Stereochemical control of the redox potential of tetracoordinate rhodium complexes

Article abstract of DOI:10.1002/anie.200353027

An apitite for reduction: The diastereoisomers of the tetrachelating ligand 1,3-bis[(5H-dibenzo[a,d]cyclohepten-5-yl)phenylphosphanyl propane, bis(tropp^Ph)propane, can be used to control the redox potentials of 16-electron rhodium(1) complexes. Whereas the R,S-isomer of the ligand enforces a more planar complex which is difficult to reduce, the R,R(S,S)-isomer favors a tetrahedrally distorted structure which is easily reduced to give a stable rhodium(0) complex (see structure, Rh silver, P yellow, C black, H white).

Full text of DOI:10.1002/anie.200353027

Angewandte
Chemie
Rhodium Redox Chemistry
planar structures while the reduced 17-valence-electron-
configured d9 rhodium(0) complexes prefer a distorted tetra-
hedral form.[2]
Stereochemical Control of the Redox Potential of
Tetracoordinate Rhodium Complexes**
Longato et al.[3] and our group[4] have prepared strongly
tetrahedrally distorted tetracoordinate 16-electron rhodium
complexes, such as [Rh(dppf)2]+ (f = 49.78; dppf = 1,1’-bis-
CØcile Laporte, Frank Breher, Jens Geier,
Jeffrey Harmer, Arthur Schweiger, and
Hansjörg Grützmacher*
(diphenylphosphanyl)ferrocene)
and
[Rh(MetroppPh)2]+
(troppPh = 5-diphenylphosphanyl-5H-dibenzo[a,d]cyclohep-
tene; f = 42.08; where f is the intersection of the planes
spanned by the rhodium atom and the two donor atoms of
each bischelate ligand) but a significant anodic shift of the
reduction potentials was not observed.[5] The EPR spectra of
these rhodium(0) complexes clearly show that the unpaired
electron is predominantly located at the metal center.[3,6]
We therefore thought of another experiment allowing the
control the redox potential. A destabilization of the structure
of the reduced species in a redox couple by the energy Etred
should lead to a cathodic shift of the reduction potential, that
is, j E0B j > j E0 j (Scheme 1; right). For this experiment we
used the stereoisomers of a ligand system; one enforcing an
unfavorable coordination sphere for the reduced metal
complex, and the other one allowing the coordination
sphere to adapt to the more favorable geometry on reduction
of the metal center. Structure models indicated, that the
diastereomers of the tetrachelating bis(tropp) ligand 4 may
fulfill these requirements. This approach allows a direct
comparison of the stereochemical influence on the redox
potential while electronic influences (i.e. different donor/
acceptor properties) are minimized.
Dedicated to Professor Helmut Werner
on the occasion of his 70th birthday
Understanding the factors which control the redox potential
of transition-metal complexes is of fundamental importance
for the design of functionality. In classical experiments, the
redox potential of CuII[1] or NiII complexes[2] was shifted
substantially to less negative values (i.e. anodic j EoA j < j E8 j
in Scheme 1) by enforcing tetrahedral structures on the
The synthesis of 4 is presented in Scheme 2. Treating 1,3-
bis(diphenylphosphanyl)propane (1) with finely divided lith-
ꢀ
ium in THF leads to cleavage of one P Ph bond in each PPh2
group and after hydrolysis furnished 2 as a mixture of
Scheme 1. Schematic representation of approaches for the stereochem-
ical control over transition-metal-complex redox potentials. Left: Desta-
bilization of the oxidized form jE0A j<jE0 j; right: destabilization of the
reduced form jE0B j>jE0 j.
usually planar MII complexes which are destabilized by the
torsion energy Eot x. Tetrahedral structures are preferred for
the reduced MI complexes. To our knowledge, this concept
has not been systematically applied to complexes of the
transition metals from the fifth and sixth periods. For such an
investigation the redox couple of tetracoordinate d8 RhI/
d9 Rh0 complexes are ideally suited because 16-valence-
electron-configured d8 rhodium(i) complexes strongly favor
[*] Dr. C. Laporte, Dr. F. Breher, J. Geier, Dr. J. Harmer,
Prof. Dr. A. Schweiger, Prof. Dr. H. Grützmacher
Department of Chemistry and Applied Biosciences
ETH-Hönggerberg
8093 Zürich (Switzerland)
Fax: (41)163-31-032
E-mail: gruetzmacher@inorg.chem.ethz.ch
[**] This work was supported by the Swiss National Science Foundation.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Scheme 2. Syntheses of the stereoisomers of the bis(tropp)ligands 4
and the corresponding rhodium(i)/(0)complexes 6 and 7.
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DOI: 10.1002/anie.200353027
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isomers.[7] Reaction with 5-chloro-5H-dibenzo[a,d]cyclohep-
In the cyclic voltammogram of pure rac-6 in THF/0.1m
[nBu4N]+PF6 only two reversible redox waves are observed
ꢀ
tene (trop-Cl, 3) gives after alkaline work-up the desired
bis(tropp) compounds R,S-4 (meso-4) and the racemic
mixture R,R-/S,S-4 (rac-4 in the following) in satisfactory
yield (ꢁ 70%). The diastereomers, meso-4 and rac-4, could
(E11=2 = ꢀ1.11 V; E12=2 = ꢀ1.42 V (Figure 2a)).
not be separated and were treated with [Rh(cod)2]+CF3SO3
ꢀ
(COD = cyclooctadiene) to yield almost quantitatively the
corresponding mixture of diastereomeric rhodium(i) com-
plexes meso-6 and rac-6 as a microcrystalline red powder.
Pure rac-6 crystallizes from an approximately 0.02m solution
in THF, while the other diastereomer, meso-6, is obtained by
layering the mother liquor with n-hexane (contaminated with
about 10% rac-6).
The structures of rac-6 and meso-6 were determined by X-
[8]
ray structure analyses (Figure 1a and band Table 1).
Figure 2. Cyclic voltammograms of a)pure rac-6, b)a mixture of rac-6
and meso-6; 0.1m [nBu4N]+PF6ꢀ, THF, T=298 K, Pt working electrode,
versus Ag/AgCl, scan rate=100 mVsꢀ1
.
A mixture of rac-6 with meso-6 in THF shows a cyclic
voltammogram with four separated redox waves (Figure 2b).
Clearly, rac-6 is much easier to reduce than meso-6 (rac-6:
Figure 1. Structures of the cation in meso-6 (a), the R,R-configured
cation in rac-6 (b), the R,R-configured enantiomer in rac-7 (c).
E11=2 = ꢀ1.12 V, E12=2 = ꢀ1.44 V; meso-6: E11=2 = ꢀ1.36 V, E21=2
=
As planned, the coordination sphere in the rac-6 cation is
significantly distorted towards a tetrahedral geometry (f ꢁ
308). The six-membered Rh-P-(CH2)3-P ring has a twist
conformation. In the meso-6 cation, however, this ring adopts
the expected chair conformation with the two phenyl groups
at P1 and P2 in the axial positions and the sterically
demanding dibenzo[a,d]cycloheptenyl (trop) units in the
equatorial positions. This arrangement permits a metal
coordination sphere which is closer to square planar (f ꢁ
198).[9]
ꢀ1.72 V). The same features and almost identical redox
potentials are obtained in a cyclic voltammogram taken in
CH2Cl2, which shows that solvent effects are not responsible
for the different redox potentials. The slight cathodic shift of
E11=2 by about 100 mV observed for rac-6 in comparison to
[Rh(troppPh)2]+ (see Scheme 1) is probably caused by replac-
ing one P-phenyl group in the troppPh ligand by the alkyl chain
in the bis(tropp) ligand.[10]
On a preparative scale, rac-6 can be reduced to give pure
rac-[Rh{bis(tropp)}]0 (rac-7) with sodium or sodium naph-
Table 1: Selected structural data for meso-6, rac-6, and rac-7.
Compound
Rh-P [Š]
Rh-Ct [Š][a]
P1-Rh-P2 [8]
P1-Rh-Ct1 [8]
P2-Rh-Ct2 [8][a]
Ct-Rh-Ct [8][a]
f [8][b]
=
C Ctrop [Š]
meso-6
rac-6
2.242
2.240
2.209
2.231
2.199
2.182
2.203
2.084
1.373
1.383
1.395
1.413
87.1
86.3
88.0
88.9
89.9
91.4
91.6
92.0
90.3
90.9
91.5
92.0
94.5
97.5
98.1
18.6
26.8
32.9
42.7
rac-6’[c]
rac-7
102.7
=
[a] Ct is the center of the coordinated C Ctrop bond. [b] f is the angle between the planes P1-Rh-Ct1 and P2-Rh-Ct2 and is a measure of the tetrahedral
distortion. [c] Crystals of rac-6 contain two independent molecules (one molecule of each enantiomer)per asymmetric unit.
2568
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Angewandte
Chemie
thalenide, [Na+(naph)Cꢀ] in THF. Alternatively, the neutral
paramagnetic complexes rac-7/meso-7 are obtained by a
comproportion reaction of rac-6/meso-6 with the fully
reduced 18-valence-electron sodium rhodates [Na(thf)n]+ -
rac/meso-[Rh{bis(tropp)}]ꢀ (rac-8/meso-8).
doublets with g = 2.0137, A1iso = 81.8 MHz and Ai2so
=
51.4 MHz. A straightforward explanation for this observation
is that the d9 valence-electron meso-7 radical is unable to
adapt to the preferred tetrahedral structure because of the
rigid chair conformation of the central Rh-P-(CH2)3-P chelate
The result of an X-ray structure analysis[8] for rac-7, which
is only the third example of the still rare mononuclear
rhodium(0) complexes, is shown in Figure 1c. As previously
ring. As a consequence, one Rh P bond elongates which also
ꢀ
lowers the energy of the SOMO.[12] However, this stabilization
is less effective than the distortion towards a tetrahedron and
as a net result, a significant cathodic shift for the potential of
the [meso-7]+/[meso-7]0 redox couple is observed.
observed,[3,5] the Rh Ct bonds (Ct = midpoint of the coordi-
ꢀ
=
=
nated C C bond) is shorter and the coordinated C Ctrop bond
slightly longer in rac-7 than in rac-6. The twist conformation
of the central six-membered chelate ring is retained, however,
the distortion towards a tetrahedron is more pronounced (f ꢁ
438) in rac-7 and equals the one observed in cis-
[Rh(troppPh)2]0 (f ꢁ 438).[5]
While it seems to be difficult to influence the redox
couples of tetracoordinate d8 M/d9 M centers (M = Rh, Ir, Pt)
using the classical approach for the 4th period metals, that is,
destabilizing the oxidized form (d8 complex) by distortion
towards a tetrahedron, the experiments reported herein (in
combination with preliminary results for M = Ir and Pt)[13]
show that the redox potentials of late-transition-metal com-
plexes can be controlled effectively by destabilization of the
reduced complex (d9 complex) by enforcing a more planar
structure.
Crystals of the other diastereomer meso-7 suitable for an
X-ray analysis could not be obtained. However, information
concerning the structure of meso-7 were obtained from
continuous wave (CW) X-band EPR spectra in a THF
solution. The EPR spectrum of pure rac-7 (Figure 3a) consists
Experimental Section
In the following, short descriptions of the syntheses and selected
NMR resonances of diagnostic value are given. 1H and 13C NMR
spectra are referenced relative to TMS, 31P NMR spectra to H3PO4,
and 103Rh NMR spectra to X = 3.16 MHz. For full details, see the
Supporting Information.
rac-4/meso-4: A solution of 2 (1 g, 3.8 mmol) in toluene (50 mL)
was added to a solution of 3 (1.74 g, 7.6 mmol) in toluene (50 mL).
After stirring for 2 h at RT, an aqueous solution of K2CO3 (40 mL;
1m) was added and the reaction mixture was stirred for 14 h at RT,
and then heated under reflux until the organic phase became yellow.
The organic layer was separated, the solvent was removed under
vacuum, and the residue was recrystallized from ethanol (30 mL) to
afford 1.6 g (66%) of a 1:1 mixture of meso-4/rac-4 as a white air-
sensitive solid. M.p. (mixture): 162–1668C. Only some of the NMR
signals of the rac-4/meso-4 mixture and could be assigned to one
1
=
isomer: H NMR (500.2 MHz, C6D6, 298 K): d = 6.92 (2d, 4H, CH),
2
=
6.86 (2d, 4H, CH), 4.09 (d, 2H, JP, H = 5.7 Hz, CHP(rac)), 4.08 (d,
2H, 2JP, H = 5.7 Hz, CHP(meso)), 1.89 (m, 2H, PCHH-CH2-
CHHP(rac)), 1.80 (m, 2H, PCHH-CH2-CHHP(meso)), 1.40 (2 m,
4H, PCHH-CH2-CHHP(meso + rac)), 1.23 (2 m, 3H, PCH2-CH2-
CH2P(meso + rac)), 0.97 ppm (m, 1H, PCH2-CHH-CH2P(meso));
13C NMR (125.7 MHz, C6D6, 298 K): d = 130.4 (2d, JP, C = 1.4 Hz,
=
1
=
CH), 130.1 (2d, JP, C = 1.4 Hz, CH), 60.8 (2d, JP, C = 19.4 Hz, CHP),
29.3–28.9 (4d, 1JP, C = 19.4 Hz, PCH2), 23.5 ppm (2t, 1JP, C = 19.6 Hz,
1JP, C = 18.8 Hz, PCH2-CH2-CH2P); 31P NMR (202.5 MHz, CDCl3,
298 K): d = ꢀ22.6 (s, rac), ꢀ22.8 ppm (s, meso).
Figure 3. CW X-band EPR spectrum of a)pure rac-7, b)a mixture of
meso-7 and rac-7 in THF, c)the simulated spectrum of rac-7, d)and
the simulated spectrum of meso-7. Experimental conditions: room
temperature, modulation frequency 100 kHz, modulation amplitude
0.05 mT, microwave frequency 9.76 GHz.
rac-6/meso-6: The phosphanes rac-4/meso-4 (137 mg, 0.2 mmol)
in CH2Cl2 (5 mL) were treated with a solution of [Rh(cod)2]OTf (5;
100 mg, 0.2 mmol; Tf = CF3SO2) in CH2Cl2 (5 mL). After stirring for
20 min, the solution was concentrated to 20% of its volume and
hexane was added to afford rac-6 and meso-6 as an orange-red air
stable microcrystalline powder (164 mg, 92%). The diastereomers
were separated by fractional crystallization from THF; rac-6 crystal-
lized first from a 0.02m solution and meso-6 crystallized after addition
of a triplet (intensity ratio 1:2:1, and g = 2.0208) with a 31P
hyperfine coupling of Aiso = 84 MHz corresponding to two
equivalent phosphorus nuclei as seen in the solid state
structure.
Subtraction of the spectrum of rac-7 from a spectrum of
the rac-7/meso-7 mixture (Figure 3b) allows the EPR param-
eters for meso-7 to be determined (Figure 3c, d).[11] In
contrast to rac-7, the meso-diastereomer has a lower symme-
try and the hyperfine couplings with two inequivalent
phosphorus nuclei lead to the observation of a doublet of
of n-hexane to the mother liquor in about 90% purity. rac-6: M.p. >
1
=
2508C. H NMR (400.1 MHz, CD2Cl2, 298 K): d = 7.31 (m, 4H, CH),
5.09 (m, 2H, CHP), 2.36 (m, 2H, PCHH-CH2-CHHP), 1.83 (m, 2H,
PCHH-CH2-CHHP), 0.82 ppm (m, 2H, PCH2-CH2-CH2P); 13C NMR
(100.7 MHz, CD2Cl2, 298 K): d = 103.3 (m, 2JP, C + 2JP’,C = 5.1 Hz,
2
1
1JRh,C = 6.3 Hz, CH), 92.4 (m, JP, C
+ =
2JP’,C = 10.8 Hz, JRh,C
=
1
3
=
5.0 Hz, CH), 52.9 (m, JP, C + JP, C = 18.1 Hz, CHP), 17.3 (m, PCH2-
Angew. Chem. Int. Ed. 2004, 43, 2567 –2570
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Communications
CH2-CH2P), 16.0 ppm (m, 1JP, C + 3JP, C = 38.7 Hz, PCH2-CH2-CH2P);
[6] S. Deblon, L. Liesum, J. Harmer, H. Schönberg, A. Schweiger,
H. Grützmacher, Chem. Eur. J. 2002, 8, 601 – 611.
[7] J. Dogan, J. B. Schulte, G. F. Schwiegers, S. B. Wild, J. Org. Chem.
2000, 65, 951 – 957.
31P NMR (162.0 MHz, CD2Cl2): d = 80.3 ppm (d, 1JRh,P = 170 Hz);
103Rh (12.7 MHz, CD2Cl2): d = ꢀ330 ppm (t); meso-6: M.p. > 2508C.
1
=
H NMR (400.13 MHz, CD2Cl2, 298 K): d = 6.70 (m, 2H, CH), 6.57
=
(m, 2H, CH), 4.72 (m, 2H, CHP), 2.50 (m, 2H, PCHH-CH2-CHHP),
[8] rac-6·CH2Cl2: C93H78Cl2F6O6P4Rh2S2; monoclinic; P2(1); a =
1334.0(5), b = 1830.2(7), c = 1793.5(7) pm, b = 104.191(8); V=
2.19 (m, 1H, PCH2-CHH-CH2P), 1.86 (m, 2H, PCHH-CH2-CHHP),
0.89 ppm (m, 1H, PCH2-CHH-CH2P); 13C NMR (100.7 MHz,
4.25(1) nm3, Z = 2, 1calcd = 1.463 mgmꢀ3
, crystal dimensions:
2
CD2Cl2, 298 K): d = 84.6 (m, 2JP, C + JP’,C = 10.0 Hz, CH), 79.3 (m,
0.15 ” 0.15 ” 0.20 mm, diffractometer Siemens CCD 1k area
detector; MoKa radiation, 293 K, measurement range: 3.14 <
2V< 56.568, 20789 independent reflections, 43894 reflections
F > 4s(F), m = 0.644 mmꢀ1; direct methods; refinement against
full matrix (versus F2) with SHELXTL (Rel. 5.1) and SHELXL-
97; 1036 parameters, R1 = 0.0568 and wR2 (all data) = 0.1094,
max/min residual electron density: 916/ꢀ429 enmꢀ3. meso-6:
C53H46F3O3P2RhS; monoclinic; P2(1); a = 1109.6(1), b =
1841.8(1), c = 1189.2(1) pm, b = 111.523(6)8, V= 2.2609(3) nm3,
=
2JP, C + JP’,C = 13.7 Hz, CH), 51.2 (m, 1JP, C + JP’,C = 24.4 Hz, JP, C
=
2
3
2
=
9.0 Hz, CHP), 21.2 (m, 2JP, C + 4JP’,C = 37.2 Hz, PCH2-CH2-CH2P),
20.5 ppm (m, PCH2-CH2-CH2P); 31P NMR (162.0 MHz, CD2Cl2,
298 K): d = 72.0 ppm (d, 1JRh,P = 163 Hz); 103Rh NMR (12.7 MHz,
CD2Cl2, 298 K): d = ꢀ576 ppm (t).
rac-8/meso-8: Sodium (3 mg, 0.13 mmol) was added to a suspen-
sion of the complexes rac-6 and meso-6 (50 mg, 0.056 mmol) in THF
(5 mL). The reaction mixture was sonicated for 12 h at 258C, and
subsequently the resulting deep red reaction mixture was filtered. The
filtrate was evaporated to dryness and washed several times with
hexane leading to a deep red highly air-sensitive powder (45 mg,
67%). rac-8: 1H NMR (400.1 MHz, [D8]THF, 298 K): d = 4.35 (m,
Z = 2, 1calcd = 1.447 mgmꢀ3
, crystal dimensions: 0.12 ” 0.11 ”
0.10 mm, diffractometer Stoe IPDS II image plate system,
MoKa radiation, 293 K, measured range: 3.94 < 2V< 55.728,
9377 independent reflections, 15169 reflections F > 4s(F), m =
2H, CHP), 4.20 (m, 2H, CH), 2.83 (m, 2H, CH), 1.40 (m, 2H,
0.55 mmꢀ1
; direct methods; refinement against full matrix
=
=
PCHH-CH2-CHHP), 1.01 (m, 2H, PCH2-CH2-CH2P), 0.81 ppm (m,
(versus F2) with SHELXTL (Rel. 5.1) and SHELXL-97; 567
parameters; R1 = 0.0403 and wR2 (all data) = 0.0888, max/min
2H, PCHH-CH2-CHHP); 13C NMR (100.7 MHz, [D8]THF, 298 K):
2
d = 57.2 (m, CHP), 53.7 (m, CH), 40.3 (d, JP, C or 2JRh,C = 11.1 Hz,
residual electron density: 426/ꢀ460 enmꢀ3
.
rac-7·2THF:
=
=
CH), 25.6 (m, PCH2-CH2-CH2P), 21.5 ppm (m, PCH2-CH2-CH2P);
C53H54O2P2Rh; monoclinic; P21/n; a = 1318.9(2), b = 1889.8(3),
c = 1807.5(3) pm, b = 109.857(4); V= 4.237(1) nm3, Z = 4,
1calcd = 1.392 mgmꢀ3, crystal dimensions: 0.08 ” 0.14 ” 0.27 mm,
diffractometer Bruker AXS SMART APEX with CCD area
detector; MoKa radiation, 180 K, phase transition at temper-
atures beyond 180 K, measurement range: 3.22 < 2V< 56.588,
10510 independent reflections, 31246 reflections F > 4s(F), m =
0.521 mmꢀ1; direct methods; refinement against full matrix
(versus F2) with SHELXTL (Rel. 5.1) and SHELXL-97; 523
parameters, R1 = 0.0570 and wR2 (all data) = 0.1396, max/min
residual electron density: 916/ꢀ625 enmꢀ3. CCDC-229384 (rac-
6·CH2Cl2), CCDC-229383 (meso-6), CCDC-229385 rac-7·2THF
contain the supplementary crystallographic data for this paper.
These data can be obtained free of charge via www.ccdc.cam.
ac.uk/conts/retrieving.html (or from the Cambridge Crystallo-
graphic Data Centre, 12 Union Road, Cambridge CB21EZ, UK;
fax: (+ 44)1223-336-033; or deposit@ccdc.cam.ac.uk).
1
31P NMR (121.5 MHz, [D8]THF, 298 K): d = 81.1 ppm (d, JP, Rh
=
179 Hz); 103Rh NMR (12.7 MHz, [D8]THF, 298 K): d = ꢀ681 ppm
(t). meso-8: 31P NMR (121.5 MHz, [D8]THF, 298 K): d = 88.1 ppm
(br).
rac-7: A solution of sodium-naphthalenide in THF (0.056m,
0.056 mmol, 1 mL) was added to a suspension of rac-6 (50 mg,
0.056 mmol) in THF (2 mL). The deep green solution was filtered.
After removing the solvent from the filtrate under reduced pressure, a
deep green highly air-sensitive powder was obtained, which was
washed several times with hexane (40 mg, 93%). EPR (THF, 298 K):
g
iso = 2.017 [t, Aiso (31P) = 84 MHz].
rac-7/meso-7: A 1:1 mixture of the compounds rac-8 and meso-8
(29 mg, 0.024 mmol) were mixed in THF (2 mL) with a 1:1 mixture of
rac-6 and meso-6 (20 mg, 0.022 mmol). An immediate color change
from red to deep green was observed. After removing the solvent
under reduced pressure, the product was isolated as highly air-
sensitive deep green powder (40 mg, 93%). meso-7: EPR (THF,
298 K): giso = 2.0145 [dd, A1iso (31P) = 81.8 MHz, A2iso (31P) = 51.4 MHz].
[9] Alternatively, the distortion can be described by the deviation of
Ct from the plane running through the P-Rh-P plane: meso-6:
25.1 pm (Ct1), ꢀ66.3 pm (Ct2); rac-6: 81.5 and 78.7 pm (Ct1),
ꢀ95.4 and ꢀ68.2 pm; rac-7: 100.2 pm (Ct1), ꢀ115.3 pm (Ct2).
[10] The redox potentials of the P-bis(cyclohexyl) substituted com-
Received: October 7, 2003
Revised: January 23, 2004 [Z53027]
plex [Rh(troppcyc)2]+/0/ꢀ1 are at E11=2 = ꢀ1.19 V and E21=2
=
ꢀ1.53 V; S. Deblon, Dissertation, ETH Zürich, 2000, No. 13920.
[11] The EPR parameters were obtained by calculating the exper-
imental spectra (least-squares-fittings using EasySpin, see:
Keywords: electrochemistry · phosphane ligands ·
.
redox chemistry · rhodium · stereochemistry
http://www.esr.ethz.ch).
31
ꢀ
[12] A correlation of Aiso( P) with Rh P separations has been clearly
established in [Rh(dppf)2]0[3] where the Rh P bond lengths show
[1] a) S. Kapp, T. P. Keenan, X. Zhang, R. Fikar, J. A. Potenza, H. J.
Schugar, J. Am. Chem. Soc. 1990, 112, 3452 – 3464, and
references therein; b) W. Kaim, J. Rall, Angew. Chem. 1996,
108, 47 – 64; Angew. Chem. Int. Ed. Engl. 1996, 35, 43 – 60.
[2] For impressive examples of this strategy see: a) C. O. Dietrich-
Buchecker, J. Guilhem, J.-M. Kern, C. Pascard, J.-P. Sauvage,
Inorg. Chem. 1994, 33, 3498 – 3502, and references therein; b) C.
Dietrich-Buchecker, J.-P. Sauvage, J.-M. Kern, J. Am. Chem. Soc.
1989, 111, 7791 – 7800.
ꢀ
a linear inverse correlation with the hyperfine couplings.
[13] Experiments with the corresponding diastereomers of [M{bis-
(tropp)}] with M = Ir or Pt show the same behavior as reported
here for M = Rh; C. Laporte, Dissertation, ETH Zürich, 2003,
No. 15137.
[3] B. Longato, R. Coppo, G. Pilloni, R. Corvaja, A. Toffoletti, G.
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