Redox-active catecholate complexes of rhodium hydrotris(pyrazolyl)-borates

Article abstract of DOI:10.1039/b010061g

The complexes [Rh(CO)LTp′] {L = CO or PPh₃, Tp′ = hydrotris(3,5-dimethylpyrazolyl)borate} reacted with the ortho-quinone o-C₆Cl₄O₂ (o-chloranil; 3,4,5,6-tetrachloro-l,2-benzoquinone) to give [Rh{C(O)OC₆Cl₄O}LTp′] (L = CO, 1; L = PPh₃, 2). X-Ray structural studies on 2 reveal CO insertion into one Rh-O bond of a rhodium-catecholate ring. Loss of the inserted CO on UV irradiation (of 1) or thermolysis (of 2) gives [Rh(o-O₂C₆Cl₄)LTp′] (L = CO 3 or PPh₃ 4); thermal substitution of the CO ligand of [Rh(o-O₂C₆Cl₄)(CO)Tp′] with L provides a second route to [Rh(o-O₂C₆Cl₄)(PPh₃)Tp′] as well as [Rh(o-O₂C₆Cl₄)LTp′] {L = AsPh₃ 5, P(OPh)₃ 6 or py 7} which are oxidised by [NO]⁺ to the monocations [Rh(o-O₂C₆Cl₄)LTp′]⁺ 4⁺-7⁺. X-Ray structural studies on the redox pair [Rh(o-O₂C₆Cl₄)(PPh₃)Tp′] (z = 0, 4 or 1, 4⁺) are consistent with catecholate ligand-based oxidation; the ESR spectra of the paramagnetic cations 4⁺-7⁺ suggest little delocalisation of unpaired electron density from the semiquinone ligand to the Rh^IIITp′ unit.

Full text of DOI:10.1039/b010061g

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Redox-active catecholate complexes of rhodium hydrotris(pyrazolyl)-
borates
Neil G. Connelly,* David J. H. Emslie, Owen D. Hayward, A. Guy Orpen and
Michael J. Quayle
School of Chemistry, University of Bristol, Bristol, UK BS8 1TS
Received 18th December 2000, Accepted 22nd January 2001
First published as an Advance Article on the web 16th February 2001
The complexes [Rh(CO)LTpЈ] {L = CO or PPh3, TpЈ = hydrotris(3,5-dimethylpyrazolyl)borate} reacted with the
ortho-quinone o-C6Cl4O2 (o-chloranil; 3,4,5,6-tetrachloro-1,2-benzoquinone) to give [Rh{C(O)OC6Cl4O}LTpЈ]
(L = CO, 1; L = PPh3, 2). X-Ray structural studies on 2 reveal CO insertion into one Rh–O bond of a rhodium–
catecholate ring. Loss of the inserted CO on UV irradiation (of 1) or thermolysis (of 2) gives [Rh(o-O2C6Cl4)LTpЈ]
(L = CO 3 or PPh3 4); thermal substitution of the CO ligand of [Rh(o-O2C6Cl4)(CO)TpЈ] with L provides a second
route to [Rh(o-O2C6Cl4)(PPh3)TpЈ] as well as [Rh(o-O2C6Cl4)LTpЈ] {L = AsPh3 5, P(OPh)3 6 or py 7} which are
oxidised by [NO]ϩ to the monocations [Rh(o-O2C6Cl4)LTpЈ]ϩ 4؉–7؉. X-Ray structural studies on the redox pair
[Rh(o-O2C6Cl4)(PPh3)TpЈ]z (z = 0, 4 or 1, 4؉) are consistent with catecholate ligand-based oxidation; the ESR spectra
of the paramagnetic cations 4؉–7ϩ suggest little delocalisation of unpaired electron density from the semiquinone
ligand to the RhIIILTpЈ unit.
Introduction
Results and discussion
Our recent studies have shown that one-electron oxidation of
the κ2-rhodium() species [Rh(CO)LTpЈ] {TpЈ = the hydro-
tris(pyrazolyl)borate ligand [HBR3]Ϫ, R = 3,5-dimethyl-
pyrazolyl} gives stable κ3-rhodium() complexes [Rh(CO)-
LTpЈ]ϩ in which the RhII is stabilised by the formation of a
third Rh–N bond, in the axial site of a square pyramidal struc-
ture.1 The isolation of such stable rhodium() complexes is
noteworthy and contrasts with the behaviour of the analo-
gous cyclopentadienyl species [Rh(CO)(PPh3)(η-C5H5)] which,
on oxidation, gave the fulvalene-bridged dication [Rh2-
Synthesis
The reaction of [Rh(CO)LTpЈ] (L = CO or PPh3) with 3,4,5,6-
tetrachloro-1,2-benzoquinone (o-chloranil) in the absence of
light gave bright yellow (1, L = CO) or orange (2, L = PPh3)
solids (Scheme 1), characterised by elemental analysis and IR
(CO)2(PPh3)2(η5 : ηЈ5-C10H8)]2ϩ 2
although the intermediate
;
rhodium() cation [Rh(CO)(PPh3)(η-C5H5)]ϩ was not detected
spectroscopically the sterically protected derivative [Rh-
(CO)(PPh3)(η-C5Ph5)]ϩ was fully characterised.3 Stable
paramagnetic rhodium complexes were also obtained by one-
electron oxidation of the rhodium() catecholate complexes
[Rh(o-O2C6Cl4)L(η-C5R5)] (L = PPh3, py, etc.; R = H or Me).
The cations [Rh(o-O2C6Cl4)L(η-C5R5)]ϩ were, however, best
formulated as semiquinone rhodium() complexes although
delocalisation of unpaired electron density onto both Rh and L
was evident from the ESR spectra.4
An analogy is often drawn5,6 between η-C5R5 and hydro-
tris(pyrazolyl)borate ligands but, as noted above, there is a
significant difference between the redox chemistry of
[Rh(CO)LTpЈ] and [Rh(CO)L(η-C5H5)]. We now therefore
compare the reactions of the ortho-quinone o-C6Cl4O2
(o-chloranil; 3,4,5,6-tetrachloro-1,2-benzoquinone) with [Rh-
(CO)LTpЈ] and [Rh(CO)L(η-C5R5)].4 Although the chemistry is
generally analogous, the TpЈ ligand stabilises the carbonyl
insertion products [Rh{C(O)OC6Cl4O}LTpЈ] and allows the
isolation of the stable but substitutionally labile carbonyl
[Rh(o-O2C6Cl4)(CO)TpЈ]. Moreover, structural studies of the
redox pair [Rh(o-O2C6Cl4)(PPh3)TpЈ]z (z = 0 or 1) and the ESR
spectra of [Rh(o-O2C6Cl4)(PPh3)TpЈ]ϩ are consistent with
increased localisation of unpaired electron density on the
semiquinone ligand of the paramagnetic cations.
Scheme 1 BN3 = hydrotris(3,5-dimethylpyrazolyl)borate.
(Table 1) and NMR (Table 2) spectroscopy. The IR spectra in
CH2Cl2 show ketonic carbonyl bands in the region 1700–1740
cmϪ1 and, in the case of 1, a band at 2108 cmϪ1 corresponding
DOI: 10.1039/b010061g
J. Chem. Soc., Dalton Trans., 2001, 875–880
This journal is © The Royal Society of Chemistry 2001
875

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Fig. 1 The molecular structure of complex 2. Hydrogen atoms have
been omitted for clarity (in all structures shown).
to a terminal CO bound to Rh; the spectra of 1 and 2 (and of
all the complexes reported herein) also show ν(BH) bands
corresponding unambiguously to κ3 co-ordination of the TpЈ
ligand to rhodium, in both solution and the solid state.7 The
spectroscopic data suggest that 1 and 2 are analogous to the
products of the reactions between [Rh(CO)L(η-C5R5)] (L = CO
or PPh3, R = H or Me) with o-C6Cl4O2,4 i.e. that 1 and 2 are
[Rh{C(O)OC6Cl4O}LTpЈ]. This suggestion was borne out by an
X-ray structural study of complex 2.
The structure of complex 2 is shown in Fig. 1 and important
bond lengths and angles, together with those for the two
crystallographically independent molecules of [Rh{C(O)-
OC6Cl4O}(PPh3)(η-C5H5)] for comparison,4 are given in Table
3. The rhodium atom of 2 is essentially octahedral, co-
ordinated to κ3-TpЈ and PPh3 ligands and bound to oxygen and
carbon atoms as part of a Rh{C(O)OC6Cl4O} metallacycle; the
metallacycle is formed, at least formally, by the insertion of CO
into one Rh–O bond of a rhodium catecholate ring. Within the
Rh{C(O)OC6Cl4O} metallacycle the C(34)–O(1) and C(34)–
O(3) lengths [1.199(3) and 1.409(2) Å] are normal for carbon–
oxygen double and single bonds [1.23(1) and 1.43(1) Å].8 The C6
ring is planar with C–C distances which vary between 1.379(3)
and 1.411(3) Å.
Though the structure of complex 2 is generally similar to that
of [Rh{C(O)OC6Cl4O}(PPh3)(η-C5H5)], there are some small
differences in the conformation of the metallacycle which is
more flattened in 2 than in the η-C5H5 complex. In 2 the Rh–
N(6) bond trans to the ketonic carbonyl group [2.247(2) Å] is
significantly longer than Rh–N(1) [2.091(2) Å], trans to oxygen,
and Rh–N(4) [2.108(2)Å], trans to phosphorus, in accord with
the high trans influence of the σ-bound carbon ligand. The Rh–
P(1) distance [2.364(1) Å] in 2 is longer [cf. 2.298(3) Å], and the
Rh–O(2) distance shorter [2.022(2), cf. 2.079(6) Å] than the
corresponding lengths in [Rh{C(O)OC6Cl4O}(PPh3)(η-C5H5)].4
Complexes 1 and 2 are significantly more stable than their
cyclopentadienyl analogues. Both are stable in solution (in the
absence of air) whereas solid [Rh{C(O)OC6Cl4O}(CO)-
(η-C5R5)] (R = H or Me) rapidly loses two molecules of CO on
dissolution, to give [{Rh(O2C6Cl4)(η-C5R5)}n], and [Rh{C(O)-
OC6Cl4O}(PPh3)(η-C5R5)] gives [Rh(o-O2C6Cl4)(PPh3)(η-C5-
R5)], albeit more slowly.4 Decarbonylation occurs on UV
irradiation for 6 hours (for 1) or heating under reflux (for 2).
Moreover, 1 only loses the CO group inserted into the Rh–O
bond, giving orange-brown [Rh(o-O2C6Cl4)(CO)TpЈ] 3 the
C5R5 analogues of which were not observed. (Subsequent
heating of a toluene solution of 3 under reflux for five
days resulted in decarbonylation to give a black solution
containing unidentified products perhaps analogous to
876
J. Chem. Soc., Dalton Trans., 2001, 875–880

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Table 2 Proton, 13C-{1H} and 31P NMR spectroscopic data for rhodium hydrotris(pyrazolyl)borate complexesa
Complex
1H
13C-{1H}
31P
[Rh{C(O)OC6Cl4O}(CO)TpЈ] 1
5.97 (s, 1H,TpЈ CH), 5.94 (s, 1H,TpЈ
CH), 5.91 (s, 1H,TpЈ CH), 2.47 (s, 3H,
TpЈ CCH3), 2.42 (s, 6H, TpЈ CCH3),
2.41 (s, 3H, TpЈ CCH3), 2.21 (s, 3H,
TpЈ CCH3), 2.20 (s, 3H, TpЈ CCH3)
180.0 (d, JCRh 59, CO), 173.2 {d, JCRh 35,
C(O)C6Cl4O}, 152.1, 151.8, 151.4, 146.9,
146.1, 145.6 (TpЈ CCH3), 152.1, 141.3,
127.1, 123.1, 122.1, 120.5 {C(O)C6Cl4O},
108.9, 108.5, 107.5 (TpЈ CH), 15.7, 14.2,
13.6, 13.2, 12.9, 12.4 (s, TpЈ CH3)
—
[Rh{C(O)OC6Cl4O}(PPh3)TpЈ]ؒ0.33
CH2Cl2 2
7.38–6.87 {m, 15H, P(C6H5)3}, 5.80 (s,
1H,TpЈ CH), 5.75 (s, 1H,TpЈ CH), 5.15
(s, 1H,TpЈ CH), 2.53 (s, 3H, TpЈ
CCH3), 2.31 (s, 3H, TpЈ CCH3), 2.30 (s,
174.5 {dd, JCRh 39, JCP 12, C(O)C6Cl4O}, 20.7 (d, JPRh 121)
155.6, 152.1 (TpЈ CCH3), 150.9 {C(O)C-
6Cl4O}, 150.7 (d, J 5, TpЈ CCH3) 146.4,
145.1 (TpЈ CCH3), 144.8 (d, J 3, TpЈ CCH3),
3H, TpЈ CCH3), 1.99 (s, 3H, TpЈ 140.3 {s, C(O)C6Cl4O}, 134.7 {d, JCP 10,
CCH3), 1.73 (s, 3H, TpЈ CCH3), 1.42 (s,
3H, TpЈ CCH3)
P(C6H5)3}, 130.8 {d, JCP 3, P(C6H5)3}, 130.7
{d, JCP 47, P(C6H5)3}, 128.3 {d, JCP 11,
P(C6H5)3}, 125.7, 123.7, 120.9, 117.1
{C(O)C6Cl4O}, 108.7, 108.6 (TpЈ CH),
108.1 (d, J 4, TpЈ CH), 18.3, 13.1, 13.0, 12.9,
12.8, 12.7 (TpЈ CH3)
[Rh(o-O2C6Cl4)(CO)TpЈ] 3
5.99 (s, 2H, TpЈ CH) 5.82 (s, 1H, TpЈ
CH), 2.64 (s, 6H, TpЈ CCH3), 2.42 (s,
176.6 (d, JCRh 58, CO), 158.4 (C6Cl4O2),
153.1, 152.46, 146.9, 145.2 (TpЈ CCH3),
—
6H, TpЈ CCH3), 2.37 (s, 3H, TpЈ 118.6 (s, C6Cl4O2), 116.5 (d, J 2, C6Cl4O2),
CCH3), 1.92 (s, 3H, TpЈ CCH3)
109.0, 108.8 (TpЈ CH), 13.3, 12.7, 12.3 (TpЈ
CH3)
[Rh(o-O2C6Cl4)(PPh3)TpЈ]ؒCH2Cl2 4
7.36–6.87 {m, 15H, P(C6H5)3}, 5.64 (d, 160.0 (d, J 1, C6Cl4O2), 155.9 (TpЈ CCH3), 8.6 (d, JPRh 124)
1H, J 1, TpЈ CH), 5.53 (s, 2H, TpЈ
CH), 2.36 (s, 6H, TpЈ CCH3), 2.20 (s,
150.4 (d, J 5, TpЈ CCH3), 147.0 (TpЈ CCH3),
144.5 (d, J 3, TpЈ CCH3), 135.2 {d, JCP 9,
3H, TpЈ CCH3), 1.91 (s, 6H, TpЈ P(C6H5)3}, 131.1 {d, JCP 48, P(C6H5)3},
CCH3), 1.81 (s, 3H, TpЈ CCH3)
130.9 {d, JCP 2, P(C6H5)3}, 128.2 {d, JCP 10,
P(C6H5)3}, 117.3, 116.7 (C6Cl4O2), 110.0
(TpЈ CH), 107.9 (d, J 5, TpЈ CH), 13.5, 12.9,
12.8, 12.8 (TpЈ CH3)
[Rh(o-O2C6Cl4)(AsPh3)TpЈ] 5
[Rh(o-O2C6Cl4){P(OPh)3}TpЈ] 6
7.4–7.0 {m, 15H, As(C6H5)3}, 5.67 (s, 160.1 (C6Cl4O2), 155.9, 151.1, 146.9, 145.0
—
1H, TpЈ CH), 5.56 (s, 2H, TpЈ CH),
2.39 (s, 6H, TpЈ CCH3), 2.25 (s, 3H,
TpЈ CCH3), 1.98 (s, 6H, TpЈ CCH3),
1.85 (s, 3H, TpЈ CCH3)
(TpЈ CCH3), 134.4 {As(C6H5)3}, 132.9 {d,
J 1, As(C6H5)3}, 130.5, 128.8 {As(C6H5)3},
117.3, 116.6 (C6Cl4O2), 109.7, 108.4 (TpЈ
CH), 13.8, 12.9, 12.8, 12.6 (TpЈ CH3)
7.05–6.61 {m, 15H, P(OC6H5)3}, 5.82 159.9 (C6Cl4O2), 155.3 (TpЈ CCH3), 150.8 74.6 (d, JPRh 204)
(s, 2H, TpЈ CH), 5.67 (d, 1H, J 3, TpЈ {d, JCP 14, P(OC6H5)3}, 150.7 (d, J 8, TpЈ
CH), 2.72 (s, 6H, TpЈ CCH3), 2.24 (s,
CCH3), 146.5 (TpЈ CCH3), 144.3 (d, J 6, TpЈ
6H, TpЈ CCH3), 2.21 (s, 3H, TpЈ CCH3), 129.8, 125.2 {P(OC6H5)3}, 119.9 {d,
CCH3), 1.86 (s, 3H, TpЈ CCH3)
JCP 5, P(OC6H5)3}, 117.2, 116.3 (C6Cl4O2),
110.1 (TpЈ CH), 107.8 (d, J 8, TpЈ CH),
14.0, 12.7, 12.6 (TpЈ CH3)
[Rh(o-O2C6Cl4)(py)TpЈ] 7
8.3–7.0 (m, 5H, NC5H5), 5.86 (s, 2H,
TpЈ CH), 5.75 (s, 1H, TpЈ CH), 2.48 (s,
159.7 (C6Cl4O2), 154–152 (broad s, NC5H5)
153.1, 152.1, 145.9, 145.6 (TpЈ CCH3), 139.0
—
6H, TpЈ CCH3), 2.38 (s, 3H, TpЈ (s, NC5H5), 127–125 (broad s, NC5H5), 117.3
CCH3), 1.76 (s, 3H, TpЈ CCH3), 1.72 (s, (C6Cl4O2), 117.2 (d, J 1, C6Cl4O2), 109.1,
6H, TpЈ CCH3)
108.9 (TpЈ CH), 13.5, 13.0, 12.7, 12.0 (TpЈ
CH3)
a Chemical shift (δ) in ppm, J values in Hz, spectra in CD2Cl2.
[{Rh(O2C6Cl4)(η-C5R5)}n].) Decarbonylation of 2 gives [Rh-
(o-O2C6Cl4)(PPh3)TpЈ] 4 which is, however, analogous to
[Rh(o-O2C6Cl4)(PPh3)(η-C5R5)].
approximately 70–100 nm shorter in wavelength than that of
the η-C5H5 analogue which ranges from 475 nm for the pyridine
complex to 558 nm for the AsPh3 complex.4 The dependence of
energy on L suggests the absorption derives from catecholate-
to-L charge transfer.
The high energy of the ν(CO) band of complex 3 (2120 cmϪ1
)
suggested carbonyl substitution would readily occur and,
indeed, heating with PPh3 in toluene at 70 ЊC provided a second
route to 4, as well as to [Rh(o-O2C6Cl4)LTpЈ] {L = AsPh3 5,
P(OPh)3 6 or py 7}. Complexes 3–7 were characterised by elem-
ental analysis and IR, UV-visible (Table 1) and NMR spec-
troscopy (Table 2); in contrast to the η-C5R5 analogues, no
spectroscopic evidence was found for dissociation of the ligand,
L, in solution. (The molecular structure of 4 is discussed below,
together with that of the monocation 4؉.)
The most intense absorption in the electronic spectra of
complexes 1–7 lies between 306 and 325 nm, in the UV region,
and is virtually independent of the nature of the ligand L. A
very similar absorption occurs for the cations 4؉–7؉ (324–328
nm) (see below). The only absorption in the visible region is
highly dependent on L, occurring from approximately 380 nm
for the bright yellow pyridine complex 7 to 494 nm for the pink
triphenylarsine complex 5. This absorption is, in each case,
Voltammetric studies
The cyclic voltammograms of the insertion products 1 and 2, in
CH2Cl2 at a platinum electrode, are relatively uninformative,
each showing an irreversible oxidation wave {(Ep)ox = 1.47 and
1.25 V respectively} and, for 1, an irreversible reduction wave
(at ca. –1.5 V); each process is accompanied by ill defined prod-
uct waves. Unlike [Rh{C(O)OC6Cl4O}(PPh3)(η-C5Me5)], 2 does
not show a product wave corresponding to the formation of the
ligand decarbonylation product 4, perhaps a further indication
of the greater stability of the Rh{C(O)OC6Cl4O} metallacycle
in the TpЈ complex.
By contrast, each of complexes 3–7 shows one reversible one-
electron oxidation wave with no evidence for dissociation of L
from [Rh(o-O2C6Cl4)LTpЈ] (cf. the detection of the oxidation
J. Chem. Soc., Dalton Trans., 2001, 875–880
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Table 3 Selected bond lengths (Å) and angles (Њ) for complex 2 and
the two crystallographically independent molecules, A and B, of
[Rh{C(O)OC6Cl4O}(PPh3)(η-C5H5)]a
2
A
B
Rh(1)–N(1)
Rh(1)–N(4)
Rh(1)–N(6)
Rh(1)–P(1)
Rh(1)–O(2)
Rh(1)–C(34)
C(35)–O(2)
C(40)–O(3)
C(34)–O(3)
C(34)–O(1)
C(35)–C(36)
C(36)–C(37)
C(37)–C(38)
C(38)–C(39)
C(39)–C(40)
C(40)–C(35)
2.091(2)
2.108(2)
2.247(2)
2.364(1)
2.022(2)
1.969(2)
1.316(3)
1.369(3)
1.409(2)
1.199(3)
1.409(4)
1.391(3)
1.392(4)
1.379(4)
1.405(3)
1.411(3)
—
—
—
—
—
—
2.302(3)
2.079(6)
1.986(11)
1.322(9)
1.341(17)
1.401(9)
1.207(16)
1.427(26)
1.413(17)
1.376(31)
1.365(28)
1.428(12)
1.427(19)
2.294(3)
2.078(4)
2.001(12)
1.309(7)
1.373(12)
1.423(7)
1.192(13)
1.403(14)
1.363(9)
1.398(16)
1.401(16)
1.371(9)
1.395(14)
Fig. 2 The molecular structure of complex 4.
O(2)–Rh(1)–C(34)
C(34)–Rh(1)–P(1)
P(1)–Rh(1)–O(2)
91.5(1)
95.8(1)
93.3(1)
85.2(3)
89.4(3)
87.1(2)
87.7(3)
88.7(3)
84.6(2)
a Data from ref. 4.
waves for [{Rh(O2C6Cl4)(η-C5R5)}n] in the CV of [Rh(o-O2-
C6Cl4)L(η-C5R5)]). The oxidation potentials, EЊЈ, are 120–190
mV more positive than those of the η-C5H5 analogues, suggest-
ing the η-C5H5 ligand to be more electron-donating than TpЈ
when bound to the Rh(o-O2C6Cl4)L fragment. As Bergman and
co-workers have recently pointed out,9 the relative donor abil-
ities of cyclopentadienyl and hydrotris(pyrazolyl)borate ligands
seem to depend on the metal centre to which they are bound so
that no useful generalisations can yet be made. This is strikingly
underlined by comparing the redox behaviour of the catechol-
ate complexes with that of the related species [Rh(CO)-
(PPh3)TpЈ]1 and [Rh(CO)(PPh3)(η-C5H5)]2 which are oxidised
at 0.31 and ca. 0.47 V (albeit irreversibly) respectively. Thus,
TpЈ appears to be more electron donating than η-C5H5 when
bound to Rh(CO)(PPh3).
Fig. 3 The molecular structure of complex 4؉.
Apart from the carbonyl 3, the dependence of EЊЈ on L for
complex 4–7 is small, varying by only 50 mV from L = AsPh3 to
P(OPh)3. This may suggest less delocalisation over the
Rh(O2C6Cl4)L unit than for [Rh(o-O2C6Cl4)L(η-C5H5)] for
which the variation in EЊЈ is 110 mV for L = AsPh3 to P(OPh)3.4
atom, only [Rh(o-O2C6Cl4){P(OPh)3}TpЈ]ϩ 6؉ shows resolved
coupling to phosphorus; at 6 G this is considerably less than
observed for [Rh(o-O2C6Cl4){P(OPh)3}(η-C5H5)]ϩ (20.8 G),
lending support to the electrochemical results described above
which suggested less delocalisation of unpaired electron density
from the semiquinone ligand to RhIII in [Rh(o-O2C6Cl4)LTpЈ]ϩ
than in [Rh(o-O2C6Cl4)L(η-C5H5)]ϩ.
Chemical oxidation reactions
Chemical oxidation of [Rh(o-O2C6Cl4)LTpЈ] with [NO][PF6]
gave the cations [Rh(o-O2C6Cl4)LTpЈ]ϩ {L = PPh3 4؉, AsPh3 5؉,
P(OPh)3 6؉ or py 7؉}, characterised as brown or purple [PF6]Ϫ
salts (Table 1), which are reversibly reduced at potentials essen-
tially identical to those for the oxidation of 4–7. Each of the
cations 4؉–7؉ exhibits strong UV-visible absorptions at 324–
328 and 482–502 nm, similar to those of the neutral complexes
1–7. An additional band at 664–711 nm may arise from
rhodium to semiquinone charge transfer.
Structures of complexes 4 and 4ϩ
The molecular structures of complexes 4 and 4؉ are shown in
Figs. 2 and 3 and important bond lengths and angles are
given in Table 4, together with selected data for the redox pair
[Ru(o-O2C6Cl4)(CO)2(PPh3)2]z (z = 0 or 1)11 for comparison.
Both complexes 4 and 4؉ are octahedral with the metal
bound to a κ3-TpЈ ligand, PPh3, and an O,OЈ-chelate. In each
case, a plane of symmetry bisects the O,OЈ-chelate; in contrast
to 2, the Rh(o-O2C6Cl4) metallacycle is nearly planar in both 4
and 4؉. The C–O bond distances define the bonding in the
Rh(o-O2C6Cl4) units. Thus, in 4 the C(40)–O(1) bond length
[1.339(4) Å] is typical of the complexed catecholate dianion
o-C6Cl4O22Ϫ (1.33–1.35 Å) whereas in 4؉ the C–O length is sig-
nificantly shorter [1.293(4)Å] and typical of the co-ordinated
semiquinone radical anion o-C6Cl4O2Ϫ (1.26–1.30 Å).12
The ν(BH) stretches of cations 4؉–7؉ lie between 2561 and
2564 cmϪ1, 10–20 cmϪ1 higher in energy than those of the
corresponding neutral complexes 4–7. Much larger shifts in
ν(BH) (ca. 85–90 cmϪ1
) are observed on oxidation of
[Rh(CO)L(κ2-TpЈ)] to [Rh(CO)L(κ3-TpЈ)]ϩ.1
The isotropic ESR spectra of cations 4؉–7؉ are very similar,
consisting of narrow lines with giso ranging from 2.000 (L =
PPh3 or py) to 2.002 {(L = P(OPh)3}, close to the free electron
value and to that of the uncomplexed o-C6Cl4O2Ϫ radical anion
(2.0053),10 indicating that the unpaired electron resides mainly
on the O-donor ligand of a rhodium() semiquinone complex.
Unlike the cyclopentadienyl analogues,4 which generally show
hyperfine coupling to both rhodium and the P- or As-donor
As in complex 2, the Rh–N(4) bond trans to PPh3 in both 4
and 4؉ [2.130(4) and 2.143(3) Å respectively] is longer than
Rh–N(1) which is trans to oxygen [2.080(3) and 2.039(2) Å],
presumably as a result of trans influence effects. The Rh–N
878
J. Chem. Soc., Dalton Trans., 2001, 875–880

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Table 4 Selected bond lengths (Å) and angles (Њ) for [Rh(o-O2C6-
Cl4)(PPh3)TpЈ] 4 and [Rh(o-O2C6Cl4)(PPh3)TpЈ][PF6] 4ϩ[PF6]Ϫ and for
trans-[Ru(CO)2(PPh3)2(O2C6Cl4)]z (z = 0 or 1)a
described were carried out under an atmosphere of dry nitrogen
using dried, distilled and deoxygenated solvents; reactions were
monitored by IR spectroscopy where necessary. Unless stated
otherwise complexes (i) were purified by dissolution in CH2Cl2,
filtration of the solution through Celite, addition of n-hexane
to the filtrate and reduction of the volume of the mixture
in vacuo to induce precipitation, and (ii) are stable under nitro-
gen and dissolve in polar solvents such as CH2Cl2, acetone and
thf to give moderately air-stable solutions. Photolysis reactions
were carried out in silica tubes placed ca. 20 cm from a 500 W
mercury vapour lamp as a source of UV irradiation.
4
4ϩ
Ru, z = 0
Ru, z = 1
Rh–N(1)
Rh–N(4)
M–P(1)
2.080(3)
2.130(4)
2.357(1)
2.039(2)
2.143(3)
2.390(1)
—
—
—
—
2.434(2)
2.414(2)
2.062(3)
2.065(3)
1.334(5)
1.326(5)
1.417(7)
1.829(5)
2.420(2)
2.437(2)
2.098(3)
2.088(3)
1.291(5)
1.289(5)
1.439(7)
1.826(5)
M–O(1)
2.021(2)
—
1.339(4)
—
1.383(4)
1.830(4)
2.048(2)
—
1.293(4)
—
1.454(6)
1.831(3)
The compounds [Rh(CO)2TpЈ]14 and [Rh(CO)(PPh3)TpЈ]1
were prepared by published methods. IR spectra were recorded
on a Nicolet 5ZDX FT spectrometer and UV-visible spectra on
a Perkin-Elmer Lambda 2 UV/VIS spectrometer. X-Band ESR
spectra were recorded on a Bruker 300ESP spectrometer
equipped with a Bruker variable temperature accessory and a
Hewlett-Packard 5350B microwave frequency counter. The field
calibration was checked by measuring the resonance of the
dpph (diphenylpicrylhydrazyl) radical before each series of
spectra. NMR spectra were recorded on JEOL GX270, GX400
and l300 spectrometers with SiMe4 as an internal standard. All
spectrometers operated in the Fourier transform mode, with
field stability maintained by an external lock system. Electro-
chemical studies were carried out in CH2Cl2 as previously
described.15 Under the conditions used, EЊЈ for the one-electron
oxidations of [Fe(η-C5H5)2] and [Fe(η-C5H4COMe)2], added to
the test solutions as internal calibrants for complexes 3–7 and
4؉–7؉ respectively, are 0.47 and 0.97 V. Microanalyses were
carried out by the staff of the Microanalytical Service of the
School of Chemistry, University of Bristol.
C(40)–O(1)
C(40)–C(40A)
P–Cav
O(1)–M-O(1A)
O(1)–M–P(1)
N(1)–M–N(4)
N(1)–M–N(1A)
83.49(13)
96.0(1)
83.75(11)
96.1(2)
80.97(12)
92.66(6)
84.36(9)
96.28(14)
80.9(1)
92.3(2)
—
78.4(1)
90.6(2)
—
—
—
a Data from ref. 11.
bonds are slightly shorter in 4؉ than in 4 perhaps as a con-
sequence of some of the positive charge residing on rhodium.
The Rh–P bond is longer in the cation [2.390(1) compared to
2.357(1) Å] as is Rh–O [2.048(2) compared to 2.021(2) Å]. The
increase on oxidation in the Rh–P bond length (0.033 Å) is
greater than that observed on oxidation of [Ru(o-O2C6Cl4)-
(CO)2(PPh3)2] (ca. 0.005 Å)11 and towards the lower end of the
range observed for metal–phosphine complex redox pairs
{increases range from 0.012(2) to 0.117(4) Å}.13 The lack of any
observable change in the mean P–C bond lengths on oxidation
is in accord with our overall view that oxidation is primarily
confined to the catecholate ligand of 4.
Preparations
[Rh{C(O)OC6Cl4O}(CO)TpЈ] 1. A solution of [Rh(CO)2TpЈ]
(0.20 g, 0.44 mmol) and o-C6Cl4O2 (0.11 g, 0.44 mmol) in tolu-
ene (45 cm3) was stirred in the absence of light for 20 min and
then evaporated to dryness in vacuo. Purification of the result-
ing solid (three times) gave the product as a bright yellow solid,
yield 165 mg (54%).
Conclusion
The reaction of [Rh(CO)LTpЈ] with o-C6Cl4O2 gives the car-
bonyl insertion products [Rh{C(O)OC6Cl4O}LTpЈ] (L = CO 1
or PPh3 2) which are considerably more stable to heat and UV
light than the η-C5R5 analogues. The complexes [Rh(o-O2C6-
Cl4)LTpЈ] {L = CO 3, PPh3 4, AsPh3 5, P(OPh)3 6 or py 7} are
formed by decarbonylating 1 or 2 either thermally or by UV
irradiation, or by the reaction of [Rh(o-O2C6Cl4)(CO)TpЈ] 3
with L. Oxidation of 4–7 gave stable monocations in which the
unpaired electron is localised on the O,OЈ-donor ligand.
[Rh{C(O)OC6Cl4O}(PPh3)TpЈ]ؒ0.33 CH2Cl2 2. A solution of
[Rh(CO)(PPh3)TpЈ] (0.20 g, 0.29 mmol) and o-C6Cl4O2 (71 mg,
0.29 mmol) in n-hexane (180 cm3) was stirred in the absence of
light for 20 min. The pale yellow mother liquors were decanted
from a bright orange-yellow solid which was purified and then
dried in vacuo, yield 183 mg (68%).
Experimental
The preparation, purification and reactions of the complexes
[Rh(o-O2C6Cl4)(CO)TpЈ] 3. A mixture of [Rh(CO)2TpЈ] (1.5
g, 3.3 mmol) and o-C6Cl4O2 (0.81 g, 3.3 mmol) in toluene (150
Table 5 Crystal and refinement data for [Rh{C(O)OC6Cl4O}(PPh3)TpЈ]ؒ2CH2Cl2 2ؒ2CH2Cl2, [Rh(o-O2C6Cl4)(PPh3)TpЈ]ؒCH2Cl2 4ؒCH2Cl2 and
[Rh(o-O2C6Cl4)(PPh3)TpЈ][PF6]ؒ3CH2Cl2 4ϩ[PF6]Ϫؒ3CH2Cl2
2ؒ2 CH2Cl2
4ؒCH2Cl2
4ϩ[PF6]Ϫؒ3CH2Cl2
Formula
Formula weight
C42H41BCl8N6O3PRh
1106.10
C40H39BCl6N6O2PRh
993.16
C42H43BCl10F6N6O2P2Rh
1307.98
Crystal system
Space group (no.)
Triclinic
P1 (2)
Orthorhombic
Pnma (62)
Orthorhombic
Pnma (62)
¯
a/Å
b/Å
c/Å
α/Њ
11.724(1)
12.302(2)
18.348(2)
93.077(9)
97.73(3)
114.924(8)
2303.5(4)
2
0.916
24121
10422
0.034
21.053(4)
14.046(4)
14.439(3)
26.150(1)
13.756(3)
14.601(1)
β/Њ
γ/Њ
V/Å3
4269.9(17)
4
0.856
26097
5068
5252.4(13)
4
0.959
32291
6252
Z
µ/mmϪ1
Reflections collected
Independent reflections
Final R1 [I > 2σ(I)]
0.046
0.0435
J. Chem. Soc., Dalton Trans., 2001, 875–880
879

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cm3) was stirred in a silica tube for 30 min in the absence of
light and then irradiated under UV light for 6 h. The resulting
orange-red solution was evaporated to dryness in vacuo to give
an orange-brown solid which was dissolved in CH2Cl2 (80 cm3),
filtered through Celite, and concentrated in vacuo before
n-hexane was added to give an orange-brown solid. The prod-
uct was purified, washed with n-hexane and dried in vacuo,
yield 0.97 g (44%).
to diffuse slowly into n-hexane at Ϫ10 ЊC. All X-ray diffraction
measurements were made at 173 K. Many of the other details
of the structure analyses are presented in Table 5. Molecules of
4 and 4؉ lie on mirror planes with one pyrazolyl ring and one
phosphine ring in the plane and hydrogens on the methyl
groups of the pyrazolyl disordered over the plane. One
dichloromethane solvate in 2ؒ2 CH2Cl2 was disordered and
refined anisotropically with two site occupancies for the
carbon and hydrogens in the ratio 62 : 38(1). Refinement of the
anion in 4؉[PF6]Ϫؒ3CH2Cl2 was hampered by disorder since
its site (in a general position of the space group) is also 50%
occupied by a dichloromethane solvate molecule whose car-
bon atom lies at the same position as the phosphorus atom
of the anion. The B–H hydrogen atom in each structure was
located in the electron density map and the co-ordinates
refined freely.
[Rh(o-O2C6Cl4)(PPh3)TpЈ]ؒCH2Cl2 4ؒCH2Cl2. A solution of
[Rh(CO)(PPh3)TpЈ] (0.10 g, 0.145 mmol) and o-C6Cl4O2 (36
mg, 0.145 mmol) in toluene (70 cm3) was stirred for 20 min in
the absence of light. The resulting orange-brown solution was
heated under reflux for 40 min and then evaporated to dryness
in vacuo. The product was purified (twice) to give a brick red
solid, yield 64 mg (49%).
CCDC reference numbers 155162–155164.
See http://www.rsc.org/suppdata/dt/b0/b010061g/ for crystal-
lographic data in CIF or other electronic format.
[Rh(o-O2C6Cl4)(AsPh3)TpЈ] 5. A solution of [Rh(o-O2C6-
Cl4)(CO)TpЈ] 3 (0.20 g, 0.30 mmol) in toluene (50 cm3) was
added dropwise over 4 h to a solution of AsPh3 (0.273 g, 0.89
mmol) in toluene (30 cm3) at 70 ЊC. After stirring the mix-
ture for 30 h at 70 ЊC, the solution was evaporated to dryness
in vacuo. The resulting oily red solid was washed with n-hexane
and purified (twice) to give a pink-purple solid, yield 74 mg
(26%).
Acknowledgements
We thank the EPSRC for Research Studentships (to D. J. H. E.,
O. D. H. and M. J. Q.).
[Rh(o-O2C6Cl4){P(OPh)3}TpЈ] 6. A solution of [Rh(o-O2C6-
Cl4)(CO)TpЈ] 3 (0.20 g, 0.30 mmol) and P(OPh)3 (80 µl, 0.30
mmol) in toluene (50 cm3) was stirred at 70 ЊC for 1 h and then
evaporated to dryness in vacuo. The resulting solid was purified
to give a pale orange solid, yield 190 mg (67%).
References
1 N. G. Connelly, D. J. H. Emslie, B. Metz, A. G. Orpen and M. J.
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[Rh(o-O2C6Cl4)(py)TpЈ] 7. A solution of [Rh(o-O2C6Cl4)-
(CO)TpЈ] 3 (0.20 g, 0.30 mmol) and pyridine (30 µl, 0.37 mmol)
in toluene (50 cm3) was stirred for 3 h at 70 ЊC and then evapor-
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filtered through Celite, and then n-hexane was added and the
solution concentrated in vacuo. The resulting bright yellow pre-
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[Rh(o-O2C6Cl4)(PPh3)TpЈ][PF6]ؒ2
CH2Cl2
4؉[PF6]؊ؒ2
CH2Cl2. Solid [NO][PF6] (26 mg, 0.15 mmol) was added to a
CH2Cl2 (30 cm3) solution of complex 4 (0.10 g, 0.10 mmol).
After 1 h the purple solution was filtered through Celite and
n-hexane was added to give deep purple crystals, yield 100 mg
(81%). [Rh(o-O2C6Cl4)(py)TpЈ][PF6]ؒCH2Cl2 7؉[PF6]ϪؒCH2Cl2
was prepared similarly. The complexes [Rh(o-O2C6Cl4)LTpЈ]-
[PF6]ؒx CH2Cl2 {L = AsPh3, x = 1.5, 5؉; L = P(OPh)3, x = 0.5,
6؉} were isolated similarly after further purification using
CH2Cl2–n-hexane and CH2Cl2–Et2O respectively.
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Crystal structure determinations of [Rh{C(O)OC6Cl4O}-
(PPh3)TpЈ]ؒ2 CH2Cl2 2ؒ2 CH2Cl2, [Rh(o-O2C6Cl4)(PPh3)-
TpЈ]ؒCH2Cl2 4ؒCH2Cl2 and [Rh(o-O2C6Cl4)(PPh3)TpЈ][PF6]ؒ3
CH2Cl2 4؉[PF6]؊ؒ3 CH2Cl2
13 A. G. Orpen and N. G. Connelly, Organometallics, 1990, 9,
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Worth, J. Chem. Soc., Dalton Trans., 1996, 3977.
Crystals of the three compounds suitable for X-ray diffraction
studies were grown by allowing concentrated CH2Cl2 solutions
880
J. Chem. Soc., Dalton Trans., 2001, 875–880