The oxidative conversion of the N,S-bridged complexes [{RhLL′(μ-X) }2] to [(RhLL′)3(μ-X)2]+ (X = mt or taz): A comparison with the oxidation of N,N-bridged analogues

Article abstract of DOI:10.1039/c1dt10930h

The structures of [{RhLL′(μ-X)}₂] [LL′ = cod, (CO)₂, (CO)(PPh₃) or {P(OPh)₃}₂; X = mt or taz], prepared from [{RhLL′(μ-Cl)}₂] and HX in the presence of NEt₃, depend on the auxiliary ligands LL′. The head-to-tail arrangement of the two N,S-bridges is accompanied by a rhodium-eclipsed conformation for the majority but the most hindered complex, [{Rh[P(OPh)₃]₂(μ-taz)}₂], uniquely adopts a sulfur-eclipsed structure. The least hindered complex, [{Rh(CO) ₂(μ-mt)}₂], shows intermolecular stacking of mt rings in the solid state. The complexes [{RhLL′(μ-X)}₂] are chemically oxidised to trinuclear cations, [(RhLL′)₃(μ-X) ₂]⁺, most probably via reaction of one molecule of the dimer, in the sulfur-eclipsed form, with the fragment [RhLL′]⁺ formed by oxidative cleavage of a second.

Full text of DOI:10.1039/c1dt10930h

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The oxidative conversion of the N,S-bridged complexes [{RhLL¢(l-X)}2] to
[(RhLL¢)3(l-X)2]+ (X = mt or taz): a comparison with the oxidation of
N,N-bridged analogues†
Robin J. Blagg,‡ Mar´ıa J. Lo´pez-Go´mez,§ Jonathan P. H. Charmant, Neil G. Connelly,* John J. Cowell,
Mairi F. Haddow, Alex Hamilton, A. Guy Orpen, Thomas Riis-Johannessen and Saowanit Saithong
Received 17th May 2011, Accepted 4th August 2011
DOI: 10.1039/c1dt10930h
The structures of [{RhLL¢(m-X)}2] [LL¢ = cod, (CO)2, (CO)(PPh3) or {P(OPh)3}2; X = mt or taz],
prepared from [{RhLL¢(m-Cl)}2] and HX in the presence of NEt3, depend on the auxiliary ligands LL¢.
The head-to-tail arrangement of the two N,S-bridges is accompanied by a rhodium-eclipsed
conformation for the majority but the most hindered complex, [{Rh[P(OPh)3]2(m-taz)}2], uniquely
adopts a sulfur-eclipsed structure. The least hindered complex, [{Rh(CO)2(m-mt)}2], shows
intermolecular stacking of mt rings in the solid state. The complexes [{RhLL¢(m-X)}2] are chemically
oxidised to trinuclear cations, [(RhLL¢)3(m-X)2]+, most probably via reaction of one molecule of the
dimer, in the sulfur-eclipsed form, with the fragment [RhLL¢]+ formed by oxidative cleavage of a second.
Introduction
During our studies of the synthesis of the scorpionate complexes
[RhL2Tx] {Tx = hydrotris(methimazolyl)borate, Tm or HB(mt)3;1
Tx = hydrotris(4-ethyl-3-methyl-5-thioxo-1,2,4-triazolyl)borate,
2
Tt = HB(taz)3 }, from rhodium precursors such as [{RhL2(m-
Cl)}2] [L2 = (CO)2, cycloocta-1,5-diene (cod), etc.] and KTx,
we occasionally observed the formation of small quantities
of by-products, apparently dimers with the rhodium atoms
bridged by N,S-donors originating from the Tx anion.
We have now directly synthesised and fully characterised
Fig. 1 Bridging N,S-donating heterocycles.
these species as [{RhLL¢(m-X)}2] (X
= methimazolyl, mt
or thioxotriazolyl, taz; Fig. 1), analogues of complexes
with other N,S-bridges, e.g. pyridine-2-thiolate (pyt),3–7
than to paramagnetic monocations [{RhLL¢(m-X)}2]+ analogous
to those isolated from the oxidation of N,N-bridged species
such as [{Rh(CO)(PPh3)(m-X)}2] (X = p-tolNNNtol-p15 or
2-t-butylpyrazolyl16).
benzothiazole-2-thiolate
(bzt),3,4,8–10
2-mercaptothiazolinate
(mtz),5,11 benzimidazole-2-thiolate (Hbzimt),12,13 pyridine-2,6-
dithiolate14 and w-thiocaprolactamate,6 showing how the bridging
and ancillary ligands affect molecular conformation. In addition,
we describe their reactions with chemical one-electron oxidants,
providing a new route to trinuclear [(RhLL¢)3(m-X)2]+ rather
Results and discussion
School of Chemistry, University of Bristol, Bristol, BS8 1TS, UK.
E-mail: Neil.Connelly@bristol.ac.uk; Tel: +44(0)117 9288162
Synthesis and characterisation of dirhodium(I) methimazolyl and
thioxotriazolyl complexes
† Electronic supplementary information (ESI) available: X-ray structures
1
of 1, 2, 6 and 12+[BF4]-·CH2Cl2; 13C-{ H} NMR spectroscopic data for 2, 4,
The complexes [{Rh(cod)(m-X)}2] (X = mt, 1 or taz, 2) were
obtained by reacting [{Rh(cod)(m-Cl)}2] with methimazole, Hmt,
in CH2Cl2 or with thioxotriazole, Htaz, in toluene to give
[Rh(cod)Cl(HX)] and then deprotonating the coordinated het-
erocycle using triethylamine. The dimers were then isolated by
either evaporating the reaction mixture to dryness and subsequent
extraction into toluene, for 1, or direct removal of the precipitated
salt [NEt3H]Cl for 2. In both cases, the yellow or orange solid
6, 7, 9+[PF6]-, 11+[PF6]- and 13+[PF6]-. CCDC reference numbers 826255–
826265. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c1dt10930h
‡ Current address: Department of Chemistry, The University of
Manchester, Oxford Road, Manchester, M13 9PL, UK. E-mail:
robin.blagg@manchester.ac.uk
§ Current address: UCR-CNRS Joint Research Chemistry Laboratory,
Department of Chemistry, University of California, Riverside, CA 92521-
0403, USA. E-mail: maria@ucr.edu
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was isolated by concentrating the toluene solution and adding
n-hexane.
The IR spectra in the carbonyl region of [{Rh(CO)2(m-mt)}2]
3 {n(CO) = 2080(s), 2059(m), 2011(s) cm-1} and [{Rh(CO)2(m-
taz)}2] 4 {n(CO) = 2086(s), 2062(m), 2019(s) cm-1} are very
similar, each showing three bands (but with a shoulder, at ca. 2010
cm-1, discernible on the lowest energy band of 4). The reported
spectra of other complexes with N,S-bridging ligands, with similar
molecular structures (see below), are surprisingly variable in the
number and energy of the bands observed, e.g. [{Rh(CO)2(m-
The preparation of [{Rh(CO)2(m-mt)}2] 3 from [{Rh(CO)2(m-
Cl)}2], Hmt and NEt3 was similar to that of 1, the red toluene
extract giving a dark blue-brown solid on treatment with n-hexane.
In this case the IR spectrum of the initial mixture of [{Rh(CO)2(m-
Cl)}2] and Hmt in CH2Cl2 showed two strong bands of equal
intensity at 2080 and 2011 cm-1, evidence for the monomeric
intermediate cis-[Rh(CO)2Cl(Hmt)]. Using a similar method to
synthesise [{Rh(CO)2(m-taz)}2] 4 was unsuccessful, although cis-
[Rh(CO)2Cl(Htaz)] was observed by IR spectroscopy {n(CO) =
2084, 2014 cm-1} following initial bridge cleavage of [{Rh(CO)2(m-
Cl)}2]. However, the tetracarbonyl 4 was synthesised in good
yield by bubbling CO through a CH2Cl2 solution of 2, thereby
substituting the cod ligands.
The bis(phosphine) derivative [{Rh(CO)(PPh3)(m-mt)}2] 5
was prepared from [{Rh(CO)2(m-mt)}2] 3 and two equivalents
of PPh3 in toluene but attempts to prepare the monosub-
stituted complex, by adding only one equivalent of PPh3,
gave a mixture of [Rh2(CO)3(PPh3)(m-mt)2] {n(CO) = 2061,
1994(br) cm-1}, the dimer 5 and unreacted tetracarbonyl 3.
(The complex [Rh2(CO)3(PPh3)(m-bzt)2] {n(CO) (in Nujol) =
2065, 1997, 1995 cm-1} has been structurally characterised in
the solid state but exists in equilibrium with [{Rh(CO)2(m-
bzt)}2] and [{Rh(CO)(PPh3)(m-bzt)}2] in solution.9) The analogue
[{Rh(CO)(PPh3)(m-taz)}2] 6 was synthesised by an alternative
route, namely by reacting [{Rh(CO)(PPh3)(m-Cl)}2] with Htaz in
CH2Cl2, to give [Rh(CO)Cl(PPh3)(Htaz)] {n(CO) = 1979 cm-1}
which was deprotonated using triethylamine.
Finally, the tetrakis(phosphite) complex [{Rh[P(OPh)3]2(m-
taz)}2] 7 was synthesised from 4 and an excess of P(OPh)3. In order
to ensure complete carbonyl substitution, the reaction mixture was
stirred for 80 min and then evaporated to dryness in vacuo. After
extraction into toluene, filtration and precipitation by adding n-
hexane, the complex was further purified by chromatography on
a silica/CH2Cl2 column. A single yellow band was eluted with
CH2Cl2 and the product, 7, precipitated using n-hexane.
Complexes 1–7 (Scheme 1) were characterised by elemental
analysis, IR and NMR spectroscopy (Table 1) and, in all cases
except 4, by X-ray crystallography.
3
pyt)}2] {n(CO) = 2080(vs), 2055(s), 2015(vs), 1980(w) cm-1} and
[{Rh(CO)2(m-mtz)}2] {n(CO) = 2094(s), 2058(m), 2045(w, sh),
2018(s) cm-1},11 though the spectrum of the N,N-bridged pyrazolyl
complex [{Rh(CO)2(m-pz)}2] {pz = pyrazolyl; n(CO) = 2090, 2079,
17
2021 cm-1} appears to be more like those of 3 and 4. The
observation of two closely spaced IR bands {n(CO) = 1974, 1967
cm-1} for [{Rh(CO)(PPh3)(m-mt)}2] 5, as in [{Rh(CO)(PPh3)(m-
bzt)}2],3 is consistent with the arrangement of the carbonyl ligands
determined crystallographically (see below); only one broad,
unresolved band was observed for [{Rh(CO)(PPh3)(m-taz)}2] 6
{n(CO) = 1975 cm-1} and [{Rh(CO)(PPh3)(m-mtz)}2] {n(CO) =
1981 cm-1}.11
NMR spectroscopy and fluxional behaviour. Proton and 31P
NMR spectroscopic data for complexes 1–7 are given in Table
1
1. (Representative 13C-{ H} NMR spectroscopic data, for the taz
complexes 2, 4, 6 and 7, are given in the ESI.†) The two bridging
ligands in each of [{Rh(cod)(m-mt)}2] 1 and [{Rh(cod)(m-taz)}2] 2
are equivalent (for example, the ring methyl substituents appear
as only one singlet). Likewise, in each complex the two cod rings
are equivalent although the eight carbon atoms in each ring are
inequivalent. Thus, at -80 (for 1) or -60 ◦C (for 2) four peaks
are observed for the alkene protons or carbons of the cod ligand,
as seen for [{Rh(cod)(m-Hbzimt)}2].12 (The 1H signals for the cod
ligands of 1 and 2 are very broad at room temperature, probably
as a result of rotation of the cod ligand about the metal-cod axis.5)
1
In contrast to 1 and 2, both the 1H and 13C-{ H} spectra of the
tetracarbonyl [{Rh(CO)2(m-taz)}2] 4 (but not [{Rh(CO)2(m-mt)}2]
3) show the presence of two isomers, 4a and 4b, in a 3 : 2 ratio {e.g.
four doublets are observed for the rhodium-bound carbonyls, two
for 4a, at 187.64 (1JCRh 64.5 Hz) and 182.86 (1JCRh 69.1 Hz) and
two for 4b, at 186.14 (1JCRh 66.2 Hz) and 185.05 (1JCRh 68.0 Hz)}.
The major spectral differences are the lower chemical shifts for the
resonances of 4a, the observation of two diastereotopic methylene
protons for each thioxotriazolyl ethyl group of 4a, and the greater
difference between the chemical shifts of the two carbonyl carbons
for 4a {D(dC) 4.78 ppm} compared with 4b {D(dC) 1.09 ppm}.
Such significant differences suggest a mixture of ‘head-to-tail’ and
‘head-to-head’ isomers. (Resolution was insufficient in CH2Cl2 to
detect the isomers by IR spectroscopy in the carbonyl region.) Why
4 should appear as a mixture of isomers while the precursor, 2, does
not is unknown. The tetracarbonyl may undergo isomerisation
more readily than the cod complex though we have no evidence to
support or disprove this.
The protons of the thioxotriazolyl rings of [{Rh[P(OPh)3]2(m-
taz)}2] 7 were generally observed at low chemical shifts due to the
shielding effect of the twelve P(OPh)3 phenyl rings which surround
the Rh(m-taz)2Rh core. The tendency for nearby aromatic rings to
shift the resonances of protons on the thioxotriazolyl ring (espe-
cially the terminal protons of the ethyl group) has been observed
in [Rh(CO)(PPh3)Tt]2 and [Rh(CO)(PPh3){HB(taz)2(pzPh)}]18 at
Scheme 1 Dirhodium complexes.
11498 | Dalton Trans., 2011, 40, 11497–11510
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Table 1 Analytical and spectroscopic data for dimeric rhodium complexes
Analysis/%a
IR/cm-1
NMRc
1H
1
Complex
Colour
Yellow
Yield/%
92
C
H
N
n(CO)b
31P-{ H}
[{Rh(cod)(m-mt)}2] 1
44.4
(44.5)
5.1
8.5
—
6.51 (d, 2H, 3JHH 1.6, mt
—
(5.3) (8.6)
4/5-H), 6.39 (d, 2H, 3JHH 1.6,
mt 4/5-H), 4.78 (br m, 2H,
cod CH), 4.08 (br m, 2H, cod
CH), 3.75 (br m, 2H, cod
CH), 3.70 (br m, 2H, cod
CH), 3.28 (s, 6H, mt 3-CH3),
2.65 (br m, 2H, cod CHHexo),
2.50 (br m, 2H, cod CHHexo),
2.29 (br m, 4H, cod CHHexo),
1.93 (br m, 2H, cod
CHHendo), 1.74 (br m, 6H,
d
cod CHHendo
)
[{Rh(cod)(m-taz)}2] 2
Orange
93
42.2
5.8
11.2
—
4.80 (br m, 2H, cod CH),
4.12 (br m, 2H, cod CH),
3.98 (br m, 2H, cod CH),
3.72 (br m, 2H, cod CH),
3.64 (q, 4H, 3JHH 7.0, taz
4-CH2CH3), 2.76–2.13 (br m,
8H, cod CHHexo), 2.04 (s,
6H, taz 3-CH3), 1.98–1.54 (br
m, 8H, cod CHHendo), 1.03 (t,
6H, 3JHH 7.0, taz 4-CH2CH3)f
—
(42.5)e
(5.5) (11.2)
[{Rh(CO)2(m-mt)}2] 3
[{Rh(CO)2(m-taz)}2] 4
Blue-brown 77
26.1
(26.5)
2.1
10.5
2080, 2059(m), 6.47 (d, 2H, 3JHH 1.7, mt
—
—
(1.9) (10.3) 2011
4/5-H), 6.39 (d, 2H, 3JHH 1.7,
mt 4/5-H), 3.33 (s, 6H, mt
3-CH3)
Purple
64
28.0
(27.9)
3.0
14.0
2086, 2062(m), 3.93 (dq, 2H, 2JHH 14.2, 3JHH
(2.7) (14.0) 2019, 2010(sh) 7.2, taz 4-CHHCH3)†, 3.93
(q, 4H, 3JHH 7.3, taz
4-CH2CH3)‡, 3.74 (dq, 2H,
2JHH 14.2, 3JHH 7.2, taz
4-CHHCH3)†, 2.29 (s, 6H,
taz 3-CH3)‡, 2.18 (s, 6H, taz
3-CH3)†, 1.24 (t, 6H, 3JHH
7.3, taz 4-CH2CH3)‡, 1.14 (t,
6H, 3JHH 7.2, taz
4-CH2CH3)†g
[{Rh(CO)(PPh3)(m-mt)}2] 5 Yellow
[{Rh(CO)(PPh3)(m-taz)}2] 6 Yellow
32
43
54.8
(54.5)
4.4
5.0
1974, 1967
1975(brd)
7.78–6.58 (m, 30H, PPh3),
5.81 (d, 2H, 3JHH 1.6, mt
4/5-H), 5.27 (d, 2H, 3JHH 1.6,
mt 4/5-H), 2.55 (s, 6H, mt
3-CH3)h
41.38 (d, 1JPRh 161,
PPh3)g
(4.0) (5.5)
53.6
(53.8)
4.2
8.0
7.71–7.15 (m, 30H, PPh3),
41.98 (d, 1JPRh 162,
(4.3) (7.9)
3.37 (dq, 2H, 2JHH 13.7, 3JHH PPh3)
6.8, taz 4-CHHCH3) 3.28
(dq, 2H, 2JHH 13.7, 3JHH 6.8,
taz 4-CHHCH3), 1.77 (s, 6H,
taz 3-CH3), 0.80 (t, 6H, 3JHH
7.0, taz 4-CH2CH3)
[{Rh[P(OPh)3]2(m-taz)}2] 7
Yellow
22
56.0
4.5
4.6
—
7.35–6.78 {m, 60H,
14.3, 3JHH 7.3, taz
4-CHHCH3) 2.64 (dq, 2H,
2JHH 14.1, 3JHH 7.2, taz
4-CHHCH3), 1.79 (s, 6H, taz
3-CH3), 0.41 (t, 6H, 3JHH 7.3,
taz 4-CH2CH3)
127.83 {dd, 1JPRh
(55.9)e
(4.4) (4.7)
P(OPh)3}, 3.23 (dq, 2H, 2JHH 265, 2JPP 70,
P(OPh)3}, 115.99
{dd, 1JPRh 265, 2JPP
70, P(OPh)3}
a Calculated values in parentheses. b In CH2Cl2; strong absorptions unless stated, m = medium, sh = shoulder, brd = broad. c Chemical shift (d) in ppm, J
values in Hz, spectra in CD2Cl2 at 20 ◦C unless otherwise stated. d At -80 ◦C. e Calculated as a 2 : 1 CH2Cl2 solvate. f At -60 ◦C. g Two isomers present in
a ratio of 3†:2‡. h In d8-toluene.
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low temperature and [{Rh(CO)(PPh3)(m-taz)}2] 6 at room temper-
ature.
1
The 31P-{ H} NMR spectra of 5 and 6 each show only one
doublet (at ca. 42 ppm, JPRh 160 Hz), consistent with the presence
of only one (head-to-tail) isomer in which two equivalent phos-
phorus atoms are each coupled to rhodium; [{Rh(CO)(PPh3)(m-
mtz)}2] similarly showed a single doublet, at 39.81 (JPRh 164 Hz)
ppm.11 The tetrakis(phosphite) [{Rh[P(OPh)3]2(m-taz)}2] 7 shows
2
two doublets of doublets, at 127.83 and 115.99 (JPRh 265, JPP
70 Hz) ppm, again consistent with only one isomer in which,
however, the two phosphites on each metal are inequivalent (one
is trans to N and the other to S).
Fig. 3 Molecular structure of [{Rh(CO)2(m-mt)}2] 3.
The X-ray structures of complexes 1–3 and 5–7. In or-
der to determine the relative orientations of the two hetero-
cyclic bridges in the new dimers (i.e. head-to-head or head-
to-tail), and to define more fully the effects of the bridg-
ing and ancillary ligands on conformation, X-ray diffrac-
tion studies were carried out on [{Rh(cod)(m-mt)}2] 1,
[{Rh(cod)(m-taz)}2] 2, [{Rh(CO)2(m-mt)}2]·toluene 3·toluene,
[{Rh(CO)(PPh3)(m-mt)}2]·toluene 5·toluene, [{Rh(CO)(PPh3)(m-
taz)}2] 6 and [{Rh[P(OPh)3]2(m-taz)}2]·CH2Cl2 7·CH2Cl2 using
crystals grown by slow diffusion of n-hexane into a concentrated
toluene (for 1, 3 and 5) or CH2Cl2 (for 2, 6 and 7) solution of each
complex at 5 ◦C. The structures of 2, 3, 5 and 7 are shown in Fig.
2–5 as representative examples (those of 1 and 6 are shown in the
ESI†) with selected bond lengths and angles for all six species in
Table 2.
Fig. 4 Molecular structure of [{Rh(CO)(PPh3)(m-mt)}2] 5.
Fig. 2 Molecular structure of [{Rh(cod)(m-taz)}2] 2. (One of two
inequivalent molecules in the unit cell; the second is shown in the ESI†. In
this and all other structures reported in this paper, ellipsoids are shown at
the 50% probability level and hydrogen atoms are omitted for clarity.)
Fig. 5 Molecular structure of [{Rh{P(OPh)3}2(m-taz)}2] 7 (phenyl rings
omitted for clarity).
effect of the sulfur donor. (The tendency of PPh3 to coordinate
to rhodium trans to sulfur rather than nitrogen has also been
6
All of the complexes 1–3, 5 and 6 have an ‘open book’ geometry
in which the two heterocycles bridge two cis positions of slightly
distorted square planar d8 metals. (The structure of 7 is very
different, see below.) Overall, the complexes have approximate C2
rotational symmetry, i.e. a C2 axis runs between the ring planes,
through the middle of the Rh-N-C-S-Rh-N-C-S eight-membered
ring.
In each complex, the two rings bridge head-to-tail so that each
rhodium atom is cis N,S-coordinated. In both PPh3 complexes, 5
and 6, the phosphine ligands are trans to the sulfur atoms, i.e. the
site of carbonyl substitution in 3 or 4 depends on the higher trans
observed in [{Rh(CO)(PPh3)(m-pyt)}2] and [{Rh(CO)(PPh3)(m-
mtz)}2].11) The mutually trans arrangement of the two phosphine
ligands (across the P–Rh . . . Rh–P unit) also minimises steric
hindrance.
The coordination planes of the two metals are inclined, at an
angle q, and twisted, by an angle b (the average of the two S–
Rh ◊ ◊ ◊ Rh–N dihedral angles, Table 2); the combined inclination
and twisting minimises steric repulsion between the ancillary
ligands, particularly cod or PPh3. In the dicarbonyl [{Rh(CO)2(m-
mt)}2] 3, q is 20.7◦ but in the cod complexes 1 and 2 (which has
two independent but essentially isostructural molecules in the unit
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◦
˚
Table 2 Selected bond lengths (A) and angles ( ) for dimeric rhodium complexes
[{Rh(cod)-
(m-mt)}2] 1
[{Rh(cod)-
(m-taz)}2] 2a
[{Rh(CO)2-
(m-mt)}2] 3
[{Rh(CO)(PPh3)-
(m-mt)}2] 5
[{Rh(CO)(PPh3)-
(m-taz)}2] 6
[{Rh[P(OPh)3]2-
(m-taz)}2] 7b
Rh(1) ◊ ◊ ◊ Rh(2)
3.847(1)
2.003(2)
2.011(2)
2.364(1)
2.096(2)
2.017(2)
2.007(3)
2.360(1)
2.093(2)
4.493(1)
87.90(8)
91.69(5)
88.63(7)
90.75(5)
-42.35(5)
-40.82(5)
3.832(1),
3.893(1)
1.996(7),
1.966(6)
2.011(7),
2.007(6)
2.362(2),
2.351(2)
2.086(5),
2.080(5)
2.003(7),
1.997(8)
2.006(7),
2.002(6)
2.360(2),
2.350(2)
2.087(5),
2.069(5)
4.496(3),
4.543(2)
88.20(3),
88.09(2)
91.73(13),
91.30(13)
87.94(3),
87.66(3)
91.36(13),
92.22(14)
-43.17(14),
-39.73(15)
-42.53(15),
-41.77(13)
51.2, 54.3
-41.8
3.049(1)
3.618(1)
2.274(1)
3.651(1)
2.261(1)
4.968(1)
2.161(1)
2.163(1)
2.408(1)
2.087(2)
—
Rh(1)–X(1)c {X(1)–O}
Rh(1)–X(2)d {X(2)–O}
Rh(1)–S(1)
1.851(6)
{1.152(7)}
1.858(6)
1.817(2)
{1.149(3)}
2.383(1)
1.826(4)
{1.148(4)}
2.394(1)
{1.129(6)}
2.382(1)
Rh(1)–N(z)e {X(3)–O}
Rh(2)–X(3)f {X(4)–O}
Rh(2)–X(4)g
2.097(4)
2.088(2)
2.266(1)
2.084(4)
2.258(1)
{1.125(7)}
1.874(6)
{1.140(6)}
1.848(6)
1.817(2)
{1.149(3)}
2.378(1)
1.817(4)
{1.146(4)}
2.395(1)
—
Rh(2)–S(2)
2.381(1)
2.086(4)
4.770(2)
92.5(2)
—
Rh(2)–N(1)
2.089(2)
4.638(2)
89.70(7)
88.91(5)
88.01(8)
89.28(5)
-42.16(5)
-42.60(5)
2.079(3)
4.662(2)
89.27(11)
87.95(8)
89.03(12)
88.62(8)
38.10(9)
38.29(8)
—
S(1) ◊ ◊ ◊ S(2)
3.776(1)
91.75(2)
89.26(5)
—
X(1)–Rh(1)–X(2)
S(1)–Rh(1)–N(z)
X(3)–Rh(2)–X(4)
S(2)–Rh(2)–N(1)
S(1)–Rh(1) ◊ ◊ ◊ Rh(2)–N(1)
S(2)–Rh(2) ◊ ◊ ◊ Rh(1)–N(z)
88.65(12)
91.8(2)
88.49(12)
-31.63(12)
-31.09(12)
—
59.85(6)
—
h
q
b
52.7
-41.6
3.445
20.7
-31.4
3.485
46.9
-42.3
3.354
46.1
38.2
3.165
86.0
59.9
3.129
i
ringcentroid–ringcentroid
3.466, 3.514
a Two independent molecules in the unit cell. b Molecular structure C2 symmetric, other bond lengths/angles equivalent by symmetry. In this structure,
Rh(2) = Rh(1¢). c X(1) = midpoint C(9)–C(10) 1; midpoint C(11)–C(12) 2; C(9) 3; P(1) 5–7. d X(2) = midpoint C(13)–C(14) 1; midpoint C(15)–C(16) 2;
C(10) 3; C(9) 5; C(11) 6; P(2) 7. e N(z) = N(3) 1, 3 and 5; N(4) 2 and 6; N(1)¢ 7. f X(3) = midpoint C(21)–C(22) 1; midpoint C(23)–C(24) 2; C(12) 3; P(2)
5 and 6. g X(4) = midpoint C(17)–C(18) 1; midpoint C(19)–C(20) 2; C(11) 3; C(10) 5; C(12) 6. h q = angle between the planes {Rh(1), X(1), X(2), S(1),
N(z)} and {Rh(2), X(3), X(4), S(2), N(1)}. i b = average of the S(1)–Rh(1) ◊ ◊ ◊ Rh(2)–N dihedral angles.
cell, 2a and 2b) and the carbonyl phosphine analogues 5 and 6 that
angle is much larger, at 46.1–54.3◦. Likewise, b varies from 31.4◦
in dicarbonyl 3 to ca. 42◦ in the cod complexes 1 and 2.
In all cases the Rh ◊ ◊ ◊ Rh distance is too long to indicate signif-
icant metal-metal interaction. However, it is affected significantly
6) is also unique in that the dimers are linked via intermolecular
p-stacking of methimazolyl rings. The rings in adjacent dimers are
˚
parallel, though offset (by ca. 0.9 A) from being perfectly eclipsed,
˚
with a distance between the ring centroids of 3.78 A. (This distance
compares with the intramolecular ringcentroid–ringcentroid distances of
˚
˚
by the nature of the ancillary ligands, increasing from 3.05 A in the
3.17–3.51 A in 1, 2, 5 and 6, Table 2.) Presumably, the sterically
˚
tetracarbonyl 3 to 3.62 and 3.65 A in the carbonyl phosphines 5
and 6 and to ca. 3.85 A in the cod complexes 1 and 2. Surprisingly,
these distances are significantly longer than those reported for
more demanding cod and PPh3 ligands of 1, 2, 5 and 6 prevent
such stacking in the solid state. It is interesting to note that
[{Rh(CO)2(m-pyt)}2] was described as dichroic green-purple in
the solid state. Its molecular structure has been reported but no
comment was made on any stacking in the crystal structure.3
Further inspection of the original crystallographic data showed
that neither the relative orientation of the pyridyl rings nor
their intermolecular ringcentroid–ringcentroid distance indicated such
stacking.
As noted above, the structure of [{Rh[P(OPh)3]2(m-taz)}2] 7 is
distinctly different from those of the other new dimers. In this
case, and presumably because of the presence of four sterically
demanding phosphite ligands, the two rhodium square planes are
approximately mutually perpendicular (q = 86◦) and twisted by
˚
analogous N,S-bridged complexes such as [{Rh(CO)2(m-pyt)}2]
3
6
˚
˚
{2.941(2) A} (cf . 3), [{Rh(CO)(PPh3)(m-pyt)}2] {3.195(1) A}
11
˚
and [{Rh(CO)(PPh3)(m-mtz)}2] {3.244(1)A} (cf . 5 and 6) and
5
˚
[{Rh(cod)(m-mtz)}2] {3.715(1) A} (cf . 1 and 2).
One other observation is of interest in this series of dimers,
namely that solid 3 is much darker (blue-brown), and with a
metallic aspect, than 1, 2 and 5–7 (yellow or orange). (The other
new tetracarbonyl, 4, was isolated as a purple powder rather than
a crystalline solid, possibly due to the presence of isomers, so has
not been structurally characterised. Note that all of 1–7 are yellow
or orange in solution.) However, the crystal structure of 3 (Fig.
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Fig. 6 p-Stacking between the methimazolyl rings in [{Rh(CO)2(m-mt)}2] 3. Symmetry operator for symmetry-generated atoms: A: x, 0.5 - y, -0.5 + z.
B: x, 0.5 - y, 0.5 + z.
ca. 60◦. Accordingly, the rhodium–rhodium distance is increased
Table 3 Electrochemical data for the oxidation of dimeric rhodium
complexesa
˚
to 4.97 A. However, the sulfur–sulfur distance is decreased to
˚
˚
3.78 A (cf . 4.49–4.77 A in 1–6) and, overall, 7 uniquely adopts
a sulfur-eclipsed ‘twist-tub’ conformation of the eight-membered
Rh-N-C-S-Rh-N-C-S ring [Fig. 7(a)] (cf . the rhodium-eclipsed
conformation in 1–3, 5 and 6 [Fig. 7(b)]).
(Ep)ox/V (Ep)red/V E◦¢/V (ip)red/(ip)ox
[{Rh(cod)(m-mt)}2] 1
[{Rh(cod)(m-taz)}2] 2
[{Rh(CO)2(m-mt)}2] 3
[{Rh(CO)2(m-taz)}2] 4
0.38
0.55
0.68
0.42c
0.26
0.39
—
—
—
0.32b
0.47
—
—
—
1.0
0.40
—
—
—
[{Rh(CO)(PPh3)(m-mt)}2] 5 0.28
[{Rh(CO)(PPh3)(m-taz)}2] 6 0.36
0.28
0.36
0.32
0.43
0.52
0.74
[{Rh[P(OPh)3]2(m-taz)}2] 7
0.50
a CV at a platinum disc electrode at scan rate, 200 mV s-1. Calibrated
vs. [Fe(h-C5Me5)2]0/1+ at E◦¢ = -0.08 V. b Second, irreversible, wave with
(Ep)ox = 0.75 V. c Broad, ill-defined wave.
Fig.
7
(a) The sulfur-eclipsed ‘twist-tub’ conformation of the
Rh-N-C-S-Rh-N-C-S ring of 7, and (b) the rhodium-eclipsed ‘twist-tub’
conformation of that ring in 1–3, 5 and 6.
The oxidation of the tetrakis(phosphite) complex
[{Rh[P(OPh)3]2(m-taz)}2] 7 appears surprisingly positive compared
with those of [{Rh(CO)2(m-taz)}2] 4 and [{Rh(CO)(PPh3)(m-
mt)}2] 6 in that carbonyl substitution by P-donors usually leads
to considerably lower potentials (cf. the triazenide complexes
[{RhL2(m-PhNNNPh)}2] {L2 = (CO)2, 0.88 V; (CO)(PPh3), 0.56 V;
The electrochemistry of complexes 1–7. The cyclic voltammo-
grams of 1–7 in CH2Cl2 are rather poorly defined relative to
those of N,N-bridged species such as [{Rh(CO)(PPh3)(m-X)}2]
(X = p-tolNNNtol-p15 or 2-t-butylpyrazolyl16) which are reversibly
oxidised to stable monocations. However, all of the new dimers do
show chemically accessible oxidation processes (Table 3) (some
with further ill-defined oxidation at more positive potentials). The
cod complex [{Rh(cod)(m-mt)}2] 1 shows an apparently chemically
reversible wave centred at 0.32 V, implying some stability for
the cation [{Rh(cod)(m-mt)}2]+. However, the oxidations of 2
[{Rh(cod)(m-taz)}2], and the carbonyl phosphine complexes 6 and
7, are only partially reversible, the reversibility increasing with
scan rate, e.g. the peak current ratio for 6 increases from ca. 0.4
at 50 mV s-1 to ca. 0.6 at 1.0 V s-1. The instability of the N,S-
bridged monocations of 1–7 is therefore in marked contrast to
the stability of the structurally characterised monocations formed
from the N,N-bridged triazenide and pyrazolyl complexes noted
above. Given the reactions described below, one might speculate
that rapid Rh-S bond cleavage occurs after the formation of 1+–7+.
19
(PPh3)2, 0.22 V} ). However, as shown above, this complex is
structurally unique in the series 1–7 so that direct comparison is
difficult.
Finally, cyclic voltammetry provides further evidence for step-
wise phosphine substitution in the carbonyl [{Rh(CO)2(m-mt)}2] 3.
After adding one equivalent of PPh3 to 3 in CH2Cl2 the irreversible
oxidation wave at ca. 0.70 V is replaced by three peaks, at ca. 0.74,
0.44 and 0.32 V, possibly corresponding to the oxidation of a
mixture of [{Rh(CO)2(m-mt)}2] 3, [Rh2(CO)3(PPh3)(m-mt)2] and
[{Rh(CO)(PPh3)(m-mt)}2] 5 respectively. After the addition of two
equivalents of PPh3, only two irreversible waves are observed, at ca.
0.30 and 0.75 V, probably corresponding first to the oxidation of
5, to give [{Rh(CO)(PPh3)(m-mt)}2]+, and then of the product of a
rapid following chemical reaction, possibly [{Rh(CO)(PPh3)}3(m-
mt)2]+ (see below).
11502 | Dalton Trans., 2011, 40, 11497–11510
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Synthesis and characterisation of [(RhLL¢)3(l-X)2]+
The observation by cyclic voltammetry of chemically accessible
oxidation processes, albeit mostly ill-defined, led us to investigate
the reactions of [{RhLL¢(m-X)}2] 1–7 with one-electron oxidants.
Rather than leading to the paramagnetic monocations [{RhLL¢(m-
X)}2]+, as found for [{Rh(CO)(PPh3)(m-X)}2] (X = p-tolNNNtol-
p15 or 2-t-butylpyrazolyl16), a new route was discovered to trinu-
clear cations of the type [(RhLL¢)3(m-X)2]+.
Treatment of 1, 3 or 5 in CH2Cl2 with two equivalents of
[Fe(h-C5H4COMe)(h-C5H5)][BF4], or of 2, 4 or 6 with [Fe(h-
C5H5)2][PF6], resulted in colour changes from yellow or or-
ange to deep red or purple and the isolation, on addition of
n-hexane, of red-purple solids characterised as the trinuclear
salts [{Rh(cod)}3(m-mt)2][BF4] 8+[BF4]-, [{Rh(cod)}3(m-taz)2][PF6]
9+[PF6]-, [{Rh(CO)2}3(m-mt)2][BF4] 10+[BF4]-, [{Rh(CO)2}3(m-
taz)2][PF6] 11+[PF6]-, [{Rh(CO)(PPh3)}3(m-mt)2][BF4] 12+[BF4]-
and [{Rh(CO)(PPh3)}3(m-taz)2][PF6] 13+[PF6]-.
Scheme 2 Trirhodium cations.
cm-1} and [{Rh(CO)(PPh3)}3(m-bzt)2]+ {n(CO) = 2000 cm-1}
though, again, previously reported spectra are surprisingly vari-
able in the number and energy of the bands observed.
4
Yields of [{Rh(CO)2}3(m-taz)2][PF6] 11+[PF6]- were poor by this
method but the dark purple solid was obtained more readily by
bubbling CO through 9+[PF6]- in CH2Cl2. By contrast, similar
carbonylation of 8+[BF4]- led to the isolation of the novel species
[Rh3(CO)4(cod)(m-mt)2][BF4] 14+[BF4]-. IR spectroscopy showed
that all three cod ligands were first replaced by CO, giving 10+, but
five minutes after gas passage was stopped the carbonyl groups on
the central rhodium atom had been substituted by the displaced
cod, giving a purple solution with IR carbonyl bands at 2080 and
2026 cm-1. Adding n-hexane to the mixture, and storage at -10 ◦C,
gave purple crystals of 14+[BF4]- (which is also formed when an
excess of cod is added to pure 10+ in CH2Cl2). (Reversible diene
substitution was also observed between CO and [{Rh(cod)}3(m-
pyt)2]+.4)
The 1H NMR spectra show the two bridging heterocycles to be
equivalent in 8+–11+ and 14+ but not in the carbonyl phosphine
complexes 12+ or 13+ because of the two different ancillary ligands
on each metal. Thus, [{Rh(CO)(PPh3)}3(m-mt)2]+ 12+ gives four
peaks for the alkene protons of the mt rings and two for the two
methyl substituents. The cations [{Rh(cod)}3(m-X)2]+ (X = mt, 8+
or taz, 9+) each showed six signals for the alkene protons and
carbons of the three cod rings, corresponding to four distinct
environments on each of the terminal [Rh(cod)]+ centres and
two on the central [Rh(cod)]+ fragment (as also found for this
fragment in [Rh3(CO)4(cod)(m-mt)2]+ 14+). Six 13C resonances are
also observed for the alkane carbons of the cod rings of 9+ (see the
ESI†). However, in all of 8+, 9+ and 14+ the hydrocarbon ligand
signals are broad at room temperature, suggesting slow rotation
about the metal-cod axis, as in the dimers 1 and 2.
Although pure [(Rh{P(OPh)3}2)3(m-taz)2][PF6] could not be
isolated, spectroscopic evidence was obtained for its formation by
The room temperature NMR spectra of [{Rh(CO)2}3(m-taz)2]+
11+ are well defined with, for example, all three carbonyl environ-
ments (trans-S–cis-S, trans-N–cis-S and trans-S–cis-N) observed
as rhodium-coupled 13C doublets (183.98, 183.84 and 180.66
1
two distinct synthetic routes. First, the 31P-{ H} NMR spectrum
of the crude solid formed by oxidising 7 with [Fe(h-C5H5)2][PF6]
in CH2Cl2 showed the presence of [(Rh{P(OPh)3}2)3(m-taz)2]+
1
2
(six separate doublets of doublets: 125.52 { JPRh 199, JPP 61},
ppm; JCRh 68.0 Hz). Both [{Rh(CO)(PPh3)}3(m-mt)2]+ 12+ and
1
1
2
1
2
123.43 { JPRh 258, JPP 69}, 111.83 { JPRh 304, JPP 69}, 109.26
[{Rh(CO)(PPh3)}3(m-taz)2]+ 13+ show three distinct signals in
1
2
1
2
1
{ JPRh 304, JPP 73}, 92.99 { JPRh 191, JPP 52}, 88.01 { JPRh
the 31P-{ H} NMR spectrum, each a doublet with coupling
1
181, JPP 62}) as well as unreacted 7. Second, the 31P-{ H}
NMR spectrum of the product of the reaction of a large excess
of P(OPh)3 with [{Rh(CO)2}3(m-taz)2][PF6] 11+[PF6]- in CH2Cl2
suggested the presence of both [(Rh{P(OPh)3}2)3(m-taz)2][PF6]
and partially substituted products, i.e. [Rh3{P(OPh)3}x(CO)6-x(m-
taz)2][PF6], also indicated by new carbonyl bands in the IR
spectrum of the crude solid.
2
1
to rhodium, as found for the analogous N,S-bridged complex
[{Rh(CO)(PPh3)}3(m-bzt)2]+.4 The two thioxotriazolyl rings are
magnetically inequivalent, the proton and carbon resonances at
lower chemical shifts for one ring possibly resulting from different
degrees of shielding from the aromatic rings of the PPh3 ligands.
Other trinuclear complexes with N,S-bridging units have
been prepared4,8 by adding a reactive fragment, [RhLL¢]+,
to an N,S-bridged dimer, e.g. [{Rh(CO)2}3(m-pyt)2]+ from
[Rh(CO)2(acetone)n]+ and [{Rh(CO)2(m-pyt)} ].4 It is therefore
proposed that oxidation of [{RhLL¢(m-X)}22] (1–7) results in
bridge cleavage, the liberated fragment [RhLL¢]+ S-coordinating
to a second molecule of [{RhLL¢(m-X)}2] to give [(RhLL¢)3(m-
X)2]+ (8+–14+). However, for such coordination to take place the
rhodium-eclipsed geometry of 1–6 must convert to the sulfur-
eclipsed form (as found in 7), as shown in Scheme 3.
Complexes 8+–13+ (Scheme 2) were characterised as [BF4]- or
[PF6]- salts by elemental analysis, IR and NMR spectroscopy
(Table 4) and, for 9+, 10+ and 12+–14+, by X-ray crystallography.
(Representative 13C-{ H} NMR spectroscopic data, for the taz
1
complexes 9+, 11+ and 13+, are given in the ESI.†)
Four IR carbonyl bands were observed for each of the hexacar-
bonyls [{Rh(CO)2}3(m-mt)2][BF4], 10+[BF4]-, and [{Rh(CO)2}3(m-
taz)2][PF6], 11+[PF6]-, and two each for 12+–14+, (closely spaced for
12+ and 13+, containing Rh(CO)(PPh3) units, and well separated
for the cis-dicarbonyl units in 14+). These spectra are comparable
with those of analogous N,S-bridged trirhodium complexes such
as [{Rh(CO)2}3(m-bzt)2]+ {n(CO) = 2088(vs), 2055(s), 2020(vs)
The electrochemistry of complexes 8+–13+. For completeness,
a brief electrochemical study was made of the trinuclear species
8+–13+ (Table 5). Again, the cod complexes showed chemically
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Table 4 Analytical and spectroscopic data for cationic trirhodium complexes
Analysis/%a
IR/cm-1
NMRc
1H
1
Complex
Colour
Purple
Yield/%
34
C
H
N
n(CO)b
31P-{ H}
[{Rh(cod)}3(m-mt)2]-
[BF4] 8+[BF4]-
40.5
(40.6)
4.9
6.0
—
6.64 (d, 2H, 3JHH 2.0, mt
—
(4.9) (5.9)
4/5-H), 6.24 (d, 2H, 3JHH 2.0,
mt 4/5-H), 5.55–5.45 (br m, 2H,
cod CH), 5.35–5.35 (br m, 1H,
cod CH), 4.95–4.80 (br m, 2H,
cod CH), 4.65–4.55 (br m, cod
CH), 4.50–4.35 (br m, 2H, cod
CH), 4.25–4.15 (br m, 2H, cod
CH), 3.92 (s, 6H mt 3-CH3),
3.40–3.30 (br m, 2H, cod CH),
3.25–2.75 (m, 6H, cod CH2),
2.70–1.95 (m, 12H, cod CH2),
1.90–1.55 (m, 6H, cod CH2)
5.63–5.54 (br m, 2H, cod CH),
4.93–5.05 (br m, 2H, cod CH),
4.90–4.81 (br m, 2H, cod CH),
4.83 (dq, 2H, 2JHH 14.5, 3JHH
7.3, taz 4-CHHCH3), 4.63–4.52
(br m, 2H, cod CH), 4.29–4.21
(br m, 2H, cod CH), 4.07 (dq,
2H, 2JHH 14.6, 3JHH 7.3, taz
4-CHHCH3), 3.51–3.40 (br m,
2H, cod CH), 3.23–2.76 (m, 6H,
cod CH2), 2.61–2.12 (m, 12H,
cod CH2), 2.14 (s, 6H, taz
[{Rh(cod)}3(m-taz)2]-
[PF6] 9+[PF6]-
Dark red
49
38.2
(38.4)
5.0
8.0
—
—
(4.9) (7.9)
3-CH3), 1.98–1.69 (m, 6H, cod
CH2), 1.63 (t, 6H, 3JHH 7.3, taz
4-CH2CH3)
[{Rh(CO)2}3(m-mt)2]-
[BF4] 10+[BF4]-
Red-orange
Dark purple
6
21.4
(21.3)
1.7
7.0
2109(ms), 2091(s), 6.95 (d, 2H, 3JHH 1.6, mt
—
—
(1.3) (7.1) 2046(s,brd), 2035
(sh)
4/5-H), 6.66 (d, 2H, 3JHH 1.6,
mt 4/5-H), 3.62 (s, 6H, mt
3-CH3)
[{Rh(CO)2}3(m-taz)2]-
[PF6] 11+[PF6]-
64
21.3
(21.2)
2.0
9.1
2109, 2088, 2049,
4.04 (q, 2H, 3JHH 7.3, taz
4-CH2CH3), 4.03 (q, 2H, 3JHH
7.3, taz 4-CH2CH3), 2.45 (s, 6H,
taz 3-CH3), 1.21 (t, 6H, 3JHH
7.3, taz 4-CH2CH3)
(1.8) (9.3) 2030(m)
[{Rh(CO)(PPh3)}3(m-
mt)2][BF4] 12+[BF4]-
Pink
9
54.2
(52.3)
3.9
3.3
2003, 1990
2005, 1995
7.74–7.10 (m, 45H, PPh3), 6.02
(s, 1H, mt 4/5-H), 5.86 (s, 1H,
mt 4/5-H), 5.35 (s, 1H, mt
40.03 (d, 1JPRh
167, PPh3),
(3.7) (3.8)
39.61 (d, 1JPRh
4/5-H), 5.34 (s, 1H, mt 4/5-H), 166, PPh3),
3.53 (s, 3H, mt 3-CH3), 3.27 (s,
3H, mt 3-CH3)
38.17 (d, 1JPRh
154, PPh3)
[{Rh(CO)(PPh3)}3(m-
taz)2][PF6] 13+[PF6]-
Dark red
12
46.4
3.8
5.0
7.77–7.14 (m, 45H, PPh3), 4.34
40.35 (d, 1JPRh
(46.6)d (3.7) (4.7)
(dq, 1H, 2JHH 14.7, 3JHH 7.3, taz 169, PPh3),
4-CHHCH3), 4.12 (dq, 1H, 2JHH 39.89 (d, 1JPRh
14.4, 3JHH 7.2, taz 4-CHHCH3), 168, PPh3),
3.60 (dq, 1H, 2JHH 14.5, 3JHH
7.0, taz 4-CHHCH3), 3.41 (dq,
1H, 2JHH 14.6, 3JHH 7.5, taz
39.31 (d, 1JPRh
154, PPh3),
-143.91
4-CHHCH3), 1.54 (s, 6H, taz
3-CH3), 1.21 (t, 3H, 3JHH 7.1,
taz 4-CH2CH3), 1.02 (t, 3H,
3JHH 7.3, taz 4-CH2CH3)
(septet, 1JPF
710, PF6)
[Rh3(CO)4(cod)(m-mt)2]- Purple
[BF4] 14+[BF4]-
80
29.3
(28.5)
3.1
6.5
2080, 2026
6.84 (br s, 2H, mt 4/5-H), 6.54
(br s, 2H, mt 4/5-H), 5.25–5.15
(br m, 2H, cod CH), 3.95–3.75
(br m, 2H, cod CH), 3.56 (s, 6H,
mt 3-CH3), 3.00–2.85 (br m, 2H,
cod CHHexo), 2.55–2.40 (br m,
2H, cod CHHexo), 2.35–2.25 (br
m, 2H, cod CHHendo), 2.00–1.80
—
(2.6) (6.7)
(br m, 2H, cod CHHendo
)
a Calculated values in parentheses. b In CH2Cl2; strong (s) absorptions unless stated, m = medium, sh = shoulder, brd = broad. c Chemical shift (d) in ppm,
J values in Hz, spectra in CD2Cl2 at 20 ◦C. d Calculated as a 1 : 2 CH2Cl2 solvate.
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Fig. 9 Structure of the cation [{Rh(CO)2}3(m-mt)2]+ 10+.
Scheme 3
reversible (8+) or near reversible (9+) oxidation waves but at much
more positive potentials than those of the analogous dimers 1 and
2. As with the dimers, the hexacarbonyl cations are the least stable
on oxidation.
The X-ray structures of complexes 9+, 10+ and 12+–14+. In
order to define their structures more fully, X-ray diffraction
studies were carried out on crystals of [{Rh(cod)}3(m-
taz)2][PF6] 9+[PF6]-, [{Rh(CO)2}3(m-mt)2][BF4] 10+[BF4]-,
[{Rh(CO)(PPh3)} (m-mt)2][BF4]·CH2Cl2
[{Rh(CO)(PPh3)}33(m-taz)2][PF6]·3CH2Cl2
12+[BF4]-·CH2Cl2,
13+[PF6]-·3CH2Cl2
and [Rh3(CO)4(cod)(m-mt)2] [BF4]·2CH2Cl2 14+[BF4]-·2CH2Cl2,
grown by slow diffusion of n-hexane into a concentrated CH2Cl2
Fig. 10 Structure of the cation [{Rh(CO)(PPh3)}3(m-taz)2]+ 13+.
◦
solution of each complex at 5 C. The structures of 9+, 10+, 13+
and 14+ are shown in Fig. 8–11 as representative examples, with
selected bond lengths and angles for all in Table 6. (The structure
of 12+ is shown in the ESI†.)
Fig. 11 Structure of the cation [Rh3(CO)4(cod)(m-mt)2]+ 14+.
bound with minimal distortion from square planar geometry, as
in the dimers 1–7, while the central metal has a more distorted
square planar arrangement with the two cis-bound sulfur atoms
displaced from the RhLL¢ plane.
Fig. 8 Structure of the cation [{Rh(cod)}3(m-taz)2]+ 9+.
To form the trinuclear cations from the dimers 1–6, the rhodium-
eclipsed geometry of the Rh-N-C-S-Rh-N-C-S eight-membered
ring distorts to the sulfur-eclipsed conformation (as observed in
the structure of 7) to accommodate the third metal. The terminal
rhodium atoms move apart (i.e. compared with their positions
in the parent dimer) so that, for example, Rh(2) . . . Rh(2)¢ in
The trirhodium skeleton in each of 9+, 10+ and 12+–14+ is bent
with Rh ◊ ◊ ◊ Rh ◊ ◊ ◊ Rh angles (104.5–112.3◦) comparable with those
4
in [{Rh(CO)2}3(m-pyt)2]+ and [Rh2(cod)2(m-bzt)2Ag(O2ClO2)]8
(112.7 and 110.5◦ respectively). The three rhodium atoms are
linked by two rings, each of which is N-bonded to one terminal
rhodium atom and S-bonded to both the other terminal and the
central rhodium atoms. Thus, the end rhodium atoms are cis N,S-
[{Rh(CO)2}3(m-mt)2]+ 10+ is 5.076(2) A and Rh(1) ◊ ◊ ◊ Rh(2) in
˚
+
+
˚
[Rh(CO)(PPh3)}3(m-mt)2] 12 is 5.062(1) A, longer than 3.049(1)
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Table 5 Electrochemical data for the oxidation of cationic trirhodium complexesa
(Ep)ox/V
(Ep)red/V
E◦¢/V
(ip)red/(ip)ox
[{Rh(cod)}3(m-mt)2][BF4] 8+[BF4]-
[{Rh(cod)}3(m-taz)2][PF6] 9+[PF6]-
[{Rh(CO)2}3(m-mt)2][BF4] 10+[BF4]-
[{Rh(CO)2}3(m-taz)2][PF6] 11+[PF6]-
[{Rh(CO)(PPh3)}3(m-mt)2][BF4] 12+[BF4]-
[{Rh(CO)(PPh3)}3(m-taz)2][PF6] 13+[PF6]-c
0.92
1.01
1.30
1.08
0.80
0.81
0.84
0.91
—
—
—
0.88b
0.97
—
—
—
1.0
0.83
—
—
—
0.69
0.75
0.96
a CV at a platinum disc electrode at scan rate, 200 mV s-1. Calibrated vs. [Fe(h-C5H5)2]0/1+ at E◦¢ = 0.47 V unless stated. b Second, irreversible, wave with
(Ep)ox = 1.32 V. c Calibrated vs. [Fe(h-C5Me5)2]0/1+ at E◦¢ = -0.08 V.
◦
˚
Table 6 Selected bond lengths (A) and angles ( ) for cationic trirhodium complexes
[{Rh(cod)}3(m-
taz)2]+ 9+
[{Rh(CO)2}3(m-mt)2]+
[{Rh(CO)(PPh3)}3(m-
mt)2]+ 12+
[{Rh(CO)(PPh3)}3(m-
taz)2]+ 13+
[Rh3(CO)4(cod)(m-
mt)2]+ 14+
10+a
Rh(1) ◊ ◊ ◊ Rh(2)
5.205(1)
3.291(1)
3.292(1)
2.016(3)
2.022(3)
2.085(3)
2.399(1)
2.021(4)
2.038(4)
2.083(3)
2.388(1)
2.032(3)
2.032(4)
2.358(1)
2.349(1)
3.517(1)
104.49(1)
87.4(1)
91.54(9)
87.0(1)
89.61(9)
87.0(1)
96.70(3)
62.6(1)
64.4(1)
63.5
5.076(2)
3.055(1)
—
5.062(1)
3.077(1)
3.034(1)
2.265(2)
1.835(9) {1.130(9)}
2.087(6)
2.406(2)
2.261(2)
1.815(8) {1.144(9)}
2.103(6)
2.410(2)
2.272(2)
1.830(8) {1.137(8)}
2.392(2)
2.403(2)
3.561(3)
111.86(2)
90.7(3)
88.61(17)
94.2(3)
89.07(17)
86.9(2)
95.92(6)
-56.0(2)
-61.23(19)
-58.6
84.2
75.4
5.019(2)
3.137(1)
3.120(1)
2.278(1)
5.042(2)
3.047(1)
3.073(2)
Rh(1) ◊ ◊ ◊ Rh(3)
Rh(2) ◊ ◊ ◊ Rh(3)
Rh(1)–X(1)b {X(1)–O}
Rh(1)–X(2)c {X(2)–O}
Rh(1)–N(z)d
1.903(7) {1.123(9)}
1.882(13) {1.120(15)}
1.873(11) {1.128(15)}
2.058(11)
2.412(3)
1.875(12) {1.133(15)}
1.871(13) {1.124(16)}
2.084(10)
2.398(3)
2.047(10)
2.055(10)
2.374(3)
2.377(3)
3.560(5)
110.92(4)
90.6(5)
90.0(3)
89.5(5)
89.3(3)
86.9(5)
97.00(11)
60.7(3)
58.3(3)
59.5
1.872(8) {1.123(10)}
1.834(5) {1.156(5)}
2.071(6)
2.090(3)
Rh(1)–S(1)
2.382(2)
—
—
—
2.433(1)
2.287(1)
Rh(2)–X(3)e {X(3)–O}
Rh(2)–X(4)f {X(4)–O}
Rh(2)–N(1)
1.838(4) {1.144(5)}
2.093(3)
Rh(2)–S(2)
—
2.434(1)
Rh(3)–X(5)g {X(5)–O}
Rh(3)–X(6)h {X(6)–O}
Rh(3)–S(1)
1.864(8) {1.147(9)}
1.846(5) {1.144(5)}
—
2.288(1)
2.406(2)
—
3.610(4)
112.34(3)
90.5(3)
88.93(15)
—
2.404(2)
2.423(1)
3.651(2)
106.68(1)
86.90(13)
88.47(10)
92.71(13)
88.10(10)
86.68(14)
98.30(4)
-65.00(12)
-67.50(12)
-66.3
86.6
77.1
78.1
3.204
Rh(3)–S(2)
S(1) ◊ ◊ ◊ S(2)
Rh(1) ◊ ◊ ◊ Rh(3) ◊ ◊ ◊ Rh(2)
X(1)–Rh(1)–X(2)
S(1)–Rh(1)–N(z)
X(3)–Rh(2)–X(4)
S(2)–Rh(2)–N(1)
X(5)–Rh(3)–X(6)
S(1)–Rh(3)–S(2)
S(1)–Rh(1) ◊ ◊ ◊ Rh(2)–N(1)
S(2)–Rh(2) ◊ ◊ ◊ Rh(1)–N(z)
—
89.6(4)
97.20(9)
57.6(2)
—
57.6
80.9
82.1
—
3.291
i
b
q(1)j
69.2
83.8
86.0
3.704
83.1
76.1
76.9
3.280
q(2)k
q(3)l
72.0
3.259
ringcentroid–ringcentroid
a Molecular structure C2 symmetric, other bond lengths/angles equivalent by symmetry. In this structure, Rh(3) = Rh(1¢). b X(1) = midpoint C(11)–C(12)
9+; C(6) 10+; P(1) 12+ and 13+; C(9) 14+. c X(2) = midpoint C(15)–C(16) 9+; C(5) 10+; C(1) 12+; C(11) 13+; C(10) 14+. d N(z) = N(4) 9+ and 13+; N(1)¢
10+; N(3), 12+ and 14+. e X(3) = midpoint C(19)–C(20) 9+; P(2) 12+ and 13+; C(11) 14+. f X(4) = midpoint C(23)–C(24) 9+; C(2) 12+; C(12) 13+; C(12)
14+. g X(5) = midpoint C(27)–C(28) 9+; C(7) 10+; P(3) 12+; C(13) 13+; midpoint C(13)–C(14) 14+. h X(6) = midpoint C(31)–C(32) 9+; C(3) 12+; P(3) 13+;
midpoint C(17)–C(18) 14+. i b = average of the S(1)–Rh(1) ◊ ◊ ◊ Rh(2)–N dihedral angles. j q(1) = angle between the planes {Rh(1), X(1), X(2), S(1), N(z)}
and {Rh(2), X(3), X(4), S(2), N(1)}. k q(2) = angle between the planes {Rh(3), X(5), X(6), S(1), S(2)} and {Rh(1), X(1), X(2), S(1), N(z)}. l q(3) = angle
between the planes {Rh(3), X(5), X(6), S(1), S(2)} and {Rh(2), X(3), X(4), S(2), N(1)}.
˚
There is a notable difference in the effect of the ancillary ligands
on the metal-metal distances in the di- and trinuclear species; steric
effects seem more important for the dimers with the Rh ◊ ◊ ◊ Rh
and 3.618(1) A for [{Rh(CO)2(m-mt)}2] 3 and [{Rh(CO)(PPh3)(m-
mt)}2] 5 respectively.
The distortion of the Rh-N-C-S-Rh-N-C-S ring on formation
of the trinuclear cations also results in a shorter distance between
the centroids of the mutually inclined bridging rings. Compare, for
˚
distance varying, for example, from 3.049 A in [{Rh(CO)2(m-
˚
˚
mt)}2] 3 to 3.618 A in [{Rh(CO)(PPh3)(m-mt)}2] 5 to 3.847 A in
[{Rh(cod)(m-mt)}2] 1. By contrast, the distance between terminal
+
+
˚
example, [{Rh(CO)2}3(m-mt)2] 10 (3.291 A) with [{Rh(CO)2(m-
+
˚
and central rhodium atoms in the trinuclear cations only varies
mt)}2] 3 (3.485 A). However, that distance in [{Rh(cod)}3(m-taz)2]
from 3.034 to 3.292 A {e.g. compare [{Rh(CO)2}3(m-mt)2] 10+
+
+
˚
˚
9 is much longer (3.704 A) apparently as a consequence of the
+
+
˚
(3.055 A) with [{Rh(CO)(PPh3)}3(m-mt)2] 12 (3.077 and 3.034
twisting of the ethyl group on one of the taz ligands so that its
terminal methyl group is positioned between the two bridging
heterocycles.
˚
A)} and that between the terminal rhodium atoms varies between
˚
5.06 to 5.20 A. {The latter distance is actually marginally longer
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in the unhindered hexacarbonyl [{Rh(CO)2}3(m-mt)2]+ 10+ (5.076
solution was filtered and treated with the second solvent, and
the mixture was then reduced in volume in vacuo to induce
precipitation. Unless stated otherwise, the new complexes are
stable under nitrogen and dissolve in solvents such as CH2Cl2,
thf and, in the case of the binuclear species, toluene to give
air-sensitive solutions. The compounds Htaz,20 [{Rh(cod)(m-
Cl)}2],21 [{Rh(CO)2(m-Cl)}2],22 [{Rh(CO)(PPh3)(m-Cl)}2],23 [Fe(h-
C5H5)2][PF6] and [Fe(h-C5H4COMe)(h-C5H5)][BF4]24 were pre-
pared by published methods. Methimazole was purchased from
Across Organics.
+
A) than in the tris(phosphine) [{Rh(CO)(PPh3)}3(m-mt)2] 12+
˚
˚
(5.062 A).}
In [{Rh(CO)(PPh3)}3(m-mt)2]+ 12+ and [{Rh(CO)(PPh3)}3(m-
taz)2]+ 13+ all three phosphorus atoms are trans to sulfur resulting
in each being chemically distinct (as observed by 31P NMR
spectroscopy). The central phosphorus atom is, of course, unique
but P(1) is trans to S(1), which is trans to CO, whereas P(2) is trans
to S(2) which is trans to PPh3. The structural study of 13+ also
suggests an explanation for the differences observed in the proton
and carbon chemical shifts (see above); the taz ring containing
S(1) and N(4) is closer to the aromatic rings of two PPh3 ligands,
P(1)Ph3 and P(3)Ph3, which give a greater shielding effect than
do the phenyl rings of the single P(2)Ph3 ligand for the taz ring
containing S(2) and N(1).
IR spectra in CH2Cl2 were recorded on a PerkinElmer 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 and at 121.4 MHz for 31P, running JEOL Delta
1
1
software. For H and 13C-{ H} spectra, either SiMe4 or residual
protio solvent was used as an internal standard. For 31P-{ H}
1
spectra, 85% H3PO4 was used as an external standard.
Conclusions
Electrochemical studies were carried out using an EG&G model
273A potentiostat linked to a computer running 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 platinum disc (1.6 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 the test compound and
In the solid state the N,S-bridged Rh(I) dimers [{RhLL¢(m-X)} ],
[X = mt or taz, LL¢ = cod, (CO)2, (CO)(PPh3) or {P(OPh)3}22],
prepared from [{RhLL¢(m-Cl)}2], HX and NEt3, have an ‘open
book’ geometry with two structural types resulting from dif-
fering conformations of the head-to-tail, eight-membered Rh-
N-C-S-Rh-N-C-S ring. In [{Rh(cod)(m-mt)}2] 1, [{Rh(cod)(m-
taz)}2] 2, [{Rh(CO)2(m-mt)}2] 3, [{Rh(CO)(PPh3)(m-mt)}2] 5 and
[{Rh(CO)(PPh3)(m-taz)}2] 6, with rhodium-eclipsed geometries,
the extent of the mutual twisting and tilting of the two rhodium
square planes depends on the nature of the ancillary ligands, the
0.1 mol dm-3 in [NBun ][PF6] as the supporting electrolyte. Under
4
the conditions used, E◦¢ for the one-electron oxidation of [Fe(h-
C5H5)2] and [Fe(h-C5Me5)2], added to the test solutions as internal
calibrants, is 0.47 and -0.08 V respectively.
˚
Rh ◊ ◊ ◊ Rh distance varying from 3.05 to 3.85 A. The least hindered
of these complexes, the dark blue-brown dicarbonyl 3, uniquely
shows intermolecular mt ring–ring stacking. By contrast, the steric
bulk of the four P(OPh)3 ligands of [{Rh[P(OPh)3]2(m-taz)}2] 7
leads to a sulfur-eclipsed conformation, with a Rh ◊ ◊ ◊ Rh distance
Microanalyses were carried out by the staff of the Microanalyt-
ical Service of the School of Chemistry, University of Bristol.
˚
of 4.97 A.
Syntheses
Chemical oxidation of [{RhLL¢(m-X)}2] with [Fe(h-C5H5)2]+
or [Fe(h-C5H4COMe)(h-C5H5)]+ provides a new route to the
complexes [(RhLL¢)3(m-mt)2]+; oxidative bridge-splitting of the
dimer liberates the monomeric rhodium(I) fragment [RhLL¢]+
which then reacts with [{RhLL¢(m-X)}2], probably in the sulfur-
eclipsed conformation, to form the trinuclear Rh(I) cations.
Substitution by cod of the carbonyl groups on the central rhodium
metal of [{Rh(CO)2}3(m-mt)2]+ 10+ gives [Rh3(CO)4(cod)(m-mt)2]+
14+.
[{Rh(cod)(l-taz)}2] 2. Solid Htaz (190 mg, 1.33 mmol) was
added to a stirred solution of [{Rh(cod)(m-Cl)}2] (326 mg, 661
mmol) in toluene (60 cm3); after 60 min, an excess (ca. 0.1 cm3)
of triethylamine was added. After a further 10 min the reaction
mixture was filtered to remove solid [NEt3H]Cl. The orange filtrate
was concentrated in vacuo to ca. 5 cm3 and then n-hexane was
added to precipitate an orange solid which was dried in vacuo,
yield 433 mg (93%).
The steric effects of L and L¢ are less marked in the trin-
uclear cations with, for example, the Rhterminal ◊ ◊ ◊ Rhcentral dis-
tances in the bent trirhodium skeletons of [{Rh(cod)}3(m-taz)2]+
9+, [{Rh(CO)2}3(m-mt)2]+ 10+, [{Rh(CO)(PPh3)}3(m-mt)2]+ 12+,
[{Rh(CO)(PPh3)}3(m-taz)2]+ 13+ and [Rh3(CO)4(cod)(m-mt)2]+ 14+
[{Rh(CO)2(l-mt)}2] 3.
A
mixture of [{Rh(CO)2(m-Cl)}2]
(500 mg, 1.29 mmol) and Hmt (295 mg, 2.58 mmol) in CH2Cl2
(50 cm3) was stirred for 10 min and then excess NEt3 (ca. 0.3 cm3)
was added. The resulting dark yellow-red solution was stirred for
15 min and then the solvent was removed in vacuo. The crude
product, a dark blue-brown solid, was extracted into toluene
(60 cm3). The extract was filtered and then concentrated in vacuo
to ca. 20 cm3; addition of n-hexane precipitated a dark blue-brown
solid which was further purified using CH2Cl2–n-hexane and dried
in vacuo, yield 538 mg (77%).
˚
varying only from 3.03 to 3.29 A.
Experimental
The preparation, purification and reactions of the complexes
described were carried out under an atmosphere of dry ni-
trogen, using solvents dried by Anhydrous Engineering double
alumina or alumina/copper catalyst columns. All solvents were
deoxygenated prior to use. Unless stated otherwise, complexes
were purified using a mixture of two solvents. The impure
solid was dissolved in the more polar solvent, the resulting
The complexes [{Rh(cod)(m-mt)}2] 1 and [{Rh(CO)(PPh3)(m-
taz)}2] 6 were prepared similarly from [{Rh(cod)(m-Cl)}2] and
[{Rh(CO)(PPh3)(m-Cl)}2] respectively.
[{Rh(CO)2(l-taz)}2] 4. Carbon monoxide was bubbled
through a solution of [{Rh(cod)(m-taz)}2] 2 (138 mg, 195 mmol) in
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CH2Cl2 (35 cm3) for ca. 15 min. The orange-red solution was then
concentrated in vacuo and n-hexane added to precipitate a purple
air-sensitive solid which was isolated by filtration and dried in
vacuo, yield 75 mg (64%).
Structure determinations
Many of the details of the structure analyses of [{Rh(cod)(m-mt)}2]
1, [{Rh(cod)(m-taz)}2] 2, [{Rh(CO)2(m-mt)}2]·toluene 3·toluene,
[{Rh(CO)(PPh3)(m-mt)}2]·toluene 5·toluene, [{Rh(CO)(PPh3)(m-
taz)}2]
6,
[{Rh[P(OPh)3]2(m-taz)}2]
7,
[{Rh(cod)}3(m-
[{Rh(CO)(PPh3)(l-mt)}2] 5. To a solution of [{Rh(CO)2(m-
mt)}2] 3 (50 mg, 90 mmol) in toluene (40 cm3) was added PPh3
(49 mg, 0.18 mmol). The yellow solution was filtered after 30 min
and then concentrated in vacuo to ca. 20 cm3; addition of n-hexane
precipitated a yellow solid which was dried in vacuo, yield 30 mg
(32%).
taz)2][PF6] 9+[PF6]-, [{Rh(CO)2}3(m-mt)2][BF4] 10+[BF4]-,
[{Rh(CO)(PPh3)}3(m-mt)2][BF4]·CH2Cl2
12+[BF4]-·CH2Cl2,
[{Rh(CO)(PPh3)}3(m-taz)2][PF6]·3CH2Cl2 13+[PF6]-·3CH2Cl2 and
[Rh3(CO)4(cod)(m-mt)2] [BF4]·2CH2Cl2 14+[BF4]-·2CH2Cl2 are
given in Table 7.
X-ray diffraction experiments on 1, 3·toluene, 9+[PF6]-,
12+[BF4]-·CH2Cl2 and 13+[PF6]-·3CH2Cl2 were carried out on a
Bruker SMART diffractometer at 173 K. Similar experiments
on 2, 5·toluene, 6, 7, 10+[BF4]- and 14+[BF4]-·2CH2Cl2 were
carried out on a Bruker APEX diffractometer at 100 K. All
[{Rh[P(OPh)3]2(l-taz)}2] 7. An excess of P(OPh)3 (ca. 0.1 cm3)
was added to [{Rh(CO)2(m-taz)}2] 4, generated in situ by adding
an excess of NEt3 (ca. 0.1 cm3) to a mixture of [{Rh(CO)2(m-
Cl)}2] (210 mg, 0.54 mmol) and Htaz (155 mg,1,08 mmol) in
CH2Cl2 (40 cm3). After stirring the mixture for 80 min, the solvent
was removed in vacuo to give a brown solid residue which was
extracted into toluene (ca. 40 cm3) and then concentrated in vacuo
to ca. 5 cm3. Addition of n-hexane precipitated a pale brown solid
which was isolated by filtration. A concentrated CH2Cl2 solution
of the solid was then added to a silica gel/CH2Cl2 chromatography
column. Elution with CH2Cl2 gave a yellow solution which was
concentrated in vacuo to ca. 5 cm3; addition of n-hexane and
cooling the mixture to -10 ◦C gave a pale yellow solid which
was dried in vacuo, yield 204 mg (22%).
˚
used Mo-Ka X-radiation (l = 0.71073 A). 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 integrated25 from several series of exposures,
each exposure covering 0.3◦ in w. Absorption corrections were
based on equivalent reflections using SADABS,26 and structures
2
were refined against all Fo data with hydrogen atoms riding
in calculated positions using SHELXL.27 The residual electron
density maps for 2, 5 and 6 showed some peaks which could not
be modelled satisfactorily; the data for these crystal structures were
modelled with PLATON/SQUEEZE,28 details for which may be
found in the cif.†
[{Rh(cod)}3(l-taz)}2][PF6] 9+[PF6]-. To an orange solution of
[{Rh(cod)(m-taz)}2] 2 (159 mg, 225 mmol) in CH2Cl2 (45 cm3)
was added [Fe(h-C5H5)2][PF6] (153 mg, 462 mmol). After 45 min
the solvent was removed from the red-black solution in vacuo
and the crude product washed with 3 ¥ 20 cm3 portions of n-
hexane. The solid was dissolved in ca. 2 cm3 CH2Cl2 and then
passed through a short alumina/CH2Cl2 column. The red CH2Cl2
eluate was concentrated in vacuo to ca. 5 cm3 and, after adding
n-hexane and cooling the mixture to -10 ◦C, a dark red solid was
precipitated, yield 78 mg (49% based on rhodium).
There was some minor disorder pertaining to the anion or
solvent molecules in four of the structures for which restraints
were imposed in order to ensure smooth refinement. In 1, the
cyclooctadiene ligand was disordered over two positions, which
converged to a 0.67 : 0.33 ratio. In 10+[BF4]-, three of the fluorine
atoms in the [BF4]- anion are disordered over two positions, each
with occupancy 0.5. In 13+[PF6]-·3CH2Cl2, the [PF6]- anion and
one of the CH2Cl2 molecules are disordered over two positions,
each in a 0.65 : 0.35 ratio, and in 14+[BF4]-·2CH2Cl2 the [BF4]-
anion is disordered over two positions in a 0.65 : 0.35 ratio.
The compound [{Rh(CO)(PPh3)}3(m-taz)}2][PF6] 13+[PF6]-
was prepared similarly from 6, as were [{Rh(cod)}3(m-
mt)2][BF4] 8+[BF4]-, [{Rh(CO)2}3(m-mt)2][BF4] 10+[BF4]- and
[{Rh(CO)(PPh3)}3(m-mt)2][BF4] 12+[BF4]- (from 1, 3, and 5 re-
spectively) but by using [Fe(h-C5H4COMe)(h-C5H5)][BF4] as the
oxidant.
Acknowledgements
We thank the EPSRC for postgraduate studentships (to R.J.B.,
M.J.L.-G., A.H. and T.R.-J) and Johnson Matthey for a generous
loan of hydrated rhodium trichloride. We also acknowledge the
use of the EPSRC’s Chemical Database Service at Daresbury.29
[{Rh(CO)2}3(l-taz)}2][PF6] 11+[PF6]-. Carbon monoxide was
bubbled through a solution of [{Rh(cod)}3(m-taz)2][PF6] 9+[PF6]-
(84 mg, 79 mmol) in CH2Cl2 (30 cm3) for ca. 30 min. The red-black
solution was then concentrated in vacuo and n-hexane added to
precipitate a dark purple air-sensitive solid which was isolated by
filtration and dried in vacuo, yield 46 mg (64%).
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