(4)–B(3)
B(11)–B(10) (longest)
B(9)–B(4)
B(10)–B(9)
1.760(5) [1.751]
1.812(5) [1.823]
1.787(5) [1.786]
1.841(5) [1.840]
1.881(5) [1.910]
1.765(5) [1.784]
1.835(5) [1.807]
1.734(5) [1.748]
1.758(10)
1.817(11)
1.764(10)
1.865(9)
1.960(10)
1.809(9)
1.811(10)
1.737(11)
B(6)–B(2) (shortest)
Compound 2: Molecule 1
B(9)–O(9)
1.376(7)
C(9)–O(9)
1.430(6)
is due to a preference for rhodium to adopt a sixteen-electron
configuration,7 as discussed below.
the cluster distances appear to be within the same ranges, apart
from the shorter distances to the nitrogen atom in comparison
with that of the sulfur atom, consistent with the smaller size of the
The crystal and molecular structures of 1 and 2 were determined
by X-ray diffraction analysis (Fig. 1). The molecular structures are
consistent with the NMR data described below, thus confirming
that the crystals selected were representative of the bulk samples.
Selected interatomic distances are summarized in Table 1 and
angles in Table 2. Crystallographic collection data are in Table 3.
Both metallaheteroboranes have the classical eleven-vertex nido
cluster geometry which may be derived from an icosahedron by the
removal of one vertex. The hetero and metal atoms are in adjacent
positions on the open face. In each case, there is a hydrogen atom
on the open-face bridging the B(9)B(10) site, as located by the
X-ray diffraction analyses and confirmed by NMR spectroscopy.
Selected dimensions of azarhodaundecaborane compound 1 may
be compared to those of the isoelectronic and isostructural
thiarhodaundecaborane [8,8-(PPh3)2-nido-8,7-RhSB9H10] 4.1 All
˚
˚
former (covalent radii: 0.70 A for N, 1.04 A for S).
Compound 2 crystallises with two independent molecules in
the unit cell, arranged as enantiomeric pairs, which differ chiefly
in the torsion angles of the methoxy group on B(9) and of one
of the phosphine ligands at the platinum centre. The presence of
the methoxy substituent results in elongation of the Pt(8)–B(9)
˚
˚
and Pt(8)–B(3) distances of 0.028 A and 0.038 A, respectively,
˚
and contraction of Pt(8)–C(7) by ca. 0.028 A, with respect to
the unsubstituted parent [8,8-(PMe2Ph)2-nido-8,7-PtCB9H11] 3.2
The other cage dimensions are similar, with B(10)–B(11) being
the longest interboron distance in both platinamonocarbaborane
˚
clusters. The value of 1.376(7) A for the O(9)–B(9) bond is within
˚
the reported range of 1.331 to 1.407 A for exopolyhedral methoxy–
boron linkages.12
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Table 2 Selected angles (◦) between interatomic vectors for 1 and 2 with standard uncertainties (s.u). Corresponding calculated angles for the model
compound [8,8-(PH3)2-nido-8,7-RhNB9H11] 1a are given in square brackets.
(a) About the metal atom
Compound 1
Compound 2: Molecule 1
P(1)–Pt(8)–P(2)
B(3)–Pt(8)–P(1)
B(3)–Pt(8)–P(2)
B(4)–Pt(8)–P(1)
B(4)–Pt(8)–P(2)
B(3)–Pt(8)–B(4)
C(7)–Pt(8)–P(1)
C(7)–Pt(8)–P(2)
C(7)–Pt(8)–B(3)
C(7)–Pt(8)–B(4)
C(7)–Pt(8)–B(9)
B(9)–Pt(8)–P(1)
B(9)–Pt(8)–P(2)
B(9)–Pt(8)–B(3)
B(9)–Pt(8)–B(4)
P(1)–Rh(8)–P(2)
B(3)–Rh(8)–P(1)
B(3)–Rh(8)–P(2)
B(4)–Rh(8)–P(1)
B(4)–Rh(8)–P(2)
B(3)–Rh(8)–B(4)
N(7)–Rh(8)–P(1)
N(7)–Rh(8)–P(2)
N(7)–Rh(8)–B(3)
N(7)–Rh(8)–B(4)
N(7)–Rh(8)–B(9)
B(9)–Rh(8)–P(1)
B(9)–Rh(8)–P(2)
B(9)–Rh(8)–B(3)
B(9)–Rh(8)–B(4)
(b) About the cage heteroatom atom
Compound 1
97.65(3) [95.27]
128.54(9) [126.49]
111.60(9) [125.64]
93.30(9) [89.91]
159.80(9) [173.16]
49.00(12) [47.54]
170.60(7) [166.37]
88.81(7) [97.78]
42.15(11) [40.93]
78.57(11) [76.78]
86.29(11) [86.34]
91.57(9) [86.55]
148.91(9) [138.24]
84.48(12) [83.33]
46.76(12) [46.41]
96.1(1)
129.5(2)
120.6(2)
95.5(2)
168.0(1)
48.3(2)
171.7(1)
87.3(2)
43.1(2)
80.9(3)
92.8(3)
90.0(2)
136.4(1)
85.8(3)
46.7(2)
Compound 2: Molecule 1
B(2)–C(7)–Pt(8)
B(3)–C(7)–Pt(8)
B(3)–C(7)–B(2)
B(2)–N(7)–Rh(8)
B(3)–N(7)–Rh(8)
B(3)–N(7)–B(2)
129.2(2)
68.18(14)
68.2(2)
126.4(4)
73.1(4)
64.1(4)
108.1(4)
67.3(4)
116.1(5)
B(11)–N(7)–Rh(8)
B(11)–N(7)–B(2)
B(11)–N(7)–B(3)
116.1(2)
67.6(2)
120.3(2)
B(11)–C(7)–Pt(8)
B(11)–C(7)–B(2)
B(11)–C(7)–B(3)
(c) About the OMe in compound 2 molecule 1
O(9)–B(9)–B(4)
O(9)–B(9)–B(10)
C(9)–O(9)–B(9)
128.0(5)
123.7(5)
119.5(5)
O(9)–B(9) Pt(8)
O(9)–B(9) B(5)
113.0(4)
129.4(5)
Table 3 Crystallographic data for [8,8-(PPh3)2-nido-8,7-RhNB9H11] (compound 1) and [9-(OMe)–8,8-(PMe2Ph)2-nido-8,7-PtCB9H10] (compound 2)
Compound
1
2
Formula
M
C36H41B9ClNP2Rh.1/2CH2Cl2
792.30
Triclinic
9.7567(12)
11.936(2)
17.698(2)
93.138(9)
94.637(9)
105.790(9)
1.335
C18H35B9OP2Pt
621.80
Crystal system
Monoclinic
18.217(2)
13.785(2)
20.778(4)
—
99.89(1)
—
1.61
5140.3(14)
P21/a
8
2431.87
Yellow blocks
0.465 × 0.25 × 0.20
5.648
4.0–50.0
9459
˚
a/A
˚
b/A
˚
c/A
a/◦
b/◦
c /◦
Dc/g cm−3
3
˚
U/A
1970.4(5)
¯
Space group
Z
F(000)
P1
2
808
Colour, habit
Crystal dimensions/mm
l/mm−1
Yellow blocks
0.6 × 0.4 × 0.15
0.610
3.0–50.0
8429
2h limits/◦
No. of reflections: Total
Unique (Rint
)
6954 (0.0397)
5884
0.7109, 0.914
502
—
7161
0.1493, 0.4873
623
Observed [I > 2r(I)]
Min., max. transmissions
No. parameters
R (observed data)
wR (observed data)
0.0347
0.0861
0.0211
0.0269
˚
Common parameters: Stoe STADI4 diffractometer, collected at 200(2) K. Mo-Ka radiation (0.71073 A), scan mode x–h, scan width 1.05 + a-doublet
splitting. Absorption correction azimuthal W scans.
These values are close to the 36.6(2)◦ angle reported for
There is a substantial tilt of the M(8) vertex out of the plane of
the open faces in 1 and 2, with the angles between {M(8)E(7)B(9)}
(where M = Rh, E = N; M = Pt, E = C) and {B(2)B(5)E(7)B(9)}
(where E = N or C) being 37.98(17)◦ and 36.36(14)◦ respectively.
[8,8-(PMe2Ph)2-nido-8,7-PtCB9H11] (3). The {M(PR3)2} moiety
(where M = Rh, R3 = Ph3; M = Pt, R3 = Me2Ph) is twisted
away from a reference plane through M(8)B(1)B(6), with the
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one resonance being larger than the corresponding coupling for the
other 31P nucleus and thereby enabling the assignment of P(1) as
trans to the more electronegative nitrogen atom.13 As mentioned
above, this increase in magnitude of the couplings is associated
with a contracted rhodium–phosphorus distance. The 31P(2)
resonance is slightly broader, indicating a larger coupling 2J(31P–
Rh–11B) to 11B(4) and 11B(9) versus any 2J(31P–Rh–14N) coupling to
14N(7). At higher temperature fluxional site exchange for the two
31P doublets is apparent, as indicated by their coalescence into one
doublet at 270 K in the 40.3 MHz spectrum. As just mentioned,
the assignment of the 11B NMR data is supported by the GIAO-
calculated nuclear magnetic chemical shielding properties on the
model compound [8,8-(PH3)2-nido-8,7-RhNB9H11] 1a (Table 6).
In contrast to compound 1, the 11B NMR spectrum of [9-(OMe)-
8,8-(PMe2Ph)2-nido-8,7-PtCB9H10] (2) at room temperature is
sharper (Table 5). This difference arises because the PMe2Ph ligand
is less bulky than PPh3: the molecule therefore tumbles more
rapidly in solution and the 11B relaxationtimes are correspondingly
longer. In this case, the assignments are readily made based
dihedral angles between {M(8)P(1)P(2)} (where M = Rh or Pt)
and {E(7)M(8)B(9)} (where E = C or N) being 31.3(1)◦ for 1
and 43.8(1)◦ for 2. It is noteworthy that the phosphine ligands,
which are almost exactly trans to the heteroatom groups (i.e.
trans to HC, HN) have substantially shorter phosphorus–metal
distances than the phosphine groups trans to the boron sides
of the E(7)B(3)B(4)B(9) faces (where E = CH, NH): 2.2332(8)
˚
˚
versus 2.4163(8) A for 1, and 2.275(3) versus 2.353(3) A for 2.
This structural feature is reflected in a large difference between the
1
coupling constants J(M–31P) (where M is 103Rh or 195Pt) within
each of 1 and 2, with the bigger value in each case corresponding
to the shorter metal-to-phosphorus distance M(8)–P(1).
The measured NMR data for [8,8-(PPh3)2-nido-8,7-RhNB9H11]
1 (Table 4, A) are consistent with the observed structure, although
the broadness of the peaks at the lower temperature of 253 K
precluded a complete resolution of the 11B spectrum at available
field strengths. At this low temperature the 11B NMR spectrum
shows six broad peaks that correspond to nine independent
resonance positions, of which three are accidentally coincident
in the observed peak at ca. +10.7 ppm and two accidentally
coincident in the observed peak at ca. −19.2 ppm, giving the
apparent 3 : 1 : 1 : 1 : 2 : 1 intensity-ratio pattern.
The assignments given in Table 4 for the resonances are
reasonably based on their relative intensities, and by compari-
son with the previously reported rhodathiaborane, [8,8-(PPh3)2-
nido-8,7-RhSB9H10].1 The assignments were supported by DFT
calculations on model compound [8,8-(PH3)2-nido-8,7-RhNB9H11]
1a (see below). These considerations, together with a detailed
examination of the variable temperature multielement NMR
properties of 1, also allowed an identification of the pairs of
exchanging boron sites which arise from the known fluxional
behaviour of these eleven-vertex systems at elevated temperatures.1
Thus, measurement at the higher temperature of 370 K reveals a
substantial change in the 11B spectrum, which now exhibits six
resonances in a 1 : 2 : 2 : 1 : 2 : 1 relative intensity ratio (Table 4,
B). Additional evidence of the fluxional nature of this compound
is gained from the 31P spectrum: the lower-temperature spectrum
(203 K) consists of two doublets of doublets, with 1J(103Rh–31P) for
1
11
on H–{ B(selective)} experiments, and by comparison with the
unsubstituted congener, [8,8-(PMe2Ph)2-nido-8,7-PtCB9H10] (3).
Boron-site exchange was not observed up to 398 K, indicating a
higher activation energy for the fluxional process in this methoxy
substituted species 2 compared to its unsubstituted equivalent 3,
in which fluxionality is observed above 313 K. The 31P spectrum
of compound 2 is assigned in a similar manner to that for 1, and,
1
31
additionally, H–{ P(selective)} experiments enable assignments
among the four non-equivalent PMe groups on the two prochiral
31
PMe2Ph phosphine ligands. Incidental to this, 1H–{ P} selective
decoupling experiments on the 195Pt satellites in the 1H spectrum
confirmed 3J(195Pt–P–C–1H) to be positive, relative to 1J(195Pt–31P)
that can be taken to be positive.14
Potential information regarding the cluster bonding in these
species may be gained from a comparative study of the shielding
patterns of compound 1, of the non-metallated neutral [nido-
6-NB9H12], of the [arachno-6-NB9H13]− anion, and also of the
5
previously reported iridaazaborane [9-Cl-8-(g -C5Me5)-nido-8,7-
IrNB9H11] 6.15 This comparison is represented in Fig. 2. Thus, the
Table 4 11B, 1H and 31P NMR data for [8,8-(PPh3)2-nido-8,7-RhNB9H11] (compound 1)
(a) Cluster data
Aa
Bb
Assignmentc
d(11B)d (ppm)
+10.7
Assignmentc
9
d(11B) (ppm)
+18.2
d(1H) (ppm)
+4.79
3, 9, 11c
6
5
−0.2
3,11
1,6
5
4,10
2
+9.3
+3.66
+2.83
+2.33
+1.96
+1.88
+2.58(br)
+0.09
−13.1
−6.1
−7.6
4
−9.9
1,10e
−19.2
−25.7
[NH]
−14.00
−25.5
[NH]
2
7
7
l (9,10)
(b) 31P dataf
Assignmentg
P(1)
P(2)
d(31P) (ppm)
+50.7
+19.5
1J(103Rh–31P)/Hz
+50.7
+19.5
2J(31P–31P)/Hz
31.74
11
a CDCl3 solution at 253 K. b C6D5CD3 solution at 370 K. c See text. d 1H–{ B(selective)} experiments were not carried out at low temperature. e Broadness
of the peaks precludes the resolution of the individual resonances. f CD2Cl2 solution at 203 K. g At higher temperature these two peaks collapse to one;
coalescence temperature 270 K in the 40.3 MHz spectrum.
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Table 5 11B, 1H and 31P NMR data for compound [9-(OMe)-8,8-(PMe2Ph)2-nido-8,7-PtCB9H10] 2 in CDCl3 at room temperature
(a) Cluster data
Assignmenta
9
11
3
6
5
4
1
d(11B) (ppm)
+28.3
+11.7
+10.0
−8.5
d(1H) (ppm)
OMeb
+3.28
+3.42
+2.06
1J(11B–1H)/Hz
c
c
133
140
192
−11.4
−13.5
−15.1
−27.4
−29.2
[CH]
+2.97
+1.74
+1.58d
+1.52f
+0.88
ca. 143d
ca. 173d
ca. 163d
2
10
7
+3.60(br)
+0.28
l (9,10)
(b) PMe2Ph data in CD2Cl2 solution at 237 K
Assignment
P(1) (sh)g
d(31P) (ppm)
−6.6
1J(195Pt–31P)/Hz
d(1H) (ppm)
A +1.92
B +1.55
A +1.67
B +1.65
3J(195Pt–1H)/Hz
32.0 Hz
26.4 Hz
23.4 Hz
23.9 Hz
2J(31P–1H)/Hz
3287 Hz
11.7 Hz
ca. 10.7 Hz
9.5 Hz
ca 9.5 Hz
P(2) (br)
−4.5
2682 Hz
a See text. b +3.71 ppm, 4J(195Pt–1H) = 2.6 Hz. c Coupling constants not measured due to broadness of peaks relative to 1J. d 3J(195Pt–1H) = 24 Hz. e Almost
coincident peaks preclude more accurate values. f 3J(195Pt–1H) = 37 Hz. g 2J(31PA–31PB) = 26.8 Hz.
Fig. 2 Stick diagrams of the chemical shifts and relative intensities in the 11B NMR spectra of (a) nido-6-NB9H12, (b) [8,8-(PPh3)2-nido-8,7-RhNB9H11]
5
1, (c) [6-arachno-NB9H13]−, and (d) [9-Cl-8-(g -C5Me5)-nido-8,7-IrNB9H11] 6.
overall shielding of compound 1 [mean d(11B) −3.6 ppm; diagram
(b)] is much nearer to the nido-species [mean d(11B) −5.7 ppm;
diagram (a)] than to the arachno anion [mean d(11B) −15.4 ppm;
diagram (c)]. The shieldings at B(2)B(5) and B(1)B(6) in compound
1, relative to the respective corresponding B(2,4) and B(1,3) posi-
tions in the non-metalled species, are of particular interest because
a crossover of 11B(2,4) at lowest field and 11B(1,3) at highest field is
characteristic of a ten-vertex nido to arachno change. In this regard,
it can be seen in Fig. 2 that compound 1 [diagram (b)] retains the
highest and lowest shieldings at B(2) and B(6), respectively [corre-
sponding to B(2) and B(3) in the non-metallated species], which is a
general nido ten-vertex feature.16 Conversely, in a comparison of [6-
arachno-NB9H13]− with compound 6, there is a crossover of 11B(1)
at highest and 11B(5) at lowest shielding [corresponding to B(1)
and B(4), respectively, in the non-metallated compounds] typical
of an arachno ten-vertex cluster [diagram (c)]. Similar trends, as
demonstrated elsewhere,2 can be observed when [8,8-(PMe2Ph)2-
nido-8,7-PtCB9H11] is compared to the non-metallated nido and
arachno [6-CB9H12]− and [6-CB9H14]− anions; these results can be
extrapolated to the methoxy derivative dealt with in this paper.
2890 | Dalton Trans., 2007, 2885–2897
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systems for which a closo-geometry is at first sight predicted
by a basic application of these rules. However, both 1 and 2
have sixteen-electron metal centres, and a commonly adopted
simplistic application of the polyhedral skeletal electron pair
theory does not accommodate square-planar 16-electron metal
groups, because, as generally interpreted, this treatment always
assumes that the metal contributes three orbitals to the cluster
bonding, which implies an eighteen-electron octahedral metal
centre. By contrast, the iridaazaborane, 6,15 in which the iridium
atom has in fact an eighteen-electron metal configuration and
conforms with the three-orbital bonding assumption, agrees with
the electron-counting formalism.
Fig. 3 compares the 11B shielding patterns for [8,8-(PPh3)2-
nido-8,7-RhNB9H11] 1 and [8,8-(PMe2Ph)2-nido-8,7-PtCB9H11] 3
at high and low temperatures, and [9-(OMe)-8,8-(PMe2Ph)2-nido-
8,7-PtCB9H10)] 2 at room temperature. It is apparent that there is a
general parallel in the 11B shielding properties of equivalent sites in
the three species. The most significant deviation from this parallel
arises from the methoxy substitution at the B(9) site [Fig. 3(c)],
which results in a deshielding of ca. 15 ppm at that position
(a to OMe) and a concomitant shielding of ca. 10 ppm at B(6)
(c to the OMe), B(4) (b to the OMe) and B(10) (b to OMe).
The shift of the B(9) resonance to higher frequencies is expected
from the known substituent effect of an OMe group, which could
arise from the inductive electron-withdrawing effect (−I, electron
acceptance). Conversely, a degree of electron donation by the
methoxy substituent could be reflected by the accompanying shift
to lower frequencies of the c and b positions, as proposed by
Herˇma´nek.17
This comparative NMR work suggests that the {EB9H10X}
moiety (where E = C or N, X = H or OMe) in compounds
1 and 2, although exhibiting some intermediate character on
the heteroatom side to which the metal is bound, has more
similarities to the non-metallated nido substrates [nido-6-EB9H12]n
(where E = C and n = −1; or E = N and n = 0), than to
the non-metallated arachno ten-vertex substrates. These metalla-
heteroboranes may therefore be regarded as complexes of sixteen-
2+
+
electron {Pt(PMe2Ph)2} and {Rh(PPh3)2} metal moieties with
2−
−
the corresponding {nido-CB9H10(OMe)} and [nido-NB9H11}
groups acting as tetrahapto ligands with bidentate character: this
implies a two-orbital metal-to-heteroborane bonding interaction,
and a corresponding basic four-orbital square-planar metal co-
ordination sphere. This view of the eleven-vertex nido-clusters 1–
4 as square-planar sixteen-electron complexes would imply that
the metal site has Lewis acidic character in these compounds,
consistent with the observed reactivity of these species with Lewis
bases.4
By contrast, the 11B shielding pattern previously reported for
5
the azaborane fragment of the eleven-vertex species [9-Cl-8-(g -
C5Me5)-nido-8,7-IrNB9H11] 6,15 also shown in Fig. 2, diagram (d),
better resembles the pattern of the [arachno-6-NB9H13]− anion,
with the retention at highest and lowest field of the B(6)B(1)
and B(2)B(5) resonances, respectively [corresponding to B(1,3)
and B(2,4) in the non-metallated arachno species; diagram (c)],
a characteristic feature of ten-vertex arachno-compounds. This
resemblance suggests that the iridium-to-azaborane bonding in
this species could be described in terms of an iridium complex
5
2+
between {Ir(g -C5Me5)} and tridentate tetrahapto {arachno-
2−
An interesting feature of eleven-vertex nido-metallahetero-
boranes that incorporate sixteen-electron transition-element cen-
tres is their fluxional behaviour. Thus, in the species based on the
nido-[MEB9H10] unit [where M = Rh, E = NH (compound 1) or
NB9H11Cl} , with a three-orbital contribution to the cluster
framework from the formally octahedral iridium(III).
From the Wade–Williams electron-counting rules and cluster-
geometry approach,3 compounds 1 and 2 are 24-electron cluster
1
Fig. 3 Stick representation of the 11B–{ H} NMR spectra of compounds (a) [8,8-(PMe2Ph)2-nido-8,7-RhCB9H11] 3 at 383 K, (b) [8,8-(PMe2Ph)2-
nido-8,7-RhCB9H11] 3 at room temperature, (c) [9-(OMe)-8,8-(PMe2Ph)2-nido-8,7-RhCB9H11] 2 at room temperature, (d) [8,8-(PPh3)2-nido-8,7-RhNB9H11]
1 at 253 K, and (e) [8,8-(PPh3)2-nido-8,7-RhNB9H11] 1 at 370 K.
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S (compound 4); and M = Pt, E = CH (compound 3)], the
six resonances corresponding to B(1) and B(6), B(3) and B(11)
and B(4) and B(10) coalesce in pairs at higher temperatures,
whereas B(2), B(5) and B(9) are unchanged. Coalescence tem-
peratures yield DG‡ values of 48.4 kJ mol−1 at 270 K for
compound 1 and >75.0 kJ mol−1 at T > 398 K for compound
Calculations
Calculations using density functional theory methods (DFT) were
carried out on the model rhodathiaboranes, [8,8-(PH3)2-nido-8,7-
RhEB9H10] [where E = NH (1a), or S (4a)]. Calculated properties
include (i) the geometries of the clusters, (ii) approximate relative
energies of ground states and intermediates, and (iii) nuclear
magnetic chemical shielding properties using the GIAO approach.
The gross initial atomic coordinates for 1a and 4a were taken,
with appropriate modifications, from unrelated eleven-vertex
metallaheteroborane models such as [(PH3)2PtB10H12], previously
calculated in our laboratories. Selected energy-optimized inter-
atomic separations and angles for the model [8,8-(PH3)2-nido-
8,7-RhNB9H10] 1a are given in parentheses in Tables 3 and 4
respectively, together with the experimentally determined data for
[8,8-(PPh3)2-nido-8,7-RhNB9H11] 1. Data for [8,8-(PH3)2-nido-8,7-
RhSB9H10] 4a are listed in the ESI.†
1
2. In addition, 31P–{ H} experiments at various temperatures
on compound 1 demonstrate a concomitant exchange of the
two triphenylphosphine ligands. These results resemble those
for previously reported [8,8-(PMe2Ph)2-nido-8,7-PtCB9H11] 3 and
[8,8-(PPh3)2-nido-8,7-RhSB9H10] 4, which have activation energies
DG‡ for the fluxional process of 62.6 kJ mol−1 at 313 K (PMe 1H
resonance coalescence) and 58 kJ mol−1 at 338 K (31P resonance
coalescence), respectively.1,2
The coalescence of the 11B resonances can be attributed to the
{M(PR3)2} group (where M = Pt, R3 = Me2Ph; Rh, R3 = Ph3)
moving across the formal {EB9H10} ‘mirror plane’, with a simul-
taneous reverse shift of the bridging hydrogen atom (Scheme 1).
Concomitant exchange of the phosphine ligands in these species
implies additional rotation of the {M(PR3)2} group (where R3 =
Ph3 or PMe2Ph). For the compound [8,8-(PMe2Ph)2-nido-8,7-
PtCB9H11] 3 it has been demonstrated that the four P-methyl
proton resonances, a pair from each PMe2Ph ligand, coalesce
in pairs giving two signals in the 1H NMR spectrum.2 This
differs from a general rotation of the {Pt(PMe2Ph)2} unit above
the {C(7)B(3)B(4)B(9)B(10)B(11)} face of the monocarbaborane
fragment, which would lead to a single methyl proton resonance.
Thus the complete dynamic process may be described as a shift
of the platinum atom across the open face of the {CB9H10}
cluster, linked with a half rotation of the {(PMe2Ph)2} ligand
sphere (Scheme 1). Whereas the prochiral nature of the PMe2Ph
phosphine in species of this type allows the identification of
this half-rotation, the PPh3 ligands in the rhodaheteroboranes
[8,8-(PPh3)2-nido-8,7-RhEB9H10] (where E = NH or S) preclude
such a complete assessment of the character of the phosphine
interchange. Nevertheless, it is reasonable to assume that the half-
rotational process occurs in a similar fashion in all these eleven-
vertex [8,8-(L)2-nido-8,7-MEB9H10] species (where M = Rh, E =
NH or S, L = PPh3; M = Pt, E = CH, L = PMe2Ph). In contrast,
the methoxy-substituted platinacarbaborane, 2, does not exhibit
dynamic behaviour in either the 11B or 31P NMR spectra up to
the maximum studied temperature of 398 K. The presence of the
OMe substituent on the site adjacent to the metal centre obviously
therefore inhibits a ready fluxionality.
The agreement between the computed interatomic distances for
1a and those for the crystallographically characterized compound
1 is satisfactory. The calculated Rh–B(3) separation in 1 is 0.09 A
˚
longer than the experimental one, whereas the Rh–B(4) and Rh–
˚
˚
B(9) distances are only 0.04 A and 0.05 A longer, but these
are not unreasonable for DFT-calculated interatomic distances
involving second-row transition elements and currently available
basis sets. A better agreement is found between the computed
and experimental N–B and B–B distances as would be expected
for first-row main-group elements. A reasonable measure of the
validity of the calculated structures of these rhodaheteroboranes
is given by a comparison of the measured 11B NMR chemical shift
values and the boron nuclear shielding properties as calculated
via the GIAO approach. Thus, the 11B chemical shifts calculated
for 4a reproduce the experimental trend, and are sufficiently in
agreement to confirm the previously reported assignments.1 The
most significant differences occur for the low-field resonances of
the boron atoms B(3), B(9) and B(11) that are adjacent to the
metal and hetero atoms.
The 11B NMR spectrum of 1 was recorded for solutions in
different solvents at 253 K and 370 K. As mentioned above,
the low temperature spectrum is rather broad and precludes
resolution of the nine individual resonances that are expected for
the eleven-vertex asymmetric structure of the rhodaazaborane.
In the first instance it is therefore more instructive to compare
the calculated values with the higher temperature spectrum that
features narrower resonances, but for which the fluxional process
Scheme 1
2892 | Dalton Trans., 2007, 2885–2897
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Table 6 Measured 11B chemical shifts (d, ppm) for [8,8-(PPh3)2-nido-8,7-RhNB9H11] 1 in C6D5CD3 solution at 370 K and [8,8-(PH3)2-nido-8,7-RhSB9H10]
4 in CD2Cl2 at 298 K, compared with the corresponding calculated chemical shifts for their related P-hydryl models 1a and 4a
1
1a
4
4a
Assignment
Exptl.
Calcd. (mean)
Assignment
Exptl.
Calcd.
9
3,11
+18.2
+9.3
+18.1
+7.9, +11.4 (+9.7)
3
9
11
6
4
5
1
10
2
+16.3
+12.6
+6.0
+4.5
+2.5
+23.2
+18.8
+10.0
+7.3
1,6
5
−7.6
−9.9
−13.4, +2.5 (−5.5)
−12.5
+2.1
−9.3
−9.2
−18.6
−21.1
−27.5
−17.5
−19.3
−28.7
4,10
2
−14.0
−25.5
−0.1, −21.0 (−10.6)
−28.2
averages the shifts of particular pairs of resonances as detailed
above. The chemical shifts listed in Table 6 reveal a good
correlation between the experimental and calculated values. This
is particularly evident in the resonances due to the exchanging
pairs of atoms B(3,11), B(1,6) and B(4,10) for which the calculated
average values are close to the measured higher-temperature 11B
chemical shifts for 1.
The satisfactory agreement of the calculated and experimental
structural and nuclear shielding properties for 1/1a and 4/4a
suggests that DFT methods should provide a useful indication
of the likely transition states in the dynamic behaviour referred
to above. Therefore, the possible non-rigid intermediates through
the proposed half-rotational metal-flip (Scheme 1) have been
investigated using the calculated rhodathiaboranes [8,8-(PH3)2-
nido-8,7-RhEB9H10] [where E is NH (1a) or S (4a)] as limiting
structures in the fluxional pathway. We thence produced an
initial model for 4a using a transition state corresponding to one
previously postulated for this type of system,1 with a {RhH(PH3)2}
moiety capping the boat-shaped, six-membered face of the cluster
(central diagrams of Scheme 1 above), with overall Cs symmetry.
An initial energy minimization was carried out using the B3LYP
methodology and the 6-31G* basis sets. This minimization settled
out into an ‘isonido’ type of cluster geometry (Fig. 4), which
features a quadrilateral open face [S(7)Rh(8)B(9)B(10)], with the
Fig. 4 DFT-calculated energies (kJ mol−1) and intermediates, computed
at the B3LYP/6-31G*/LANL2DZ level for the fluxional process of
[8,8-(PH3)2-nido-8,7-RhSB9H10] 4a.
˚
Rh(8)–B(10) distance being non-bonding at 2.83 A (the initial
nido numbering as in Fig. 1 is maintained). A frequency analysis
showed no imaginary frequencies, suggesting that this structure
represents a stable local minimum close to the starting postulated
Cs transition-state model. Using this ‘isonido’ form in the STQN
calculation thence produced a transition state (designated TS,
Fig. 4) slightly higher in energy by ca. 1 kJ mol−1, with the
{RhH(PH3)2} moiety symmetrically positioned above the cluster,
featuring a Rh(8)–H–B(9) bridging bond in a Cs closo-type
eleven-vertex geometry (for simplicity the numbering for the
nido clusters is maintained). The highest point of the computed
reaction coordinate for the fluxionality therefore corresponds to
the formation of a transient eleven-vertex closo-rhodathiborane,
TS, with a saddle point of order 1: the standard condition-free
energy for TS is 49 kJ mol−1 above the ground state (Fig. 4).
For the rhodaazaborane model [8,8-(PH3)2-nido-8,7-
RhNB9H11] 1a, by contrast, no saddle point was derivable
in the STQN calculation, with the ‘transition state’ settling out
as a local minimum with no imaginary frequencies: the energy
differences between the ‘closo’ and ‘isonido’ conformations are
too small to be significant at the level of calculation used. This
finding now implies an activation energy of 23 kJ mol−1 above
the ground state, 26 kJ mol−1 lower than the calculated value of
49 kJ mol−1 for the sulfur compound 4a. This qualitatively follows
the experimentally measured trend: DG‡ for 4 being 58 kJ mol−1,
i.e. 10 kJ mol−1 higher than the 48 kJ mol−1 determined
experimentally for 1. For 1a, by contrast, the calculations did
not find an isonido-structured intermediate close to the transition
state, a further indication of the small differences of energy
that can occur between the isonido and closo geometries in this
eleven-vertex cluster area, as found for 4a. An important factor
behind the discrepancy between calculated and experimental DG‡
values is the different nature of the phosphine ligands in the
model compounds and the synthesized compounds; on simplistic
steric grounds, a higher activation energy may be expected for
PPh3 versus PH3.
The calculated intermediates in Fig. 4 represent points on
a geometrical continuum of distortion from the eleven-vertex
nido geometry to eleven-vertex closo-configuration as the metal
centre undergoes a half-rotational flip from side to side above
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atom by a CO ligand (2 × H versus CO implies the same number of
skeletal electrons) results in closure of the cluster to give the closo-
structured compound, [1-(CO)-1,3-(PPh3)2-closo-1,2-RhSB9H8].4
An analogous structural change is observed when the B(9)–B(10)
bridging hydrogen atom of either [8,8-(PPh3)-nido-8,7-RhSB9H10]
4 or [8,8-(dppe)-closo-8,7-RhSB9H10] 5 is removed as a proton
by strong bases, affording the corresponding closo rhodathiab-
orane anions, [1,1-(PPh3)2-1,2-RhSB9H9]− and [1,1-(dppe)-closo-
1,2-RhSB9H9]−,6 with no change in cluster-electron count. On
the other hand, incorporation of the bidentate phosphine dppm,
which bridges the rhodium atom and the B(3)-boron vertex,
favours the isonido configuration in 7 (schematic V above).19
A further example of the importance that ligands and hetero-
borane constituents have on the observed structure [and, by
analogy, the bonding] in the eleven-vertex system is shown by the
behaviour of the isoelectronic [9,9-(PEt3)2-nido-9,7,8-RhC2B8H11]
mentioned above. This compound exists as its closo isomer, [1,1-
(PEt3)2-1-(H)-1,2,3-RhC2B8H10] in the solid state, although the
nido form is observed to be in an interesting equilibrium with
the closo species in solution over a wide range of temperatures
(schematics III, IV).11
It has been suggested,6 and, interestingly, it seems to be currently
accepted in recent primary organometallic reference works,5 that
an ‘agostic’ interaction of an electron-pair in the phosphine-bound
ortho-hydrogen atoms (one electron from each PPh3 ligand) with
the rhodium atom in the rhodathiaborane 4 plays an important
and decisive role in the stabilization of nido versus closo geometries
in this type of compound. Moreover, it is proposed that, on
deprotonation of 4 and 5, the two Rh · · · H–C ‘agostic’ interactions
somehow switch off, affording a closo-cluster geometry. However,
it is not clear how two such interactions should be necessary to pro-
vide only two extra cluster electrons, since each of the two defined
agostic interaction should involve a C–H sigma electron pair, thus
involving a total of four electrons. As an alternative rationale, it has
been pointed out that there is no need to invoke such ‘agostic’ in-
teraction if the transition-element centre concerned is comfortable
with a sixteen-electron configuration.7 Germane to this question,
the calculations presented here for 1a and 4b, with model PH3
ligands, and which reasonably mirror the observed structures and
NMR parameters of 1 and 4, demonstrate that the clusters exhibit
a nido structure in the ground state, i.e. as isolated molecules, with-
out any possible contribution from agostic interactions from P–
aryl hydrogen atoms or from other molecules. On the other hand,
the experimental evidence indicates that the {MEB9} eleven-vertex
system is rather flexible, the observed structures of the polyhedral
clusters resulting from the balance of electronic changes promoted
by either metal-bound ligands, boron substituents, or the presence
of one or more bridging hydrogen atoms.
the {S(6)B(3)B(4)B(9)B(11)B(10)} face, this face resembling a
cyclohexane boat-type conformation. In this process, the B–H–B
bridging hydrogen atom shifts to a Rh–H–B bridging position to
attain a partial metal–hydride character in the transition state. To
assess the alternative open-face rotation, for 4a we calculated for
a half-rotation of the {Rh(PH3)2} centre, in which the bridging
hydrogen atom moves across through the sulfur side of the
pentagonal open face (rather than motion via the boron side as
illustrated in Scheme 1 and Fig. 4). The results of this calculation
showed that this route may be disregarded, as the intermediate
lies at considerably higher energy above the ground state, by ca.
92 kJ mol−1. This is not unexpected, because the lower-energy
process (Fig. 4) allows the moving hydrogen atom to continue to
be associated with the B(9) vertex in a B–H–M pseudo-bridging
manner (H–B distance 1.54 A; H–M distance 1.62 A in TS) and
thus retain the electron-pair in the Rh–H–B linkage in the cluster
bonding. This has similarities to the situation observed in the
crystallographic analyses and DFT calculations of the recently
published series of ten-vertex isonido-iridadecaborane clusters,
[(PPh3)2HIrB9H9(PPh3)].18
˚
˚
In terms of the calculated isonido-structured intermedi-
ates of Fig. 4, it may be noted that a directly analogous
eleven-vertex isonido species, [1-PPh3-{1,3-(l-dppm)}-isonido-1,2-
RhSB9H8] (7), which features a quadrilateral {Rh(1)S(2)B(7)B(4)}
open face, has previously been experimentally observed (schematic
V).19 The ‘isonido’ type of distortion is also observed in formally
closo {1,2,4-MC2B9} species as exemplified by [1,1,1-(PPh3)2H-
1,2,4-IrC2B8H10] (closo eleven-vertex numbering system), and
indicated by the hatched lines in schematic III above.20 It is
apparent that different ligands at the rhodium centre, often
allied with variation of substituents on the boron cage, can
stabilize closo geometries (versus nido) in the solid state, as
1
2
for example in [3-(g -dppm)-1,1-(g -dppe)-closo-1,2-RhSB9H8].19
These experimentally observed isonido and closo clusters can be
notionally considered as “frozen snapshots” of the intermediates
in the fluxional mechanism proposed here. There is also relevance
to the reported cluster rearrangement that renders all the vertices
equivalent on the NMR time scale in the closo-[B11H11]2− binary
borane dianion.21
Overall, the observed nido-type geometries of 1, 2, 3 and 4,
with the B–H–B bridging hydrogen atom on the B(9)–B(10)
edge, represent one of the limiting structures in an eleven-
vertex nido–isonido–closo structural continuum that exhibits a
rather smooth progression in the calculated structure/potential-
energy surface. Thus, it is not surprising that changes in cluster
substituents and constituents can lead to significant perturbations
of the energy surface, favouring different configurations in the
structural continuum. This is nicely exemplified by the parent
rhodathiaborane 4, in which a formal substitution of the BH(9)
terminal hydrogen atom and the B(9)–H–B(10) bridging hydrogen
Conclusions
Two new examples, [8,8-(PPh3)2-nido-8,7-RhNB9H11] 1 and [9-
(OMe)-8,8-(PPh3)2-nido-8,7-PtCB9H10] 2 of nido-type eleven-
vertex cluster geometry have been prepared and characterized.
Their structures, NMR properties and fluxionalities have been
compared with previously reported and closely related eleven-
vertex 12-skeletal-electron-pair species, such as the platinacarba-
borane 3 and the rhodathiaborane 4. As part of this work,
2894 | Dalton Trans., 2007, 2885–2897
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it has been established that the tetramethylnaphthalene-1,8-
diammonium salt of the [nido-6-NB9H11]− anion, viz. [tmndH]-
[NB9H11], is an air-stable reagent that provides a convenient
entry in the sparsely-examined area of metallaazaboranes. Thus,
compound 1 represents the first structurally characterized eleven-
vertex nido-rhodaazaborane.
in air. Dried and degassed solvents were used throughout.
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 pates of dimensions 20 × 20 cm.
Mass spectrometry was done on a V. G. AUTOSPEC instrument
using fast-atom bombardment (FAB; Cs+-ion bombardment of
a target solution in 3-nitrobenzyl alcohol; pressure gas phase
10−6 mmHg).
The previously proposed inter-enantiomeric fluxional be-
haviour in this eleven-vertex system, now nicely extended to
include the nitrogen-containing cluster compound 1, has been
studied by DFT calculations. These studies reveal that isonido-
shaped and closo-shaped clusters are points along the reaction co-
ordinate in the fluxional process that interchanges the two ground-
state enantiomeric eleven-vertex nido-cages. The potential-energy
profiles of the system demonstrate that, for the compounds exam-
ined, the nido cluster configuration is more stable than either of the
isonido or closo arrangements. The energy differences are small,
however, and this rationalises some of the general observations
within the eleven-vertex nido–isonido–closo structural continuum
for sets of cluster compounds with ostensibly formally equivalent
skeletal electron counts. The precise structures observed are largely
governed by the variation of either the metal-bound ligands or
the heteroborane cluster constituents or cluster substituents; as
such, observed species do not necessarily adhere rigidly to the
Wade–Williams cluster-geometry/electron-counting paradigm as
it is often commonly interpreted. Prime factors are (a) the nature
of the metal centre; preference for sixteen-electron versus eighteen-
electron transition-element configurations can be a dominant
factor here; and (b) the incidence or otherwise of hydrogen atoms
that can act either as terminal metal hydrides or be localised in
bridging positions on a cluster open face. In the systems studied,
we found no evidence to support the proposed importance of
Rh · · · C–H ‘agostic’ contacts or related interactions in dictating
nido structures versus otherwise predicted closo geometries. In
these cases, rather, the observed nido structures often reflect
the preference of the relevant metal centres for sixteen-electron
transition-element configurations rather than eighteen-electron
configurations. For the fluxionalities, in the pathway between the
limiting nido-shaped configurations, it seems likely that an internal
switch between sixteen-electron and eighteen-electron transition-
element centres may well play a significant role, involving a
transient increase by +2 in the formal oxidation state of the metal
centre (compare reference 22).
NMR spectroscopy
NMR spectroscopy was performed at 2.35 and/or 9.35 T on
commercially available instrumentation, with the general tech-
niques being essentially as detailed in other papers from our
laboratories,26 and using the general approach as most recently
summarised in reference 27. Chemical shifts d are given to high
frequency (low field) of N 100 MHz (SiMe4) for 1H (quoted
0.05 ppm), N 40.480 730 MHz (nominally 85% H3PO4) for 31P
(quoted 0.5 ppm) and N 32.083 971 MHz (nominally F3BOEt2 in
CDCl3) for 11B (quoted 0.5 ppm), N being defined as in reference
28. The chemical shifts were calibrated using solvent deuteron
or residual proton resonances as internal secondary standards.
Values for observed splittings arising from coupling constants
1J(11B–1H) are given 4 Hz.
Single-crystal X-ray diffraction analysis
Crystals of [8,8-(PPh3)2-8,7-nido-8,7-RhNB9H11] (compound 1)
and [9-(OMe)-8,8-(PMe2Ph)2-8,7-nido-8,7-PtCB9H10] (compound
2) suitable for the X-ray analyses were grown from concentrated
CH2Cl2 solutions by liquid–liquid diffusion of hexane. For crys-
tallographic details see Table 3.
CCDC reference numbers 637884 and 637885.
For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b702767b
Computations
Calculations carried out in this study were performed using the
Gaussian 98 and Gaussian 03 packages.29 Structures were initially
optimised with the STO-3G* basis-sets for B, C, P, S, N, H
and with the LANL2DZ basis-set for the metal atoms, using
standard ab initio methods. The final optimisations, including
frequency analyses to confirm the true minima, together with
GIAO NMR nuclear-shielding predictions, were performed using
B3LYP methodology, with the 6-31G* and LANL2DZ basis-sets.
GIAO NMR nuclear shielding predictions were performed on
the final optimised geometries. In each case the compounds were
modelled using hydrogen atoms rather than phenyl groups on the
phosphine ligands in order to reduce computation time. In the
results of all the structural calculations it is noted that all the
distances from metal to boron are somewhat longer than those
observed experimentally. We have found this to be a general feature
in DFT calculations of the structure of metallaboranes of second-
and third-row transition elements using the same basis-sets,
for which we have reported on ruthenaboranes,30 iridaboranes18
and platinaboranes.30 Other calculated intercluster connectivities,
e.g. interboron distances, much more closely match those deter-
mined experimentally. Transition-state optimizations were carried
out using the STQN method (synchronous transit-guided
In twelve-vertex closo-metallaheteroborane compounds,
a
5
metal-to-heteroborane g -rotation may be considered as a general
fluxional process:23 the fluxional mechanism delineated in this
present work may similarly be regarded as a common behavioural
process within the eleven-vertex nido-{MEB9}-cluster area when
the metal is formally a square-planar sixteen-electron centre.
Experimental
General
The
reagents
nido-6-NB9H12,10
[Me4N][nido-6-CB9H12],24
[RhCl(PPh3)3]25 and [cis-PtCl2(PMe2Ph)2],25 were prepared
by literature methods. N,N,Nꢀ,Nꢀ-Tetramethylnaphthalene-1,8-
diamine (tmnd) was obtained from commercial sources. The
reactions were carried out under an inert atmosphere of dry
nitrogen, although subsequent manipulations were carried out
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quasi-Newton), which finds a transition state that connects two
local minima on the potential energy surface.31
thank the Basque Government for a predoctoral Grant to RM,
and the Spanish Ministry of Education for financial support under
the “Ramo´n y Cajal Program”. Computing work performed by JB
was self-financed.
Syntheses
Synthesis of [tmndH][6-nido-NB9H11]. tmnd (1.7 g; 8 mmol)
was added to a colourless solution of [nido-6-NB9H12] (1 g; 8 mmol)
in hexane (ca. 50 ml). A pale yellow precipitate formed instantly.
This product was separated by filtration and dried under vacuum.
The yield of [tmndH][nido-6-NB9H11] was 2.0 g (74%). The salt is
a yellow solid that, by contrast with its neutral precursor, is air
stable.
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(compound 2). [cis-PtCl2(PMe2Ph)2] (100 mg; 0.18 mmol) and
[Me4N][nido-6-CB9H12] (39 mg; 0.2 mmol) were stirred in dry
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