A novel route to rhodaboratranes [Rh(CO)(PR3){B(taz) 3}]+via the redox activation of scorpionate complexes [RhLL′Tt]

Article abstract of DOI:10.1039/b910108j

The reaction of a mixture of the sodium salts of dihydrobis(4-ethyl-3- methyl-5-thioxo-1,2,4-triazolyl)borate, NaBt, and hydrotris(4-ethyl-3-methyl-5- thioxo-1,2,4-triazolyl)borate, NaTt, with [{Rh(cod)(μ-Cl)}₂] gave [Rh(cod)Bt] and [Rh(cod)Tt]

Full text of DOI:10.1039/b910108j

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A novel route to rhodaboratranes [Rh(CO)(PR3){B(taz)3}]+ via the redox
activation of scorpionate complexes [RhLL¢Tt]†
Robin J. Blagg,‡ Christopher J. Adams, Jonathan P. H. Charmant, Neil G. Connelly,* Mairi F. Haddow,
Alex Hamilton, James Knight, A. Guy Orpen and Benjamin M. Ridgway
Received 22nd May 2009, Accepted 3rd August 2009
First published as an Advance Article on the web 28th August 2009
DOI: 10.1039/b910108j
The reaction of a mixture of the sodium salts of dihydrobis(4-ethyl-3-methyl-5-thioxo-1,2,4-triazolyl)-
borate, NaBt, and hydrotris(4-ethyl-3-methyl-5-thioxo-1,2,4-triazolyl)borate, NaTt, with
[{Rh(cod)(m-Cl)}2] gave [Rh(cod)Bt] and [Rh(cod)Tt], which separately react with CO gas to give the
unstable dicarbonyl [Rh(CO)2Bt] and an equilibrium mixture of two isomers of [Rh(CO)2Tt] and
[(RhTt)2(m-CO)3], respectively. Tertiary phosphorus donor ligands react with the mixture of
[Rh(CO)2Tt] and [(RhTt)2(m-CO)3] to give [Rh(CO)(PR3)Tt] (R = Cy, NMe2, Ph or OPh) and
[Rh{P(OPh)3}2Tt] in which rhodium is bound to two sulfur atoms of the scorpionate ligand; the B–H
bond is directed towards the metal to give an agostic-like B–H ◊ ◊ ◊ Rh interaction. Dinuclear
3
[(RhTt)2(m-CO)3] has k [S3]-bound Tt ligands with a rhodium–rhodium bond bridged by three
carbonyls. In solution the mononuclear Tt complexes undergo rapid dynamic interchange of the three
3
thioxotriazolyl rings, probably via k [S3]-coordinated intermediates. The monocarbonyls
[Rh(CO)(PR3)Tt] (R = Cy, NMe2 or Ph) react with two equivalents of [Fe(h-C5H5)2][PF6] in the
presence of triethylamine to give the monocationic rhodaboratranes [Rh(CO)(PR3){B(taz)3}]+, with
boron NMR spectroscopy providing evidence for the boron–rhodium bond. In the solid state, rhodium
is bound to the three sulfur atoms and the boron of the B(taz)3 fragment, forming a tricyclo[3.3.3.0]
cage. The phosphine is trans to the Rh–B bond, the long Rh–P bond indicating a pronounced trans
influence for the coordinated boron. The cation [Rh(CO)(PPh3){B(taz)3}]+ reacts with [NBun ]I to give
4
[Rh(PPh3)I{B(taz)3}], in which the halide is trans to the Rh–B bond, and a second species, possibly
[Rh(CO)I{B(taz)3}]. The dirhodaboratrane [Rh2(PCy3){B(taz)3}2][PF6]2, a minor byproduct in the
synthesis of [Rh(CO)(PCy3){B(taz)3}][PF6], has a distorted square pyramidal rhodium atom with a
vacant site trans to the Rh–B bond. The second metal has four coordination sites filled by the sulfur
and boron atoms of a second B(taz)3 unit, the remaining octahedral sites occupied by two of the sulfur
atoms of the first B(taz)3 unit which therefore bridges the two rhodium atoms.
Introduction
The
archetypal
scorpionate
ligands,
the
N-donor
hydrotris(pyrazolyl)borates (e.g. Tp¢, Fig. 1), have been widely
used in coordination and organometallic chemistry, commonly
as analogues of the cyclopentadienyl ligand.1 However, in
recent years S-donor scorpionates have been reported, including
2
Fig. 1 The scorpionate ligands Tp¢, Tm and Tt.
hydrotris(2-thio-1-methyl-imidazolyl)borate {Tm or HB(mt)3}
and
hydrotris(4-ethyl-3-methyl-5-thioxo-1,2,4-triazolyl)borate
{Tt or HB(taz)3}; the latter is capable of acting as an S- or
N-donor depending on the metal centre.3 In addition to the
electronic effects of changing the donor atoms, there are steric
differences resulting from N- and S-donation, with two atoms
between the bridgehead (boron) and the bound metal for the
former and three for the latter.
Rhodium(I) complexes of Tp¢, e.g. [RhL2Tp¢] (Fig. 2), can adopt
a variety of structures, an extensive computational investigation
identifying four major isomers and the mechanism of interchange
3
between them.4 Two structures feature k -coordination of the scor-
School of Chemistry, University of Bristol, Bristol, UK BS8 1TS. E-mail:
Neil.Connelly@bristol.ac.uk; Fax: +44 (0)117 9290509; Tel: +44 (0)117
9288162
† Electronic supplementary information (ESI) available: Structures of 2,
7, 8, 12+ and 13+; proton NMR data for 2 and 8 at -80 ◦C. CCDC
reference numbers 733499–733503 and 733505–733509. For ESI and
crystallographic data in CIF or other electronic format see DOI: 10.
1039/b910108j
pionate, giving trigonal bipyramidal (TBP) or square pyramidal
(SPY) metal centres; in these Tp¢ can be viewed as analogous to a
cyclopentadienyl ligand, i.e. an anionic, face-capping, six-electron
donor. Interchange between the TBP and SPY isomers can result
in exchange of the donor pyrazolyl rings through a ‘reverse’
Berry pseudorotation. The other structures have Tp¢ coordinated
through two pyrazolyl rings to a square-planar metal centre,
‡ Present address: Department of Chemistry and Biochemistry, University
of Sussex, Falmer, Brighton, UK, BN1 9QJ. E-mail: rb250@sussex.ac.uk
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NaBH4. Performing the reaction under the conditions previously
reported3 invariably resulted in the generation of sodium dihy-
drobis(thioxotriazolyl)borate, NaBt, a species indistinguishable
from NaTt by either NMR spectroscopy or elemental analysis3,10
but distinguished by negative-ion electro-spray ionisation mass
spectrometry. Modification of the synthesis, by increasing the
reaction temperature to 175 ◦C and reducing the reaction time,
resulted in a solid comprising mainly of NaTt but still with 5–10%
NaBt, as suggested by mass spectrometry. Further improvement
of the product purity did not prove possible, and the presence of
a mixture of ligands therefore had to be accounted for during the
synthesis of Tt rhodium complexes.
Fig. 2 Structures of [RhL2Tp¢].
Synthesis and characterisation of rhodium(I)
(thioxotriazolyl)borate complexes
with the uncoordinated ring either pseudo-parallel to the square
plane (SP1) or directed away from the metal centre (SP2); these
structures are linked by inversion of the six-membered B(NN)2Rh
ring.
The complexes [Rh(cod)Bt] 1 and [Rh(cod)Tt] 2 were prepared
by reacting [{Rh(cod)(m-Cl)}2] with a mixture of the sodium
salts of the bis- and tris(thioxotriazolyl)borate scorpionates in
dichloromethane for 2.5 h, before separating the two species
by column chromatography and isolating the solid products.
Bubbling carbon monoxide through a CH2Cl2 solution of 1 gave
the unstable dicarbonyl [Rh(CO)2Bt] 3 {n(CO) 2067, 2002 cm-1},
while the reaction of CO with 2 under the same conditions
gave a mixture of three species in equilibrium: two isomers
of [Rh(CO)2Tt], 4a {n(CO) 2071, 2005 cm-1} and 4b {n(CO)
2050, 1982 cm-1}, and the carbonyl-bridged dirhodium complex
[(RhTt)2(m-CO)3] 5 {n(CO) 1826 cm-1}. Concentration of this
solution in vacuo drove the equilibrium towards 5, which was
then isolable in good yield. Addition of stoichiometric amounts of
tertiary phosphines to the CH2Cl2 solution of 4a, 4b and 5 gave,
over 1–2 h, the complexes [Rh(CO)(PR3)Tt] (R = Cy 6, NMe2 7,
Ph 8 or OPh 9) and [Rh{P(OPh)3}2Tt] 10, which were isolated as
yellow or orange solids.
A recent review details the wide ranging chemistry of rhodium
hydrotris(pyrazolyl)borates,5 including examples of C–H acti-
vation and of polymerisation catalysis. Studies6 of the redox
chemistry of rhodium(I) Tp¢ complexes resulted in the isolation
of rhodium(II) species stabilised through coordination of the third
pyrazolyl ring to rhodium. In contrast, rhodium complexes of
the S-donor ligands hydrotris(2-thio-1-R-imidazolyl)borate (R =
Me, Pri or But), made from either rhodium(I) or rhodium(III)
precursors, can undergo B–H bond activation to give octahedral
rhodaboratranes (with rhodium–boron bonds).7,8
We have briefly reported9 that the rhodium(I) complex
3
[Rh(CO)(PPh3)Tt] has a k [S2H]-coordinated scorpionate in the
solid state and that two-electron oxidation gives the rhod-
aboratrane complex [Rh(CO)(PPh3){B(taz)3}]+, the first to be
synthesised directly from a rhodium scorpionate precursor un-
der controlled reaction conditions. We now give details of the
synthesis, characterisation and electrochemistry of a wider range
of complexes of the form [RhLL¢Tt], and their two-electron
oxidation to yield a series of fully characterised rhodaboratranes,
the reactivity of which has also been explored.
Complexes 1, 2, 5–8 and 10 were characterised by X-ray
crystallography and, with 9, by elemental analysis, IR spec-
1
1
1
troscopy (Table 1) and 1H, 13C-{ H}, 31P-{ H} and 11B-{ H} NMR
spectroscopy (Table 2). Cyclic voltammetry has been used to probe
the redox processes of the Tt complexes 2 and 6–10.
The carbonyl stretching frequencies of 6–9 vary in accordance
with Tolman’s electronic parameter for phosphine ligands,11 as also
observed for their Tp¢ analogues.6 The two isomers of [Rh(CO)2Tt]
4 observed by IR spectroscopy are proposed to involve different
coordination modes of the Tt ligand, i.e. coordination of either two
Results and discussion
The scorpionate ligand Tt was synthesised as its sodium salt by the
reaction of 4-ethyl-3-methyl-5-thioxo-1,2,4-triazole, H(taz), with
Table 1 Analytical and IR spectroscopic data for [Rh(cod)Bt] 1, [RhLL¢Tt] 2, 6–10 and [(RhTt)2(m-CO)3}] 5
Analysis (%)a
Complex
Colour
Yield (%)
C
H
N
n(CO)/cm-1
[Rh(cod)Bt] 1
Yellow
6
42.2 (42.5)
42.3 (42.5)
33.9 (34.0)
48.2 (48.1)
34.0 (33.8)b
46.1 (45.9)b
46.2 (46.4)
53.0 (52.7)
5.8 (5.9)
5.9 (5.7)
4.4 (4.3)
7.0 (6.9)
5.4 (5.5)
4.5 (4.6)
4.4 (4.6)
4.7 (4.8)
16.2 (16.5)
19.1 (19.4)
21.8 (21.6)
14.9 (14.8)
20.5 (20.6)
13.7 (13.8)
14.4 (14.3)
10.5 (10.8)
—
—
[Rh(cod)Tt] 2
Yellow-orange
Yellow-brown
Yellow
Yellow-orange
Yellow
Yellow-orange
Yellow
33
84
61
65
75
40
65
[(RhTt)2(m-CO)3}] 5
[Rh(CO)(PCy3)Tt] 6
[Rh(CO){P(NMe2)3}Tt] 7
[Rh(CO)(PPh3)Tt] 8
[Rh(CO){P(OPh)3}Tt] 9
[Rh{P(OPh)3}2Tt] 10
1826
1956
1968
1977
2004
—
a Calculated values in parentheses. b Calculated as a 1 : 1 CH2Cl2 solvate.
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1
1
1
Table 2 Proton, 13C-{ H}, 31P-{ H} and 11B-{ H} NMR spectroscopic data for [Rh(cod)Bt] 1, [RhLL¢Tt] 2, 6–10 and [(RhTt)2(m-CO)3}] 5a
1
1
1
Complex
1H
13C-{ H}
31P-{ H}
11B-{ H}
[Rh(cod)Bt] 1
4.03 (br s, 4H, cod CH), 3.87
(q, 4H, 3JHH 7.2, Bt
165.90 (s, Bt C5), 149.14 (s, Bt
C3), 78.99 (d, 1JCRh 12.3, cod
CH), 39.83 (s, Bt 4-CH2CH2),
31.82 (s, cod CH2), 13.71 (s,
Bt 3-CH3), 11.11 (s, Bt
—
-7.93 (s, Bt BH2)b
-5.24 (s, Tt BH)
-5.17 (s, Tt BH)
4-CH2CH3), 2.46–2.30 (br m,
4H, cod CH2), 2.25 (s, 6H, Bt
3-CH3), 1.79 (q, 4H, JHH 8.2,
cod CH2), 1.19 (t, 6H, 3JHH
7.2, Bt 4-CH2CH3)b
4-CH2CH3)b
[Rh(cod)Tt] 2
4.27 (br s, 4H, cod CH), 4.01
148.61 (s, Tt C3), 80.99 (d,
1JCRh 10.9, cod CH), 39.88 (s,
Tt 4-CH2CH2), 31.99 (s, cod
CH2), 13.96 (s, Tt 3-CH3),
11.67 (s, Tt 4-CH2CH3)
—
(q, 6H, 3JHH 7.0, Tt
4-CH2CH3), 2.60–2.46 (br m,
4H, cod CH2), 2.33 (s, 9H, Tt
3-CH3), 1.81 (q, 4H, JHH 8.1,
cod CH2), 1.28 (t, 9H, 3JHH
7.0, Tt 4-CH2CH3)
[Rh(CO)(PCy3)Tt] 6
[Rh(CO){P(NMe2)3}Tt] 7
[Rh(CO)(PPh3)Tt] 8
4.06 (q, 6H, 3JHH 7.2, Tt
4-CH2CH3), 2.35 (s, 9H, Tt
3-CH3), 2.18-1.51 (br m, 33H,
167.36 (s, Tt C3), 28.66–26.36
(m, PCy3), 14.24 (s, Tt
3-CH3), 11.87 (s, Tt
50.63 (d, 1JPRh 155, PCy3)
PCy3), 1.32 (t, 6H, 3JHH 7.2, Tt 4-CH2CH3)
4-CH2CH3)
4.05 (q, 6H, 3JHH 7.3, Tt
4-CH2CH3), 2.74 {d, 18H,
3JHP 10.6, P(NMe2)3}, 2.35 (s,
9H, Tt 3-CH3), 1.31 (t, 6H,
3JHH 7.3, Tt 4-CH2CH3)
148.74 (s, Tt C3), 39.84 (s, Tt
126.14 {d, 1JPRh 207, P(NMe2)3} -5.37 (s, Tt BH)
4-CH2CH2), 39.33 {d, 2JCP
6.3, P(NMe2)3}, 14.05 (s, Tt
3-CH3), 11.73 (s, Tt
4-CH2CH3)
7.84–7.29 (m, 15H, PPh3), 3.88 148.76 (s, Tt C3), 134.55 (d,
40.79 (d, 1JPRh 158, PPh3)
118.44 {d, 1JPRh 272, P(OPh)3}
115.36 {d, 1JPRh 284, P(OPh)3}
—
-4.95 (s, Tt BH)
-4.78 (s, Tt BH)
-5.95 (s, Tt BH)
-2.46 (br s, Tt BH)
(br q, 6H, 3JHH 7.1, Tt
4-CH2CH3), 2.31 (s, 9H, Tt
3-CH3), 1.21 (t, 9H, 3JHH 7.1,
Tt 4-CH2CH3)
JCP 12.9, PPh3), 130.15 (d, JCP
2.3, PPh3), 128.37 (d, JCP
10.4, PPh3), 39.89 (s, Tt
4-CH2CH2), 13.99 (s, Tt
3-CH3), 11.67 (s, Tt
4-CH2CH3)
[Rh(CO){P(OPh)3}Tt] 9
[Rh{P(OPh)3}2Tt] 10
[(RhTt)2(m-CO)3}] 5
7.38–7.10 {m, 15H, P(OPh)3}, 149.09 (s, Tt C3), 129.95 {s,
3.93 (q, 6H, 3JHH 7.2, Tt
4-CH2CH3), 2.30 (s, 9H, Tt
3-CH3), 1.21 (t, 9H, 3JHH 7.2,
Tt 4-CH2CH3)
P(OPh)3}, 125.05 {s,
P(OPh)3}, 121.48 {d, JCP 5.8,
P(OPh)3}, 39.83 (s, Tt
4-CH2CH2), 14.09 (s, Tt
3-CH3), 11.67 (s, Tt
4-CH2CH3)
7.28–7.17 {m, 30H, P(OPh)3}, 152.24 (s, Tt C3), 129.70 {s,
3.81 (br s, 6H, Tt 4-CH2CH3), P(OPh)3}, 124.18 {s,
2.15 (s, 9H, Tt 3-CH3), 1.10
(br t, 9H, 3JHH 7.0, Tt
4-CH2CH3)
P(OPh)3}, 121.26 {d, JCP 2.9,
P(OPh)3}, 39.74 (s, Tt
4-CH2CH2), 13.97 (s, Tt
3-CH3), 11.50 (s, Tt
4-CH2CH3)
4.30 (dq, 6H, 2JHH 14.2, 3JHH
166.19 (s, Tt C5), 149.94 (s, Tt
7.0, Tt 4-CHHCH3), 3.90 (dq, C3), 39.73 (s, Tt 4-CH2CH2),
6H, 2JHH 14.2, 3JHH 7.0, Tt
4-CHHCH3), 2.41 (s, 18H, Tt
3-CH3), 1.29 (t, 18H, 3JHH 7.0,
Tt 4-CH2CH3)
14.54 (s, Tt 3-CH3), 11.67 (s,
Tt 4-CH2CH3)
a Chemical shift (d) in ppm, J values in Hz; spectra recorded in CD2Cl2 at 20 ◦C unless otherwise stated. b In CDCl3.
or three thioxotriazolyl rings to rhodium (4a and 4b respectively).
A comparison with n(CO) values calculated for various isomers
of [Rh(CO)2Tp¢]4 suggests that 4a has an SP1 or SP2 structure
while 4b has an SPY-type structure. The n(BH) absorbances,
measurement of which might have assisted assignment of the
scorpionate bonding mode, were not observed in solution or the
solid state for any of these species.
by allowing n-hexane to diffuse slowly into a concentrated CH2Cl2
solution of the complex at ca. -10 ◦C.
The rhodium atom of the Bt complex 1 (Fig. 3) is bound to
the scorpionate ligand through the two sulfur atoms, and to the
cod ligand by the two alkene bonds. One of the B–H bonds is
directed towards the rhodium centre, leading to an agostic-like
B–H ◊ ◊ ◊ Rh interaction, as also observed for the related complex
[Rh(cod){H2B(mt)2}], which was described as having a three-
centre–two-electron B–H ◊ ◊ ◊ Rh bond.12
The Tt complexes 2 (Fig. 4) and 6–8 (see Fig. 5 for 6 as a
representative example) adopt broadly similar geometries in the
solid state. As in 1, the rhodium atom is bound in a plane to a
Structures of complexes 1, 2, 5–8 and 10
The molecular structures of 1, 2, 5–8 and 10 were determined by
X-ray crystallography (Table 3). In each case, crystals were grown
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◦
˚
Table 3 Selected bond lengths (A) and angles ( ) for [Rh(cod)Bt] and [RhLL¢Tt]
[Rh(CO)(PCy3)-
Tt] 6
[Rh(CO){P(NMe2)3}-
Tt] 7
[Rh(CO)(PPh3)-
[Rh{P(OPh)3}2-
Tt] 10
[Rh(cod)Bt] 1
[Rh(cod)Tt] 2a
Tt] 8b
Rh(1)–X(1)c
Rh(1)–X(2)d
Rh(1)–S(1)
Rh(1)–S(2)
Rh(1) ◊ ◊ ◊ S(3)
Rh(1) ◊ ◊ ◊ B(1)
C(1)–O(1)
B(1)–N(1)
B(1)–N(4)
B(1)–N(7)
2.006
2.040
2.443(1)
2.375(1)
—
3.115
—
1.552(3)
1.550(3)
2.009, 2.017
2.003, 2.005
2.462(1), 2.372(1)
2.380(1), 2.445(1)
4.709, 4.953
3.233, 3.120
—
1.544(6), 1.534(6)
1.548(6), 1.541(6)
1.518(6), 1.544(6)
1.822(7)
2.239(2)
2.421(2)
2.385(2)
5.001
1.823(5)
2.251(1)
2.393(1)
2.458(1)
4.998
1.817(3)
2.264(1)
2.431(1)
2.414(1)
5.236
2.181(3)
2.188(2)
2.401(2)
2.395(2)
5.856
3.995
—
1.593(10)
1.560(10)
1.522(11)
3.362
3.465
3.239
1.152(8)
1.553(9)
1.550(10)
1.531(10)
1.122(7)
1.561(5)
1.557(10)
1.530(6)
1.152(4)
1.542(5)
1.537(5)
1.536(5)
S(1)–Rh(1)–S(2)
X(1)–Rh(1)–X(2)
S(1)–Rh(1)–X(1)
S(2)–Rh(1)–X(2)
95.34(3)
87.02
158.02
173.22
95.71(4), 94.40(5)
87.61, 87.17
170.52, 169.78
171.59, 163.12
95.38(8)
88.6(2)
1.778(2)
164.14(7)
95.18(4)
86.33(16)
174.95(15)
166.18(4)
93.62(4)
90.11(12)
162.59(12)
168.86(3)
96.10(8)
90.19(9)
171.04(8)
161.51(8)
N(1)–B(1) ◊ ◊ ◊ Rh(1)–S(1)
N(4)–B(1) ◊ ◊ ◊ Rh(1)–S(2)
N(7)–B(1)–Rh(1)–S(3)
S(3)–N(7)–B(1) ◊ ◊ ◊ Rh(1)
15.24
5.62
—
-31.66, -11.77
-29.82, -19.62
-13.22, -49.21
16.43, 66.10
-20.68
-23.98
-42.53
54.19
32.91
31.87
22.19
26.48
20.39
49.88
39.93
28.64
15.38
—
-27.38
-68.30
-18.36
a Two distinct molecules in the unit cell. b Data from reference 9. c X(1): 1, midpoint of C(12)–C(13); 2, midpoint of C(17)–C(18); 6–8, C(1); 10, P(1).
d X(2): 1, midpoint of C(16)–C(17); 2, midpoint of C(21)–C(22); 6–8, P(1); 10, P(2).
Fig. 3 Molecular structure of [Rh(cod)Bt] 1. (In this structure, and all
others in this paper, ellipsoids are shown at the 50% probability level; most
hydrogen atoms are omitted for clarity.)
Fig. 4 Molecular structure of [Rh(cod)Tt] 2. (Only one of the two distinct
molecules in the unit cell is shown.)
sulfur atom of each of two thioxotriazolyl rings and to the ancillary
ligand(s), either cod or CO and PR3. The Tt scorpionate has the
uncoordinated thioxotriazolyl ring directed away from, and the
B–H bond directed towards, the rhodium centre. By analogy with
their Tp analogues, these structures can be described as SP2-type
(Fig. 2).
The unit cell of 2 contains two independent but broadly
similar molecules. Though there are negligible differences in the
rhodium coordination sphere, the scorpionate ligands of the two
molecules have different conformations, specifically involving the
orientation of the uncoordinated thioxotriazolyl ring. Rotation
about the B(1)–N(7) bond leads to different positioning of the
uncoordinated sulfur atom, shown by the dihedral angles S(3)–
N(7)–B(1) ◊ ◊ ◊ Rh(1) (16.5 and 66.1◦ for the two molecules).
The bis(phosphite) complex 10 (Fig. 6) has, as with 2 and
6–8, an SP2-type structure with the approximately square planar
rhodium centre bound to two sulfur atoms and the two P(OPh)3
ligands. However, both phosphite ligands are displaced from the
idealised square plane to minimise steric interference; P(1) is raised
˚
by 0.25 A above the plane defined by Rh(1), S(1) and S(2) but P(2)
˚
is 0.69 A below the plane. (Similar distortions involving the two
phosphorus ligands of the rhodium(II) scorpionates [Rh(PPh3)2L]+
{L = Tp, Tp¢ or B(pz)4} result in ESR spectra showing coupling
of the unpaired electron to only one phosphorus nucleus.6) In
addition, the conformation of the scorpionate results in the
B–H bond being nearly parallel to the rhodium square plane, in a
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◦
˚
Table 4 Selected bond lengths (A) and angles ( ) for [(RhTt)2(m-CO)3] 5
Rh(1)–Rh(1¢)
Rh(1)–S(1)
Rh(1)–C(1)
C(1)–O(1)
B(1)-N(1)
Rh(1) ◊ ◊ ◊ B(1)
2.560(1)
2.481(1)
2.019(5)
1.161(7)
1.542(4)
4.063
S(1)–Rh(1)–S(1¢)
C(1)–Rh(1)–C(1¢)
S(1)–Rh(1)–C(1)
S(1)–Rh(1)–C(1¢)
S(1)–Rh(1)–C(1¢¢)
Rh(1)–C(1)–O(1)
95.60(3)
84.11(17)
87.15(8)
92.71(8)
170.95(10)
140.67(11)
N(1)–B(1) ◊ ◊ ◊ Rh(1)–S(1)
46.00
Fig. 5 Molecular structure of [Rh(CO)(PCy3)Tt] 6.
Fig. 7 Molecular structure of [(RhTt)2(m-CO)3] 5.
proton-decoupled 11B NMR spectrum shows a single resonance
at ca. dB -8 ppm, broadened in the proton-coupled 11B NMR
spectrum.
The Tt complexes 2 and 6–10 display the same general
features in their room temperature NMR spectra, with the three
thioxotriazolyl rings equivalent. Throughout, the BH resonances
are not observed. Proton-decoupled boron spectra show single
resonances at ca. dB -5 ppm, and the phosphorus spectra for
the carbonyl phosphines 6–9 are consistent with those of their Tp¢
analogues.6 The 31P NMR spectrum of the bis(phosphite) 10 shows
dP (115.4 ppm) lower by ca. 3 ppm, and 1JPRh (284 Hz) larger, than
for the carbonyl phosphite 9 (118.4 ppm and 272 Hz respectively).
At ca. -80 ◦C the three thioxotriazolyl rings of the Tt
ligand of 2 and 6–10 are no longer equivalent on the NMR
spectroscopic timescale, as previously observed for their hy-
drotris(pyrazolyl)borate analogues;6,15 two (for 2 and 10) or three
(for 6–9) thioxotriazolyl ring environments are observed in the
1H NMR spectra, corresponding to the different environments
observed in the solid state.
Fig. 6 Molecular structure of [Rh{P(OPh)3}2Tt] 10. (The phenyl rings
are omitted for clarity.)
˚
greater Rh ◊ ◊ ◊ B distance (ca. 4.0 A) than in the other Tt complexes
˚
(3.1–3.5 A), and in a reduced B–H ◊ ◊ ◊ Rh interaction.
The molecular structure of [(RhTt)2(m-CO)3] 5 (Fig. 7, Table 4)
3
is nearly D3h symmetric, with the Tt ligands k [S3]-coordinated to
two rhodium atoms which are linked by a metal–metal bond and
bridged by three carbonyls. The same crystallographically char-
acterised Rh(m-CO)3Rh units are found in [{MeGa(pz)3Rh}2(m-
CO)3]13 and [(CpCo{P(OEt)2O}3Rh)2(m-CO)3].14
NMR spectroscopy and fluxional behaviour
For 2 (Fig. 8), signals corresponding to the uncoordinated
thioxotriazolyl ring are at a higher chemical shift than those of
the coordinated rings, by ca. 0.1 ppm. The two methylene protons
of the ethyl groups on the coordinated rings are diastereotopic,
appearing as two doublets of quartets, because of slow rotation
about the N–CH2 bond. The low temperature 1H NMR spectrum
In solution, the two thioxotriazolyl rings of [Rh(cod)Bt] 1 are
equivalent, as observed in the solid state; the 1H NMR spectrum
shows six distinct proton resonances, three for cod and three for
the thioxotriazolyl rings; the BH2 protons are not observed. The
13C NMR spectrum shows all seven carbon environments and the
8728 | Dalton Trans., 2009, 8724–8736
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overlap of resonances renders complete assignment impossible.
The carbonyl phosphite complex 9 does not show significant
shielding effects on the thioxotriazolyl ring cis to P(OPh)3 as is
observed for that ring in [Rh(CO)(PPh3)Tt] 8; the phenyl rings of
P(OPh)3 in 9 are further from the metal centre than those of PPh3
in 8.
Overall, in solution the rhodium(I) Tt complexes undergo rapid
dynamic interchange of the three thioxotriazolyl rings, possibly
as shown in Scheme 1. Two square-planar isomers, SP1 and SP2,
the latter observed in the solid state, are linked by inversion of
the eight-membered Rh(SCN)2B ring. Coordination of the third
3
ring would give the k [S3]-coordinated SPY isomer (as observed in
solution for 4b) from which the three thioxotriazolyl rings might
undergo interchange through Berry pseudorotation via trigonal
bipyramidal intermediates.
1
Fig. 8 Variable temperature H NMR spectra of [Rh(cod)Tt] 2. (Peaks
marked with an asterisk are due to residual solvent/water.)
of the bis(phosphite) complex 10 is similar to that of 2 although the
methyl groups of the coordinated rings show significantly lower
chemical shifts than those of the uncoordinated ring (DdH 0.3–
0.4 ppm), perhaps due to shielding from the six phenyl rings
surrounding the metal centre.
It is possible to assign some of the resonances in the 1H NMR
◦
spectrum of 8 (Fig. 9) at -80 C to specific thioxotriazolyl rings.
By comparison with 2, the methyl groups of the uncoordinated
thioxotriazolyl ring are most probably those at 2.39 and 1.32 ppm.
For each thioxotriazolyl proton environment, one set of reso-
nances has a significantly lower chemical shift than the other two,
assigned to the ring cis to PPh3 due to the shielding effect of the
aromatic rings, i.e. resonances at 3.29, 3.11, 2.21 and 0.86 ppm.
The remaining methyl protons can therefore be assigned to the
ring trans to triphenylphosphine, although the methylene protons
cannot be unambiguously assigned.
Scheme 1 Mechanism for ring-interchange in [RhLL¢Tt].
3
Variable temperature NMR spectra of the k [S3]-coordinated,
carbonyl-bridged complex 5 did not show evidence of dynamic
processes, consistent with a rigid structure. The proton-decoupled
boron spectrum displayed a single resonance at a more positive
3
chemical shift than for any of the k [S2H]-coordinated ligands.
The redox chemistry of [RhLL¢Tt] and the synthesis and
characterisation of rhodaboratranes
Though repeatable, the cyclic voltammograms of analytically pure
samples of the rhodium(I) tris(thioxotriazolyl)borate complexes
2 and 6–10 are not as well defined as those of the rhodium(I)
tris(pyrazolyl)borate complexes [RhLL¢Tp¢].6,15 The CVs of 2 and
6–9 are broadly similar with two irreversible oxidation waves
separated by ca. 0.25 V; the dominant second wave has a peak
potential, (Ep)ox, of 0.39–0.49 V. An irreversible reduction wave at
ca. 0.0–0.2 V may be linked to the first oxidation process. The CV
of 10 shows only one irreversible oxidation wave, with (Ep)ox
0.46 V.
=
The reaction of complexes 6–8 with two equivalents of the one-
electron oxidant [Fe(h-C5H5)2][PF6] in the presence of an excess of
triethylamine gave, after 30 min, moderate yields of the monoca-
tionic rhodaboratrane complexes [Rh(CO)(PR3){B(taz)3}]+ (R =
Cy 11+, NMe2 12+ or Ph 13+) as their [PF6]- salts. These were
Fig. 9 Variable temperature 1H NMR spectra of [Rh(CO)(PPh3)Tt] 8 in
CD2Cl2. (Peaks marked with an asterisk are due to residual solvent.)
Although the variable temperature NMR spectra of 6, 7 and 9
also show three distinct thioxotriazolyl rings at low temperature,
1
1
characterised by elemental analysis, IR (Table 5), H, 13C-{ H},
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Table 5 Analytical and IR spectroscopic data for [Rh(CO)(PR3){B(taz)3}][PF6]
Analysis (%)a
C
Complex
Colour
Yield (%)
H
N
n(CO)/cm-1
[Rh(CO)(PCy3){B(taz)3}][PF6] 11+[PF6]-
[Rh(CO){P(NMe2)3}{B(taz)3}][PF6] 12+[PF6]-
[Rh(CO)(PPh3){B(taz)3}][PF6] 13+[PF6]-
Orange
Orange
Orange
29
24
26
40.8 (41.1)
30.0 (30.1)
42.1 (41.9)
5.5 (5.8)
5.0 (4.8)
4.2 (4.0)
12.8 (12.7)
19.1 (19.2)
12.9 (12.9)
2050
2055
2064
a Calculated values in parentheses.
1
1
1
Table 6 Proton, 13C-{ H}, 31P-{ H} and 11B-{ H} NMR spectroscopic data for [Rh(CO)(PR3){B(taz)3}][PF6]a
1
1
1
Complex
1H
13C-{ H}
31P-{ H}
11B-{ H}
[Rh(CO)(PCy3){B(taz)3}][PF6]
11+[PF6]-
3.92 (q, 4H, 3JHH 7.3, taz
4-CH2CH3), 3.88 (q, 2H, 3JHH
7.3, taz 4-CH2CH3), 2.46 (s,
6H, taz 3-CH3), 2.30 (s, 3H,
3-CH3), 2.12–1.24 (br m,
33H, PCy3), 1.36 (t, 9H, 3JHH
7.3, taz 4-CH2CH3)
155.12 (br s, taz C3), 40.50 (s,
2C, taz 4-CH2CH3), 40.38 (s,
1C, taz 4-CH2CH3), 34.11 (d,
JCP 9.21, PCy3), 30.07 (s,
PCy3), 27.87 (d, JCP 9.79,
PCy3), 26.62 (s, PCy3), 14.06
(s, 2C, taz 3-CH3), 13.94 (s,
1C, taz 3-CH3), 11.90 (s, 2C,
taz 4-CH2CH3), 11.82 (s, 1C,
taz 4-CH2CH3)
17.18 (v.br s, PCy3),
-143.94 (septet, 1JPF
710, PF6)
7.79 (d, 1JBRh 75.3,
B–Rh)
[Rh(CO){P(NMe2)3}{B(taz)3}][PF6] 3.91 (q, 2H, 3JHH 7.2, taz
155.12 (s, taz C3), 39.30 (s,
2C, taz 4-CH2CH3), 39.16 (s,
1C, taz 4-CH2CH3), 37.17
{d, 2JCP 6.9, P(NMe2)3},
12.88 (s, 2C, taz 3-CH3),
12.81 (s, 1C, taz 3-CH3),
10.73 (s, 2C, taz 4-CH2CH3),
10.65 (s, 1C, taz 4-CH2CH3)
114.13 {v.br s,
P(NMe2)3}, -143.92
(septet, 1JPF 714,
PF6)
7.58 (d, 1JBRh 105.9,
B–Rh)
12+[PF6]-
4-CH2CH3), 3.90 (q, 2H, 3JHH
7.2, taz 4-CH2CH3), 3.88 (q,
2H, 3JHH 7.2, taz 4-CH2CH3),
2.69 {d, 18H, 3JHP 9.9,
P(NMe2)3}, 2.45 (s, 6H, taz
3-CH3), 2.30 (s, 3H, 3-CH3),
1.35 (t, 6H, 3JHH 7.2, taz
4-CH2CH3), 1.12 (t, 3H, 3JHH
7.2, taz 4-CH2CH3)
[Rh(CO)(PPh3){B(taz)3}][PF6]
13+[PF6]-
7.56–7.43 (m, 15H, PPh3),
3.88 (q, 4H, 3JHH 7.4, taz
4-CH2CH3), 3.87 (q, 2H, 3JHH
7.4, taz 4-CH2CH3), 2.45 (s,
6H, taz 3-CH3), 2.32 (s, 3H,
3-CH3), 1.32 (t, 9H, 3JHH 7.2,
taz 4-CH2CH3)
133.80 (d, JCP 12.1, PPh3),
132.47 (d, JCP 29.9, PPh3),
131.31 (br s, PPh3), 129.55 (d,
JCP 9.2, PPh3), 40.63 (s, 2C,
taz 4-CH2CH3), 40.54 (s, 1C,
taz 4-CH2CH3), 14.22 (s, 2C,
taz 3-CH3), 14.12 (s, 1C, taz
3-CH3), 12.01 (s, 3C, taz
4-CH2CH3)
7.94 (v.br s, PPh3),
-143.93 (septet, 1JPF
714, PF6)
7.52 (d, 1JBRh 80.0,
B–Rh)
a Chemical shift (d) in ppm, J values in Hz; spectra recorded in CD2Cl2 at 20 ◦C.
1
1
31P-{ H} and 11B-{ H} NMR spectroscopy (Table 6) and X-ray
cis-phosphine, a difference also observed for the trans and cis
isomers of [Rh(PPh3)(CNR){B(mt)3}]+.7
crystallography.
The IR spectra of 11+–13+ show carbonyl bands in the
range 2050–2065 cm-1, compared with 1955–1980 cm-1 for their
rhodium(I) precursors 6–8. The large increase in n(CO) is con-
sistent with a decreased d-electron count at the metal centre.
The boron NMR spectra provide direct evidence for the
presence of a boron–rhodium bond, with doublets showing 1JBRh
between 75 and 105 Hz. Previously reported spectra for other
rhodaboratranes showed only a broad singlet, with a half height
width of ca. 50 Hz.16
The 1H and 13C-{ H} NMR spectra of the rhodaboratranes
1
11+–13+ show two distinct thioxotriazolyl ring environments,
two mutually trans and one trans to an ancillary ligand. The
31P NMR spectra showed very broad resonances for the co-
ordinated phosphines which were not further resolved at low
temperature. This broadening (half height widths ca. 200 Hz) is
proposed to be due to the quadrupole effect of the coordinated
boron nucleus, as is also observed for [Rh(PPh3)(CNR){B(mt)3}]+
and [Rh(PR3)(PR¢3){B(mt)3}]+ with half height widths of 60–
300 Hz.7 The resonances of the phosphine ligands are shifted to
more negative chemical shifts (DdP 10–30 ppm) than those of the
precursors 6–8. In the complexes [Rh(PR3)2{B(mt)3}]+ the trans-
phosphine resonance is less positive by 20–40 ppm than for the
The structures of complexes 11+–13+
The structures of the cationic rhodaboratranes 11+–13+ (see Fig. 10
for 11+ as an example) were determined by X-ray crystallography
(Table 7). Crystals were grown by allowing n-hexane to diffuse
slowly into a concentrated CH2Cl2 solution of the [PF6]- salt of
the complex at ca. -10 ◦C. The structures are broadly similar
with rhodium bound to the three sulfur atoms and the boron of
the B(taz)3 fragment, forming a tricyclo[3.3.3.0] cage. Of the two
ancillary ligands the phosphine is trans to the Rh–B bond in all
cases.
8730 | Dalton Trans., 2009, 8724–8736
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◦
+
˚
Table 7 Selected bond lengths (A) and angles ( ) for [Rh(CO)(PR3){B(taz)3}]
[Rh(CO)(PCy3){B(taz)3}]+ 11+
[Rh(CO){P(NMe2)3}{B(taz)3}]+ 12+
[Rh(CO)(PPh3){B(taz)3}]+ 13+a
Rh(1)–B(1)
Rh(1)–S(1)
Rh(1)–S(2)
Rh(1)–S(3)
Rh(1)–P(1)
Rh(1)–C(1)
C(1)–O(1)
B(1)–N(1)
B(1)–N(4)
B(1)–N(7)
2.173(5)
2.402(1)
2.388(1)
2.386(1)
2.550(1)
1.892(5)
1.101(6)
1.548(7)
1.548(6)
1.535(6)
2.176(2)
2.411(2)
2.372(1)
2.383(1)
2.485(1)
1.867(3)
1.118(3)
1.550(3)
1.545(3)
1.535(7)
2.153(5)
2.387(1)
2.386(1)
2.397(1)
2.495(1)
1.857(5)
1.146(6)
1.546(6)
1.534(6)
1.533(6)
B(1)–Rh(1)–S(1)
B(1)–Rh(1)–S(2)
B(1)–Rh(1)–S(3)
B(1)–Rh(1)–P(1)
B(1)–Rh(1)–C(1)
S(1)–Rh(1)–C(1)
S(2)–Rh(1)–S(3)
87.15(15)
83.75(14)
84.00(14)
173.08(15)
87.0(2)
172.87(14)
166.12(4)
86.19(8)
86.24(7)
84.58(7)
178.79(7)
86.19(10)
172.09(7)
167.01(2)
88.01(13)
84.37(13)
84.14(13)
176.60(13)
87.10(19)
175.10(15)
165.19(4)
N(1)–B(1)–Rh(1)–S(1)
N(4)–B(1)–Rh(1)–S(2)
N(7)–B(1)–Rh(1)–S(3)
0.6(3)
-25.7(3)
22.7(3)
3.37(13)
-20.81(13)
24.31(13)
0.8(3)
-22.6(3)
25.4(3)
a Data from reference 9.
A comparison of the structures of 11+–13+ with those of rhodab-
oratranes derived from Tm ligands, e.g. [Rh(PPh3)Cl{B(mt)3}],16
and of the ruthenaboratrane [Ru(CO)(PPh3){B(mt)3}]17 shows the
rigid tricyclo[3.3.3.0] cages B(taz)3Rh and B(mt)3M to have similar
bond lengths and angles. The ruthenium complex, isoelectronic
with [Rh(CO)(PPh3){B(taz)3}]+ 13+, also has a long M–P bond
17
˚
(2.435 A) trans to the M–B bond, cf . M–P bond lengths
between PPh3 and a six-coordinate metal centre (2.321 and
18
˚
2.370 A for Rh and Ru respectively). A long Rh–P bond
˚
(2.471 A) is also observed for the phosphine trans to boron in
[Rh(PMe3)2{B(mt)3}]+ {the cis bond (2.293 A) is significantly
˚
shorter}; the cations [Rh(PPh3)(CNR){B(mt)3}]+ also have the
phosphine, rather than the stronger p-acidic isocyanide CNR
(isoelectronic with CO), trans to boron.7
The structure of the dirhodaboratrane [Rh2(PCy3){B(taz)3}2]-
[PF6]2 142+2[PF6]- (Fig. 11 and Table 8) was determined from a
◦
˚
Table 8 Selected bond lengths (A) and angles ( ) for [Rh2(PCy3)-
{B(taz)3}2]2+ 142+
Fig. 10 Structure of the cation [Rh(CO)(PCy3){B(taz)3}]+ 11+.
Rh(1)–P(1)
Rh(1)–B(1)
Rh(1)–S(1)
Rh(1)–S(2)
Rh(1)–S(3)
Rh(2)–S(1)
Rh(2)–S(2)
Rh(2)–B(2)
Rh(2)–S(4)
Rh(2)–S(5)
Rh(2)–S(6)
B(1)–N(1)
B(1)–N(4)
B(1)–N(7)
B(2)–N(10)
B(2)–N(13)
B(2)–N(16)
2.360(1)
2.084(5)
2.419(1)
2.384(1)
2.369(1)
2.384(1)
2.632(1)
2.098(5)
2.389(2)
2.358(1)
2.369(2)
1.560(6)
1.531(6)
1.390(5)
1.545(6)
1.549(6)
1.546(6)
B(1)–Rh(1)–S(1)
B(1)–Rh(1)–S(2)
B(1)–Rh(1)–S(3)
B(1)–Rh(1)–P(1)
B(2)–Rh(2)–S(1)
B(2)–Rh(2)–S(2)
B(2)–Rh(2)–S(4)
B(2)–Rh(2)–S(5)
88.99(13)
87.21(14)
93.02(4)
103.27(13)
95.77(13)
173.80(14)
84.68(14)
88.52(13)
86.71(14)
95.21(4)
89.86(4)
-13.06(3)
-12.93(3)
7.0(3)
The Rh–P bond lengths in the rhodaboratranes 11+–13+ are
longer than those of their rhodium(I) precursors, 6–8, by up to
˚
0.3 A indicating the significant trans influence of the coordinated
boron; the Rh–S bonds are generally somewhat shorter. The
rhodium and boron coordination geometries are slightly distorted,
from octahedral and tetrahedral respectively. The two trans
thioxotriazolyl rings bridging the rhodium and boron centres are
twisted {as shown by the dihedral angles N(4)–B(1)–Rh(1)–S(2)
and N(7)–B(1)–Rh(1)–S(3)} to ensure the two centres optimise
their preferred geometry (octahedral and tetrahedral respectively).
The third ring (cis to the other two) shows negligible twisting {as
shown by the dihedral angle N(1)–B(1)–Rh(1)–S(1), in the range
0.0–3.5◦}. There is also little variation in the rigid cage structure
of the Rh(taz)3B unit in 11+–13+.
B(2)–Rh(2)–S(6)
Rh(1)–S(1)–Rh(2)
Rh(1)–S(2)–Rh(2)
Rh(1)–S(1)–Rh(2)–S(2)
Rh(2)–S(2)–Rh(1)–S(1)
N(1)–B(1)–Rh(1)–S(1)
N(4)–B(1)–Rh(1)–S(2)
N(7)–B(1)–Rh(1)–S(3)
N(10)–B(2)–Rh(2)–S(4)
N(13)–B(2)–Rh(2)–S(5)
N(16)–B(2)–Rh(2)–S(6)
-20.6(3)
33.8(3)
30.6(3)
6.9(3)
-17.9(3)
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31P NMR spectrum showed a single coordinated phosphine (dP
1
31.43 ppm, JPRh 120 Hz) corresponding to ca. 50% of the
phosphorus present, the remainder accounted for as free PPh3
1
and O PPh3. The 11B-{ H} NMR spectrum showed two sharp
=
singlets at dB 8.15 and 7.13 ppm, i.e. chemical shifts in the
region corresponding to rhodaboratranes. (However, the signal
at 8.15 ppm may be half of a doublet, with 1JBRh 95 Hz, the other
component of which is obscured by the more intense peak at
7.13 ppm.) The analogous reaction of 11+[PF6]- with [NBun ]I
4
showed no apparent change in the carbonyl IR spectrum but the
31P and 11B NMR spectra of the solid residue showed a mixture of
new species.
The crude solid obtained from the reaction of 13+[PF6]- with
[NBun ]I was purified by slow diffusion of n-hexane into a
4
concentrated CH2Cl2 solution of the product at ca. -10 ◦C,
giving a mixture of orange crystals and a yellow-orange solid.
X-Ray studies identified the orange crystals as the rhodaboratrane
[Rh(PPh3)I{B(taz)3}] 15. The possible nature of the yellow-orange
solid is briefly discussed below.
The molecular structure of [Rh(PPh3)I{B(taz)3}] 15 (Fig. 12,
Table 9) is very similar to that of [Rh(PPh3)Cl{B(mt)3}].16 The
octahedral rhodium centre is bound to the boron and three sulfur
atoms of the B(taz)3 unit, giving a tricyclo[3.3.3.0] cage, along with
the phosphine and halide ligands. The iodide is trans to the Rh–B
Fig. 11 Structure of the dication [Rh2(PCy3){B(taz)3}2]2+ 142+
.
single crystal selected from a sample of 11+[PF6]-. (The dication
may therefore be a decomposition product of 11+.)
The dication 142+ has one distorted square pyramidal rhodium
atom, Rh(1), bound to one phosphine and the three sulfur atoms
and boron of a B(taz)3 unit; there is a vacant coordination site
trans to the Rh–B bond. Space-filling models show the vacant
coordination site surrounded by cyclohexyl and ethyl groups, thus
providing significant steric bulk which, with the trans influence of
the coordinated boron, deters coordination of another ligand. The
second rhodium atom, Rh(2), is octahedral with four coordination
sites filled by the sulfur and boron atoms of a second B(taz)3 unit.
The remaining two sites are occupied by two of the sulfur atoms
of the first B(taz)3 unit which therefore bridges the two rhodium
atoms. The trans influence of coordinated boron is also observed
˚
bond with Rh(1)–I(1) (2.936(1) A), much longer than the mean
˚
Rh–I bond distance (2.715 A) quoted in ref. 18, reflecting the trans
influence of the coordinated boron. Accordingly, the Rh–P bond
˚
length (Rh(1)–P(1) = 2.335(1) A) is much closer to that in the
˚
˚
scorpionate complex [Rh(CO)(PPh3)Tt] 8 (2.264(1) A) than in the
+
+
boratrane [Rh(CO)(PPh3){B(taz)3}] 13 (2.495(1) A) where the
phosphine is trans to boron.
˚
in the increased length of the Rh(2)–S(2) bond {2.632(1) A},
compared with the other rhodium–sulfur bonds present (ca.
˚
2.4 A).
The only other dirhodaboratrane complex so far reported is
[Rh2{B(mt)3}2Tm]Cl, obtained in low yield from the decomposi-
tion of [Rh(PPh3)Cl{B(mt)3}] or by reacting [Rh(cod){B(mt)3}]Cl
with an excess of NaTm in CHCl3 for five days.7 The cation
[Rh2{B(mt)3}2Tm]+ and 142+ show similar structural features,
particularly of the [Rh2(BR3)2]2+ (R = mt or taz) core unit.
However, the P-bound and vacant sites on one rhodium of the
latter are chelated by a Tm ligand in the former.
Reactions of the rhodaboratranes
In order to obtain Tt-derived rhodaboratranes which
can be directly compared with known species such as
[Rh(PPh3)Cl{B(mt)3}],16 stoichiometric amounts of the halide
Fig. 12 Molecular structure of [Rh(PPh3)I{B(taz)3}] 15.
salt [NBun ]I were added to dilute CH2Cl2 solutions of the
4
rhodaboratrane salts 11+[PF6]- and 13+[PF6]-. Upon completion
of the reaction, as judged by IR spectroscopy, the solvent was
removed in vacuo to give crude solid products which were analysed
by IR and NMR spectroscopy.
The IR and NMR spectra of the mixture of orange crystals and
yellow-orange solid noted above showed the continued presence
of two species after purification of the initial product of the
reaction between [Rh(CO)(PPh3){B(taz)3}][PF6] 13+[PF6]- and
1
The reaction of 13+[PF6]- with [NBun ]I over one hour resulted
[NBun ]I. The single doublet in the 31P-{ H} NMR spectrum can
4
4
in the observation of a single n(CO) band, at 2055 cm-1. The
be assigned to 15, the absence of the quadrupole effect of the
8732 | Dalton Trans., 2009, 8724–8736
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and a two-centre–two-electron rhodium–boron dative bond.22
However, Parkin noted that coordination of a formally Lewis
acidic ligand (BR3 in this case) to a metal centre results in
the transfer of two electrons from a non-bonding d-orbital into
Table 9 Selected bond lengths (A) and angles (◦) for
[Rh(PPh3)I{B(taz)3}] 15
˚
Rh(1)–B(1)
Rh(1)–S(1)
Rh(1)–S(2)
Rh(1)–S(3)
Rh(1)–P(1)
Rh(1)–I(1)
B(1)–N(1)
B(1)–N(4)
B(1)–N(7)
2.117(5)
2.407(1)
2.372(1)
2.379(1)
2.335(1)
2.936(1)
1.561(6)
1.539(4)
1.527(6)
the metal–boron bond, resulting in a change in the metal d
23
configuration from dn to dn-2
.
Adopting the latter approach,
formal oxidation of the d8 metal centre of 6–8 would give the
d6 octahedral rhodaboratrane complexes 11+–13+, consistent with
the need for two equivalents of a one-electron oxidant to synthesise
the boratrane cations, and with the accompanying large increase
in energy of n(CO) of ca. 90 cm-1 (e.g. from 1956 cm-1 in 6 to
2050 cm-1 in 11+).
B(1)–Rh(1)–S(1)
B(1)–Rh(1)–S(2)
B(1)–Rh(1)–S(3)
B(1)–Rh(1)–P(1)
B(1)–Rh(1)–I(1)
S(1)–Rh(1)–P(1)
S(2)–Rh(1)–S(3)
88.18(14)
83.49(14)
84.79(14)
95.53(14)
165.29(13)
174.73(4)
168.12(4)
DFT calculations8 on [IrCl(PH3){B(mt)3}] show a d6 configu-
ration for the metal centre {formally Ir(III), though see ref. 8 for a
discussion of the relationship between d configuration and oxida-
tion state in boratrane and related species} though it is interesting
to note that similar calculations for [AuCl{BPh(C6H4PR2-2)2-
N(1)–B(1)–Rh(1)–S(1)
N(4)–B(1)–Rh(1)–S(2)
N(7)–B(1)–Rh(1)–S(3)
-3.73
-28.73
23.00
3
k B,P2}] (R = Ph or Pri), coupled with 197Au Mossbauer studies,
showed24 a d10 [Au(I)] configuration.
In both DFT studies, analysis of the frontier orbitals revealed
a three-centre B–M–Cl interaction, i.e. the metal–boron is part
of a three-centre–four-electron (3c–4e) bond also involving the
trans ligand. These two examples appear to represent the extremes
with respect to the electron distribution within an M–B bond of
a boratrane and, not unexpectedly, suggest that this distribution
depends on the metal and its ancillary ligands. In the end, one
might argue that a definitive description of the M–B bond is
less important than the chemistry which novel boratrane and
related species might undergo though, of course, a more detailed
understanding should eventually lead to more systematic synthetic
applications.
boron nucleus on the cis-coordinated phosphine leading to a sharp
signal, in contrast to the broad resonances observed for the trans-
coordinated phosphines in 11+–13+. One of the two sharp singlets
1
in the 11B-{ H} NMR spectrum of the mixture can also be assigned
to 15, leaving the other, and the carbonyl IR band at 2055 cm-1, to
be assigned to a second rhodaboratrane, reasonably assumed to
be [Rh(CO)I{B(taz)3}] 16.
The mechanism of conversion of rhodium scorpionate complexes to
rhodaboratranes by electron transfer, and comments on the
metal–boron bond in the latter
The formation of the boratranes [Rh(CO)(PR3){B(taz)3}]+ 11+–
13+ from the scorpionates [Rh(CO)(PR3)Tt] 6–8 requires re-
action with two equivalent of the one-electron oxidant [Fe(h-
C5H5)2]+ in the presence of a base, i.e. the overall pro-
cess involves the loss of two electrons and one proton. In
similar redox-induced C–H activation reactions, e.g. where
[Ru2(m-CH)(m-CO)(m-Ph2PCH2PPh2)(h-C5H5)2]+ and [Mo2(m-
C8Me7CH2)(h-C5H5)2]+ are formed from [Ru2(m-CH2)(m-CO)(m-
Ph2PCH2PPh2)(h-C5H5)2]19 and [Mo2(m-C8Me8)(h-C5H5)2]20 re-
spectively, a mechanism involving proton loss after two-electron
oxidation is supported by well-defined cyclic voltammetric studies.
However, in the absence of as clearly defined voltammetry
(see above), the point at which the B–H hydrogen atom is
lost in the scorpionate-boratrane conversion is not known, nor
whether that loss is metal-assisted, i.e. it is preceded by hydrogen
migration from boron to rhodium. Reversible such migration
is observed in, for example, the reaction of [PtH{P(C6H4Me-
4)3}{B(mt)3}]+ with phosphines to give [PtH(PR3){B(mt)3}]+ via
[Pt(PR3)2{HB(mt)3}]+.21
Conclusions
The
hydrotris(4-ethyl-3-methyl-5-thioxo-1,2,4-triazolyl)borate
(Tt) ligands of [Rh(CO)(PR3)Tt] (R = Cy, NMe2, Ph or
2
OPh) are k [S2]-bound in the solid state with the B–H bond
part of an agostic-like B–H ◊ ◊ ◊ Rh interaction. In solution the
three thioxotriazolyl rings undergo rapid exchange at room
temperature, probably via k [S3]-coordinated intermediates and
Berry pseudorotation.
With two equivalents of [Fe(h-C5H5)2][PF6] in the presence
of triethylamine [Rh(CO)(PR3)Tt] (R = Cy, NMe2 or Ph) give
the monocationic rhodaboratranes [Rh(CO)(PR3){B(taz)3}]+ in
which three sulfur atoms and the boron of the B(taz)3 fragment,
form a tricyclo[3.3.3.0] cage with the metal. The long Rh–P bond
trans to the coordinated boron indicates a pronounced trans
influence which also results in a longer Rh–S bond trans to boron
in the dirhodaboratrane [Rh2(PCy3){B(taz)3}2]2+.
3
Experimental
That boratranes can be formed from scorpionates by electron
transfer, at least for rhodium, has possible implications for the
way in which the M–B bond in the former might be described.
There has been some debate about the appropriate description
of such bonds, including in Tm-derived rhodaboratranes.22,23 Hill
has proposed the notation (M→B)n to represent the metal–
boron bonding unit with, for example, [RhClL{B(mt)3}](Rh→B)8
indicating eight electrons in the rhodium non-bonding d-orbitals,
The preparation, purification and reactions of the complexes
described were carried out using Schlenk techniques under an
atmosphere of dry nitrogen, using solvents dried by Anhydrous
Engineering double alumina or alumina/copper catalyst columns
and deoxygenated prior to use. Unless stated otherwise, complexes
are stable under nitrogen and dissolve in polar solvents such as
CH2Cl2 and thf to give air-sensitive solutions. The compounds
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[{Rh(cod)(m-Cl)}2],25 H(taz)3 and [Fe(h-C5H5)2][PF6]26 were pre-
pared by published methods. A mixture of the compounds NaBt
and NaTt was prepared by a modification of the literature
procedure.3
[Rh(CO)(PCy3)Tt] 6. Carbon monoxide was bubbled through
a solution of 2 (154 mg, 181 mmol) in CH2Cl2 (50 cm3) for 15 min.
The phosphine PCy3 (60 mg, 214 mmol) was then added and the
mixture stirred for a further 60 min. Rapid addition of n-hexane
and concentration of the mixture in vacuo gave a yellow-orange
solid, isolated by filtration. The crude solid was redissolved in
CH2Cl2 then filtered, treated with n-hexane and cooled to -10 ◦C.
The precipitate was isolated by filtration and dried in vacuo to give
a yellow solid, yield 121 mg (61%).
Solution IR spectra in CH2Cl2 were recorded on a Perkin Elmer
Spectrum One FT-IR spectrometer. NMR spectra were recorded
on a JEOL Eclipse 300 spectrometer, operating at 299.9 MHz for
1H, at 75.4 MHz for 13C, at 96.2 MHz for 11B and at 121.4 MHz
1
for 31P, using JEOL Delta software. For 1H and 13C-{ H} spectra,
The complexes [Rh(CO){P(NMe2)3}Tt] 7, [Rh(CO)(PPh3)Tt] 8
either SiMe4 or residual protio solvent was used as an internal
1
standard. For 31P-{ H} spectra, 85% H3PO4 was used as an
and [Rh(CO){P(OPh)3}Tt] 9 were prepared similarly.
1
external standard. For 11B-{ H} spectra, BF3·OEt2 was used as an
[Rh{P(OPh)3}2Tt] 10. Carbon monoxide was bubbled
through a solution of 2 (89 mg, 137 mmol) in CH2Cl2 (30 cm3)
for 15 min. An excess (ca. 0.1 cm3) of P(OPh)3 was then added and
the reaction mixture stirred for a further 75 min. Evaporation of
the mixture to dryness gave a yellow-orange solid. The crude solid
was redissolved in CH2Cl2 then filtered, treated with n-hexane and
cooled to -10 ◦C. The precipitate was isolated by filtration and
dried in vacuo to give a yellow solid, yield 104 mg (65%).
external standard. Electrochemical studies were carried out using
an EG&G model 273A potentiostat linked to a computer using
either EG&G Model 270 Research Electrochemistry software or
PAR Power Suite software, in conjunction with a three-electrode
cell. The working electrode was a glassy carbon disc (3.0 mm
diameter) and the counter-electrode a platinum wire. The reference
was an aqueous saturated calomel electrode separated from the
test solution by a fine porosity frit and an agar bridge saturated
with KCl. Solutions in CH2Cl2 were 1.0 ¥ 10-3 mol dm-3 in
[(RhTt)2(l-CO)3] 5. Carbon monoxide was bubbled through
a solution of 2 (152 mg, 234 mmol) in CH2Cl2 (25 cm3) for 30 min.
The solvent was then removed in vacuo, giving a yellow-brown
solid. The crude solid was redissolved in CH2Cl2 then filtered,
treated with n-hexane and cooled to -10 ◦C. The precipitate was
isolated by filtration and dried in vacuo to give a yellow-brown
solid, yield 114 mg (84%). All manipulations of this light-sensitive
complex were carried out in the absence of light.
the test compound and 0.1 mol dm-3 in [NBun ][PF6] as the
4
supporting electrolyte. Under the conditions used, E◦¢ for the
one-electron oxidation of [Fe(h-C5H4COMe)(h-C5H5)] or [Fe(h-
C5Me5)2], added to the test solutions as an internal calibrant is
0.74 and -0.08 V, respectively.
Microanalyses were carried out by the staff of the Microanalyt-
ical Service of the School of Chemistry, University of Bristol.
[Rh(CO)(PCy3){B(taz)3}][PF6] 11+[PF6]-. To a stirred solu-
tion of 6 (101 mg, 119 mmol) in CH2Cl2 (65 cm3) was added ca.
0.1 cm3 triethylamine and [Fe(h-C5H5)2][PF6] (79 mg, 239 mmol).
After 20 min the solvent was removed in vacuo and the crude
product washed with 3 ¥ 20 cm3 portions of n-hexane and then
dried in vacuo. The solid was dissolved in ca. 2 cm3 CH2Cl2 and
filtered through a short activated (grade III) alumina/CH2Cl2
chromatography column. A yellow CH2Cl2 solution was eluted
which was concentrated in vacuo to ca. 5 cm3. Addition of n-
hexane to the concentrate and cooling the mixture to -10 ◦C gave
an orange solid which was dried in vacuo, yield 34 mg (29%).‡
The compounds [Rh(CO){P(NMe2)3}{B(taz)3}][PF6] 12+[PF6]-
and [Rh(CO)(PPh3){B(taz)3}][PF6] 13+[PF6]- were prepared simi-
larly, from 7 and 8 respectively.
Syntheses
NaBt and NaTt. A mixture of H(taz) (3.93 g, 27.5 mmol) and
NaBH4 (331 mg, 8.75 mmol) was heated to ca. 175 ◦C for 3 h;
hydrogen gas was evolved above 130 ◦C. The white solid product
was cooled to room temperature, washed in a Soxhlet apparatus
with hot chloroform for 6 h, then dried in vacuo, to give 3.13 g of
a mixture comprising 90–95% NaTt and 5–10% NaBt.
[Rh(cod)Bt] 1 and [Rh(cod)Tt] 2. A solid mixture of NaBt and
NaTt (1.831 g) was added to a yellow solution of [{Rh(cod)(m-
Cl)}2] (944 mg, 1.91 mmol) in CH2Cl2 (50 cm3). The mixture was
stirred for 2.5 h then filtered through Celite, giving an orange
solution which was concentrated in vacuo to ca. 5 cm3. The con-
centrate was loaded onto an activated (grade III) alumina/CH2Cl2
chromatography column. Using CH2Cl2 as the mobile phase,
yellow and orange-red bands separated. The first yellow band
was eluted as a yellow CH2Cl2 solution; the second orange-red
band was eluted as an orange CH2Cl2 : thf (3 : 1) solution. For
each solution, the solvent was removed in vacuo and the residue
redissolved in a minimum volume of CH2Cl2. The solution was
then filtered, treated with n-hexane and cooled to -10 ◦C. The
precipitated product was isolated by filtration and dried in vacuo.
Complex 1 was obtained from the first band as a yellow solid,
yield 120 mg (6%, based on rhodium) and complex 2 from the
second band as a yellow-orange solid, yield 821 mg (33%, based
on rhodium).§
Structure determinations
Many of the details of the structure analyses of [Rh(cod)Bt]
1, [Rh(cod)Tt]·0.5H2O 2·0.5H2O, [(RhTt)2(m-CO)3}] 5,
[Rh(CO)(PCy3)Tt]·CH2Cl2 6·CH2Cl2, [Rh(CO)(PPh3)Tt]·CH2Cl2
8·CH2Cl2, [Rh{P(OPh)3}2Tt]·1.5CH2Cl2 10·2CH2Cl2, [Rh(CO)-
{P(NMe2)3}{B(taz)3}][PF6] 12+[PF6]-, [Rh(CO)(PPh3){B(taz)3}]-
[PF6]·2CH2Cl2 13+[PF6]-·2CH2Cl2, [Rh2(PCy3){B(taz)3}2][PF6]2·
4CH2Cl2 142+2[PF6]-·4CH2Cl2, and [Rh(PPh3)I{B(taz)3}] 15
{carried out on Bruker SMART or APEX diffractometers
˚
using Mo-Ka X-radiation (l = 0.71073 A)} and of [Rh(CO)-
{P(NMe2)3}Tt]·2CH2Cl2 7·2CH2Cl2 and [Rh(CO)(PCy3)-
{B(taz)3}][PF6]·3CH2Cl2 11+[PF6]-·3CH2Cl2 {carried out on a
Bruker PROTEUM diffractometer using Cu-Ka X-radiation
˚
(l = 1.54178 A)} are listed in Table 10. The structures of
§ Low yields are partially accounted for by adsorption of the product onto
the alumina of the chromatography column.
[Rh(CO)(PPh3)Tt]·CH2Cl2 8·CH2Cl2 (CSD refcode: QEKDAM)
8734 | Dalton Trans., 2009, 8724–8736
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Table 10 Crystal and refinement data for [Rh(cod)Bt] 1, [Rh(cod)Tt]·0.5H2O 2·0.5H2O, [(RhTt)2(m-CO)3}] 5, [Rh(CO)(PCy3)Tt]·CH2Cl2 6·CH2Cl2,
[Rh(CO){P(NMe2)3}Tt]·2CH2Cl2 7·2CH2Cl2, [Rh{P(OPh)3}2Tt]·1.5CH2Cl2 10·2CH2Cl2, [Rh(CO)(PCy3){B(taz)3}][PF6]·3CH2Cl2 11+[PF6]-·3CH2Cl2,
[Rh(CO){P(NMe2)3}{B(taz)3}][PF6] 12+[PF6]-, [Rh2(PCy3){B(taz)3}2][PF6]2·4CH2Cl2 142+2[PF6]-·4CH2Cl2, [Rh(PPh3)I{B(taz)3}] 15
[Rh(cod)Tt]·0.5H2O
2·0.5H2O
[Rh(CO)(PCy3)Tt]·
CH2Cl2 6·CH2Cl2
[Rh(CO){P(NMe2)3}Tt]·
2CH2Cl2 7·2CH2Cl2
[Rh(cod)Bt] 1
[(RhTt)2(m-CO)3}] 5
Empirical formula
M
C18H30BN6RhS2
508.32
Orthorhombic
C46H76B2N18ORh2S6 C33H50B2N18O3Rh2S6
C35H60BCl2N9OPRhS3 C24H47BCl4N12OPRhS3
1317.11
Triclinic
P1
1166.71
934.69
Triclinic
¯
P1
902.44
Triclinic
¯
P1
Crystal system
Space group
Trigonal
¯
¯
Pbca
R3c
˚
a/A
12.548(2)
13.774(2)
24.862(3)
90.00
90.00
90.00
4297.1(12)
8
1.006
100
29091
11.263(1)
15.558(1)
17.056(1)
79.575(1)
89.885(1)
89.542(1)
2939.1(4)
2
12.896(2)
12.896(2)
74.468(15)
90.00
90.00
120.00
10725(3)
6
0.674
11.530(2)
13.237(3)
14.645(3)
87.27(3)
86.12(3)
79.85(3)
2193.8(8)
2
11.697(2)
13.580(3)
14.533(3)
71.41(3)
68.99(3)
74.88(3)
2014.4(9)
2
˚
b/A
˚
c/A
a (◦)
b (◦)
g (◦)
3
˚
V/A
Z
m/mm-1
0.827
0.730
8.008
T/K
173
30872
100
34932
100
24642
100
15056
Reflections collected
Independent reflections (Rint) 4925 (0.0485)
Final R indices [I > 2s(I)]: 0.0346, 0.0729
R1,wR2
13432 (0.0901)
0.0529, 0.1036
2673 (0.0831)
0.0501, 0.1543
9963 (0.166)
0.0819, 0.1841
6792 (0.0623)
0.0649, 0.1795
[Rh(CO)(PCy3)-
[Rh2(PCy3)-
[Rh{P(OPh)3}2Tt]· {B(taz)3}][PF6]·
{B(taz)3}2][PF6]2·
1.5CH2Cl2
10·1.5CH2Cl2
3CH2Cl2 11+[PF6]-·
[Rh(CO){P(NMe2)3}-
4CH2Cl2 142+2[PF6]-· [Rh(PPh3)I{B(taz)3}]·
3CH2Cl2
{B(taz)3}][PF6] 12+[PF6]- 4CH2Cl2
3CH2Cl2 15·3CH2Cl2
Empirical formula
C105H116B2Cl6N18- C37H63BCl6F6N9OP2- C22H42BF6N12OP2RhS3 C52H89B2Cl8F12N18-
C36H45BCl6IN9PRhS3
O12P4Rh2S6
2578.54
Triclinic
¯
P1
RhS3
P3Rh2S6
1990.72
Triclinic
M
1248.50
Orthorhombic
Pbca
876.52
Triclinic
¯
P1
1184.33
Monoclinic
P21/c
10.490(1)
20.742(1)
21.960(1)
90.00
92.185(1)
90.00
4774.3(3)
4
Crystal system
Space group
¯
P1
˚
a/A
10.309(2)
14.935(3)
19.807(4)
102.89(3)
91.48(3)
94.73(3)
2959.8(10)
1
24.339(5)
15.281(3)
28.840(6)
90.00
90.00
90.00
10726(4)
8
7.508
11.254(4)
13.158(3)
13.214(13)
113.91(2)
90.92(2)
95.45(2)
1777.5(19)
2
13.412(3)
14.307(3)
24.049(5)
90.85(3)
101.11(3)
107.75(3)
4299.3(18)
2
˚
b/A
˚
c/A
a (◦)
b (◦)
g (◦)
3
˚
V/A
Z
m/mm-1
0.639
0.818
0.905
1.543
100
50648
T/K
173
29160
100
75672
100
20527
100
49319
Reflections collected
Independent reflections (Rint) 29161 (0.0000)
Final R indices [I > 2s(I)]: 0.1098, 0.2990
R1,wR2
10147 (0.0951)
0.0523, 0.1432
8130 (0.0223)
0.0286, 0.0840
19642 (0.0343)
0.0545, 0.1688
10955 (0.0740)
0.0473, 0.1089
and [Rh(CO)(PPh3){B(taz)3}][PF6]·2CH2Cl2 13+[PF6]-·2CH2Cl2
(CSD refcode: QEKDEQ) have been published previously.9
All data were collected using a CCD area-detector, from
a single crystal coated in paraffin oil or high-vacuum grease
mounted on a glass fibre. Intensities were integrated27 from
several series of exposures, each exposure covering 0.3◦ in w.
Absorption corrections were based on equivalent reflections using
merohedral twinning of the crystal (which also led to high final
R values).
Acknowledgements
We thank the EPSRC for postgraduate studentships (to
R.J.B., M.F.H. and A.H.), and Johnson Matthey for a generous
loan of hydrated rhodium trichloride. We acknowledge the use of
the EPSRC’s Chemical Database Service at Daresbury.31
SADABS,28 and structures were refined against all Fo data with
2
hydrogen atoms bonded to carbon atoms riding in calculated
positions using SHELXTL.29 The residual electron density map
for 5, 8 and 142+2[PF6]- showed some peaks which could not be
modelled satisfactorily; the data for these crystal structures were
modelled with PLATON/SQUEEZE.30 The data completeness
for 7 is rather low (89%) due to the difficulties in obtaining a
complete sphere of reflections for a triclinic crystal using copper
radiation, whilst that for 10 is rather high (213%) due to non-
Notes and references
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zolylborate Ligands, Imperial College Press, London, 1999.
2 M. D. Spicer and J. Reglinski, Eur. J. Inorg. Chem., 2009, 1553.
3 M. Careri, L. Elviri, M. Lanfranchi, L. Marchio`, C. Mora and M. A.
Pellinghelli, Inorg. Chem., 2003, 42, 2109.
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The Royal Society of Chemistry 2009
Dalton Trans., 2009, 8724–8736 | 8735
©

View Article Online
4 C. E. Webster and M. B. Hall, Inorg. Chim. Acta, 2002, 330, 268.
5 I. R. Crossley, Adv. Organomet. Chem., 2008, 56, 199.
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Linehan, A. G. Orpen, M. J. Quayle and P. H. Rieger, J. Chem. Soc.,
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7 I. R. Crossley, A. F. Hill and A. C. Willis, Organometallics, 2006, 25,
289.
8 V. K. Landry, J. G. Melnick, D. Buccella, K. Pang, J. C. Ulichny and
G. Parkin, Inorg. Chem., 2006, 45, 2588.
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