Strain in organometallics: synthesis of rhodium and iridium complexes with a novel rigid tetrachelating phosphine olefin ligand and their redox properties

Article abstract of DOI:10.1016/S0022-328X(01)01362-6

The new tetrachelating diphosphanes 1,2-[(5H-dibenzo[a,d]cyclohepten-5-yl)phenylphosphano]ethane, R,S-11 [R,S-bis(tropp^Ph)], and the corresponding enantiomers R,R-bis(tropp^Ph), R,R-12, and S,S-bis(tropp^Ph), S,S-12, were sy

Full text of DOI:10.1016/S0022-328X(01)01362-6

Journal of Organometallic Chemistry 641 (2002) 227–234
www.elsevier.com/locate/jorganchem
Strain in organometallics: synthesis of rhodium and iridium
complexes with a novel rigid tetrachelating phosphine olefin ligand
and their redox properties
Ce´cile Laporte, Carsten Bo¨hler, Hartmut Scho¨nberg, Hansjo¨rg Gru¨tzmacher *
Laboratory of Inorganic Chemistry, Department of Chemistry, ETH-Ho¨nggerberg, CH-8093 Zurich, Switzerland
Received 24 September 2001
Dedicated to Professor Rolf Gleiter on the occasion of his 65th birthday
Abstract
The new tetrachelating diphosphanes 1,2-[(5H-dibenzo[a,d]cyclohepten-5-yl)phenylphosphano]ethane, R,S-11 [R,S-
bis(troppPh)], and the corresponding enantiomers R,R-bis(troppPh), R,R-12, and S,S-bis(troppPh), S,S-12, were synthesised from
1,2-bis(phenylphosphano)ethane (9), and 5-chloro-5H-dibenzo[a,d]cycloheptene (10). With the meso form R,S-11, the rhodium(I)
complex [Rh{R,S-bis(troppPh)}]PF6 (15), and the iridium(I) complex [Ir(cod){R,S-bis(troppPh)}]O3SCF3 (16), were prepared. The
cation of the 16-electron rhodium complex 15 has a square planar structure, which is markedly distorted towards a square
pyramid with the rhodium centre in the apex. The structure of the cation of the 18-electron complex of 16 is trigonal bipyramidal
and unprecedented a phosphorus and an olefin unit occupy the axial positions. Cyclic voltammetry data of 15 indicate, that
considerable strain energy (ca. 30 kJ mol−1) is build up when the [Rh{R,S-bis(troppPh)}]+ cation is reversibly reduced in THF
by one electron to the paramagnetic neutral complex [Rh{R,S-bis(troppPh)}]0. © 2002 Published by Elsevier Science B.V.
Keywords: Cyclic voltammetry; Iridium; Phosphanes; Rhodium; Redox chemistry; Strain energy
shown in Scheme 1 were isolated and fully character-
ised. By NMR and EPR spectroscopic studies, it was
shown that the diamagnetic complexes 1, 2, 5 and 6 are
stereochemically rigid, i.e. the 16-electron complex
cations 1 and 2 exist as mixture of trans- and cis-iso-
mers which do not inter-convert in weakly co-ordinat-
ing solvents on the NMR time scale and the 18-electron
metalates 5 and 6 exist cis-isomers exclusively. On the
other hand, the paramagnetic 17-electron complexes 3
and 4 are highly dynamic. The observed trans–cis-iso-
merisation proceeds most likely via an intra-molecular
rotation of the tropp ligands [4]. In the solid state, all
complexes in Scheme 1 show a square planar structure,
which is severely distorted towards a tetrahedron. This
distortion is defined by the intersection angle € of the
planes p1 and p2 running through the phosphorus
atom, the centroid of the co-ordinated olefin indicated
by black circles in Scheme 1 and the metal centre. In
the 16-electron cations, € lies in the range between 0
and 30° in trans-1 depending on the counter anion. In
1. Introduction
We developed the synthesis of the R1troppR ligand
system (tropp=tropylidene phosphane; IUPAC 5-
diphenylphosphanyl-dibenzo[a,d]cycloheptene; R indi-
cates the substituent bonded to the phosphorus, R1 the
substituent bonded to the central olefinic unit) which
contains a seven-membered ring in a rigid boat confor-
mation such that a phosphanyl and olefinic binding site
are optimally placed to co-ordinate with various metals
(Scheme 1) [1].
It turned out that the 16-electron trans- and cis-
[M(troppph)2]+ cations 1, 2 (M=Rh, Ir) containing the
Pꢀphenyl substituted ligand troppph are reversibly re-
duced in two successive single electron transfer steps at
remarkably low negative potentials [2,3]. All complexes
* Corresponding author. Tel.: +41-1-6322855; fax: +41-1-
6321090.
E-mail address: gruetzmacher@inorg.chem.ethz.ch (H. Gru¨tz-
macher).
0022-328X/02/$ - see front matter © 2002 Published by Elsevier Science B.V.
PII: S0022-328X(01)01362-6

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C. Laporte et al. / Journal of Organometallic Chemistry 641 (2002) 227–234
tions in 7, which exists as cis-isomer exclusively, induce
a severe distortion from a square planar towards a
tetrahedral configuration (€=42°). Thereby, the
ground state of the 16-electron cation—preferring a
planar structure—should be destabilised with respect to
the reduced forms and an even lower reduction poten-
tial should result. However, although examples for this
approach are well documented [6–8], only very small
effects on the redox potentials of 7 were established [5].
As a next step, we became interested in the influence
of a chelating bi(tropp)–ligand system on the redox
properties. Such a constraint in the ligand allows only
cis-isomers to be formed and in general impedes the
adoption of the coordination sphere to the formal
oxidation state of the metal. We report here on the
synthesis of such a ligand in which both phosphorus
centres are covalently linked, the preparation of
rhodium and iridium complexes and first studies con-
cerning their redox properties.
the paramagnetic species 3 and 4, € values of 34 and
43° were observed and the d10 valence configurated
metalates 5 and 6 show with €=53–58° the strongest
deviations.
A methyl substituent at the central CꢁC double bond
was introduced and the complex 7 containing the
metroppph ligand [5] could be prepared. Steric interac-
2. Results and discussion
We chose a chelating tropp system derived from
1,2-bis-(phenylphosphano)ethane (dppe) (8) to start
with. In this context we mention that the redox chem-
istry of the complexes [M(dppe)2]+ (M=Rh, Ir) has
been intensively studied [9–12], the fluxionality of
[Rh(dppe)2]0 [12,13] and the reactivity of [Rh(dppe)2]−
[14] has been investigated. Note, that high negative
potentials must be employed (B −2 V) to generate the
reduced species. According to a recently published pro-
cedure, the bis(phosphane) 8 can be easily converted
into the secondary bis(phosphane) 9 by reductive cleav-
age of one Pꢀphenyl bond on each phosphorus atom
and subsequent hydrolysis of the intermediate bis(lith-
iophosphanyl)ethane [15] (Scheme 2).
Scheme 1. Rhodium, 1, 3, 5, 7 and iridium, 2, 4, 6, R1troppR
complexes.
Following a previously described protocol [1], reac-
tion of 9 with 5-chloro-5H-dibenzo[a,d]cycloheptene
(10) gives a mixture of 65% of the meso form, R,S-1,2-
[(5H-dibenzo[a,d]cyclohepten-5-yl)diphenylphosphino]-
ethane (11), and 35% of the racemate R,R-/S,S-12 [16].
The total yield of both diastereomers amounts about
60%. Fortunately, the meso and the racemic forms 11
and 12, respectively, could be easily separated by simple
fractional crystallisation. From toluene, the R,S-11 iso-
mer crystallised first in spectroscopically pure form
(31P, l= −16.52) and after addition of acetonitrile the
R,R-/S,S-12 isomer (31P, l= −18.41) crystallised in
about 95% purity.
As the 31P-NMR spectra showed, reactions with the
mixture of the chiral bis(tropp) ligands R,R/S,S-12 and
various organometallic precursors of rhodium(I) and
iridium(I) are rather complex and did not give the
expected products. Only one product was obtained in
Scheme 2. Synthesis of the bis(tropp) ligands R,S-11 and R,R-/S,S-
12.

C. Laporte et al. / Journal of Organometallic Chemistry 641 (2002) 227–234
229
plings were determined. The chemical shifts are typical
for tropp complexes with a cis conformation [5,17] and
are displaced by about Dl=25 to lower frequencies
when compared with the uncomplexed ligand R,S-11
(l=127.7 and another absorption l\127.9 which
could not be distinguished from the resonance signals
of the aromatic carbon nuclei).
When [Ir(cod)2]O3SCF3 14 is mixed with bis(tropp)
R,S-11 in methylene chloride, an orange–yellow solu-
tion is obtained from which yellow crystals precipitated
after adding n-hexane. The re-dissolved product 16, as
well as the reaction mixture, shows two sharp singlets
in the 31P-NMR spectrum (l=12.7, 66.58) which is
incompatible with the formation of a [Ir{bis(tropp)}]
complex like 15. The 1H- and 13C-NMR spectra are
extremely complex (for that reason they are not re-
ported here) and it was not possible to conclude from
the spectral data on the structure of complex 16. Vary-
ing the reaction conditions (i.e. longer reaction times,
higher temperatures up to 120 °C, other solvents) or
using [Ir2(m2-Cl)2(cod)2] in presence of TlPF6 also led to
the formation of 16 which eventually decomposed un-
der formation of elemental iridium under harsh reac-
tion conditions.
Scheme 3. Synthesis of rhodium(I) 15 and iridium(I) bis(tropp)
complexes 16.
3. Molecular structures of 15 and 16
Crystals of 15 and 16 suitable for X-ray analyses
were grown by diffusion of n-hexane into concentrated
methylene chloride solutions. The molecular structures
of the cations of 15 and 16 are shown in Figs. 1 and 2
and selected bond lengths and angles are listed in
Tables 1 and 2, respectively. Crystallographic data are
compiled in Table 4, which is given in Section 5.
The steric stress imposed by the ligand system is
clearly reflected in the structure of the rhodium(I) com-
plex 15. The centroids Ct1 and Ct1a of the co-ordi-
nated olefin units C4ꢁC5 and C4aꢁC5a, respectively, of
each tropp ligand and the two phosphorus atoms P1
and P1a span a square planar co-ordination sphere
around the metal centre which is distorted towards a
Fig. 1. Molecular structure of 15. The PF6 counter anion and the
hydrogen atoms were omitted for clarity. Selected bond lengths and
angles are given in Table 1.
good to excellent yields with the meso-isomer 11 when
the dinuclear complex [Rh2(m2-Cl)2(cod)2] 13 shown in
Scheme 3, TlPF6 and R,S-11 are mixed in equimolar
ratios in THF, the solution becomes deep red and the
expected reaction product 15 is formed in 94% isolated
yield.
The 31P-NMR spectrum of a methylene chloride
solution of 15 shows only one sharp doublet in the l
,
square pyramid such that the Rh1 atom lies 0.278 A
above the P1, P1a, Ct1, Ct1a plane [see Fig. 1 and (a)
on top of Scheme 4].
range typical for these of complexes (l=107.6; 1JRhP
=
177 Hz) [2,5]. Concerning the binding of the olefinic
units of the ligand, the 1H-NMR spectrum is little
informative, because, these protons are buried within
the range of absorptions for the aromatic protons
(l=6.67–7.63). Noteworthy is the high frequency shift
(l=2.40) of one pair of the diastereotopic protons of
the CH2ꢀCH2-bridge (the other pair is observed at and
l=1.52) compared with the uncomplexed ligand (R,S-
11: l=1.16). In the 13C-NMR spectrum, two multiplets
centred l=97.5 and 103.1 are assigned to the co-ordi-
nated olefins; neither the 103Rh13C nor the 31P13C cou-
This type of distortion seems to be rare while tetrahe-
dral distortions in d8 metal valence electron configu-
rated complexes are frequently observed [see (b) on top
of Scheme 4 and Ref. [5]]. The latter distortion is
obviously not possible for 15 due to the ethylene
bridge. The C22ꢀC22a single bond of this bridge is
,
slightly compressed to 1.47(2) A and the CH2 units are
located in an eclisped position (torsion angle
P1aꢀC22aꢀC22ꢀP1=10.1°) within the five membered
RhP2C2 heterocycle which has an envelope conforma-
tion folded by 29° along the P1ꢀP1a vector. Further-

230
C. Laporte et al. / Journal of Organometallic Chemistry 641 (2002) 227–234
Fig. 2. Molecular structure of 16. The CF3SO3 counter anion and the hydrogen atoms were omitted for clarity. Selected bond lengths and angles
are given in Table 2.
more, the steric congestion in the complex cation of 15
is reflected by the relatively long RhꢀC bonds [2.31(1)–
The P1 and the centroid Ct3 of the C23ꢁC30 bond
take the axial positions [P1ꢀIrꢀCt3 174.0°] and the P1a,
the centroid Ct1 of the C4ꢁC5 bond, and the centroid
Ct2 of the C26ꢁC27 bond occupy the equatorial posi-
tions [P1aꢀIrꢀCt1 110.3°, Ct1ꢀIrꢀCt2 128.6°,
Ct2ꢀIrꢀP1a 120.6°]. To our knowledge, this is the first
structurally characterised example of a complex with
tbp structure, in which an olefin and phosphanyl group,
reside in the axial positions. The length of the axial
,
2.36(1) A; distances to the olefin centroids: RhꢀCt1 2.23
,
,
,
A, RhꢀCt1a 2.21 A], which are about 0.2 A longer than
usual RhꢀC bond lengths in comparable compounds
,
(ca. 2.15 A) [18]. However, such long MꢀC bonds seem
to be typical for cis-configurated [M(tropp)2] complexes
and even longer ones have been observed in 17 (Scheme
1) [5,17]. On the other hand, the rhodium phosphorus
,
,
bonds [2.208(2), 2.227(2) A] have normal lengths.
IrꢀP1 bond [2.278(1) A] is very similar to other
The structure of the iridium complex cation, [Ir(-
cod){R,S-bis(troppPh)}]+, is shown in Fig. 2 and re-
veals it to be penta co-ordinated. The co-ordination
sphere corresponds to a trigonal bipyramid (tbp) as is
expected for a d8 valence electron configurated metal
centre and a graphical sketch is given in the end of
Scheme 4.
[Ir(tropp)2X] complexes with a tbp structure [19] and
also does not differ much from MꢀP lengths in the tetra
co-ordinated [M(tropp)2] species [1–3,5] (by Z ca. +
,
0.05 A compared with 15 for example). However, the
,
equatorial IrꢀP1a bond [2.401(1) A] is unusually long.
The IrꢀC distances to the olefins in equatorial position
,
lie in the expected range from 2.175(5) A (IrꢀC23) to
Table 1
,
Selected bond lengths (A) and angles (°) for structure 15
Rh(1)ꢀP(1)
2.208(2)
2.227(2)
2.36(1)
2.31(1)
2.331(9)
2.327(9)
1.36(2)
1.38(2)
1.85(1)
1.821(7)
1.87(1)
1.85(1)
1.799(6)
1.85(1)
1.47(2)
P(1)ꢀRh(1)ꢀP(1a)
P(1)ꢀRh(1)ꢀC(5)
80.23(9)
92.3(2)
156.3(3)
145.4(3)
89.8(3)
83.7(4)
167.2(3)
87.0(3)
99.8(3)
34.0(4)
85.6(2)
162.0(3)
34.4(4)
108.2(4)
106.8(3)
Rh(1)ꢀP(1a)
Rh(1)ꢀC(4)
Rh(1)ꢀC(5)
Rh(1)ꢀC(4A)
Rh(1)ꢀC(5A)
C(4A)ꢀC(5A)
C(4)ꢀC(5)
P(1)ꢀC(1)
P(1)ꢀC(16)
P(1)ꢀC(22)
P(1A)ꢀC(1A)
P(1A)ꢀC(16A)
P(1A)ꢀC(22A)
C(22A)ꢀC(22)
P(1a)ꢀRh(1)ꢀC(5)
P(1)ꢀRh(1)ꢀC(5A)
P(1a)ꢀRh(1)ꢀC(5A)
C(5)ꢀRh(1)ꢀC(5A)
P(1)ꢀRh(1)ꢀC(4A)
P(1a)ꢀRh(1)ꢀC(4A)
C(5)ꢀRh(1)ꢀC(4A)
C(5A)ꢀRh(1)ꢀC(4A)
P(1)ꢀRh(1)ꢀC(4)
P(1a)ꢀRh(1)ꢀC(4)
C(5)ꢀRh(1)ꢀC(4)
C(5A)ꢀRh(1)ꢀC(4)
C(4A)ꢀRh(1)ꢀC(4)
Scheme 4. Schematic depiction of the co-ordination spheres around
the metal centres in the complex cations of 15 and 16. The phospho-
rus atoms are represented as white circles, the centroids Ct1(C4ꢁC5),
Ct1a(C4aꢁC5a) of the co-ordinated olefin units in 15 and
Ct1(C4ꢁC5), Ct2(C23ꢁC30), and Ct3(C26ꢁC27) in 16 as black circles.
,
,
,
,
RhꢀCt1 2.226 A, RhꢀCt1a 2.212 A; IrꢀCt1 2.122 A, IrꢀCt2 2.072 A,
IrꢀCt1 2.218 A.
,

C. Laporte et al. / Journal of Organometallic Chemistry 641 (2002) 227–234
231
Table 2
Selected bond lengths (A) and angles (°) for structure 16
bond activations [21], or to the ring opening polymeri-
sations (ROP) of ansa-ferrocenes to yield organometal-
lic plastics [22]. The ‘wide bite angle’-effect caused by
rigid chelating diphosphanes in transition metal
catalysed reactions [23], or, on the contrary, effects
evoked by narrow bite angles in four-membered hetero-
cycles like metalladiphosphetanes, [M(h2-R2PCH2PR2]
[24], may also be cited in this context. On the other
hand, it was demonstrated that cyclic organometallics,
such as metallacyclobutanes, do are much less strained
that was a priori assumed [25]. While in most of these
studies, strain effects were correlated with reactivity,
thermodynamic data of strained organometallic
molecules seem to be scarce. Indeed, one can anticipate
that in comparison to classical strained organic
molecules, less strain energy can be loaded into an
organometallic species. Especially for MꢀL type com-
plexes (L stands for a neutral two electron donor) this
may become difficult, because of the relatively weak
MꢀL binding energies. Hence, too much strain will
provoke bond fission and in a simple case a larger
heterocyclic binuclear complex may form at the expense
of a small and strained mono-nuclear complex (of
course the entropy loss has to be overcome in this
reaction). This means, that the energy differences be-
tween comparable strained and unstrained molecules
are presumably quite small and hence difficult to mea-
sure. In this respect, electrochemical data may be a very
valuable tool and thermodynamic relations between the
redox active species can be easily extracted from cyclic
voltammograms, provided the redox waves are at least
quasi-reversible.
,
Ir(1)ꢀP(1)
Ir(1)ꢀP(1A)
C(4)ꢀC(5)
C(4A)ꢀC(5A)
Ir(1)ꢀC(23)
Ir(1)ꢀC(26)
Ir(1)ꢀC(27)
Ir(1)ꢀC(30)
Ir(1)ꢀC(4)
2.278(1)
2.401(1)
1.434(7)
1.307(9)
2.303(5)
2.175(5)
2.203(4)
2.343(4)
2.228(5)
2.252(4)
1.859(6)
1.861(6)
1.827(5)
1.913(5)
1.821(6)
1.833(5)
1.522(7)
C(26)ꢀIr(1)ꢀP(1)
C(27)ꢀIr(1)ꢀP(1)
C(4)ꢀIr(1)ꢀP(1)
93.8(1)
89.9(1)
87.6(1)
88.8(1)
78.3(2)
92.0(2)
78.7(2)
85.4(2)
163.9(1)
84.9(2)
76.8(2)
112.5(2)
107.8(2)
159.9(1)
34.5(2)
139.4(1)
101.9(1)
129.0(1)
92.2(1)
83.9(4)
111.3(1)
84.2(1)
37.5(2)
91.2(2)
128.3(2)
128.3(2)
165.6(2)
37.3(2)
C(5)ꢀIr(1)ꢀP(1)
C(26)ꢀIr(1)ꢀC(23)
C(27)ꢀIr(1)ꢀC(23)
C(4)ꢀIr(1)ꢀC(23)
C(5)ꢀIr(1)ꢀC(23)
P(1)ꢀIr(1)ꢀC(23)
C(26)ꢀIr(1)ꢀC(30)
C(27)ꢀIr(1)ꢀC(30)
C(4)ꢀIr(1)ꢀC(30)
C(5)ꢀIr(1)ꢀC(30)
P(1)ꢀIr(1)ꢀC(30)
C(23)ꢀIr(1)ꢀC(30)
C(26)ꢀIr(1)ꢀP(1A)
C(27)ꢀIr(1)ꢀP(1A)
C(4)ꢀIr(1)ꢀP(1A)
C(5)ꢀIr(1)ꢀP(1A)
P(1)ꢀIr(1)ꢀP(1A)
C(23)ꢀIr(1)ꢀP(1A)
C(30)ꢀIr(1)ꢀP(1A)
C(26)ꢀIr(1)ꢀC(27)
C(26)ꢀIr(1)ꢀC(4)
C(27)ꢀIr(1)ꢀC(4)
C(26)ꢀIr(1)ꢀC(5)
C(27)ꢀIr(1)ꢀC(5)
C(4)ꢀIr(1)ꢀC(5)
Ir(1)ꢀC(5)
P(1)ꢀC(1)
P(1)ꢀC(16)
P(1)ꢀC(22)
P(1A)ꢀC(1A)
P(1A)ꢀC(16A)
P(1A)ꢀC(22A)
C(22)ꢀC(22A)
,
2.228(5) A (IrꢀC4) while the axial IrꢀC23/30 bonds are
,
significantly longer [2.303(5), 2.343(4) A, respectively]
(see also the distances to the olefin centroids given in
the caption of Scheme 4). In contrast to 15, the five
membered IrP2C2 heterocycle in 16 has a twist confor-
In contrast to the complexes [Rh(troppph)2]+ 1 and
[Rh(metroppph)2]+ 17 (Scheme 1), the [Rh{R,S-
bis(troppPh)}]+ complex cation of 15 could not be
reversibly reduced in CH2Cl2 or acetonitrile as solvents
under our conditions (scan rates 100 to −5000 mV
,
mation and the CH2ꢀCH2 unit [C22ꢀC22a 1.522(7) A]
shows staggered conformation [torsion angle
a
P1aꢀC22aꢀC22ꢀP1 −44°].
The olefin unit of the tropp ligand, which binds via
P1a to the iridium centre, is turned away from the
metal centre. One might assume that a complex like 16
is an intermediate on the reaction pathway for the
formation of the tetrachelate complex 15. Although, it
is well known that penta co-ordinated complexes of
rhodium are much less stable than for iridium, the
reluctance of 16 to give up the co-ordination to the
remaining cod ligand and to form the desired tetra
co-ordinated [Ir{R,S-bis(troppPh)}]+ complex is some-
what surprising.
s
−1, T= −30–20 °C). However, in THF as solvent
quasi-reversible redox waves were obtained (Fig. 3)
which allow the determination of the redox potentials
of the couples [Rh{R,S-bis(troppPh)}]+/[Rh{R,S-
4. Electrochemical investigations
The effect of strain energy imposed by the ligand on
the properties of a transition metal complex is an
important issue in organometallic chemistry. As perti-
nent recent examples one may refer to the ansa-effect in
metallocenes used for olefin polymerisations [20] and
Fig. 3. Cyclic voltammogram of 15 in THF, 6=100 mV s−1
T=20 °C.
,

232
C. Laporte et al. / Journal of Organometallic Chemistry 641 (2002) 227–234
Table 3
atmosphere. Solvents were distilled from appropriate
drying agents.
Half-wave peak potentials E11/2, E12/2 and potential difference DE=
E11/2−E21/2 of 15 1, 7 and [Rh(troppcyc)2]+ vs. [Ag/AgCl] at a scan
rates, 6=100 mV s−1
5.2. Cyclo6oltammetry
E11/2 (V)
E21/2 (V)
DE (V)
The electrochemical investigations were performed
on an apparatus designed by Heinze et al. [26]. Work-
ing electrode: planar platinum electrode (approx. sur-
face area 0.785 mm2); reference electrode: silver;
counterelectrode, platinum wire; tetrahydrofuran,
methylene chloride or acetonitrile as solvent. At the end
of each measurement, ferrocene was added as internal
standard for calibration (+0.352 V vs. Ag/AgCl).
15
1
7
−1.405
−0.917
−0.882
−1.189
−1.772
−1.308
−1.298
−1.532
0.367
0.391
0.416
0.343
[Rh(troppcyc)2]+
Working electrode: Pt-wire; electrolyte: THF–0.1 M n-Bu4NPF6;
T=20 °C.
bis(troppPh)}]0 and [Rh{R,S-bis(troppPh)}]0/[Rh{R,S-
bis(troppPh)}]−1
.
5.3. NMR
The half-wave peak potentials E11/2 and E12/2 obtained
for a scan rate 6=100 mV s−1 are listed in Table 3.
For comparison, the values for 1, 7 and the Pꢀalkyl
substituted complex [Rh(troppcyc)2]+ (cyc=cyclohexyl
[17]) are included.
1
Bruker DPX Avance Series 250–300 MHz. H- and
13C-chemical shifts are calibrated against the solvent
1
signal (CDCl3, H-NMR: 7.27 ppm; 13C-NMR: 77.23
1
ppm; CD2Cl2, H-NMR: 5.32 ppm; 13C-NMR: 54.00
In contrast to 1 and 7, the cation [Rh{R,S-
bis(troppPh)}]+ of 15 is reduced at a potential about
500 mV more negative which corresponds to an energy
difference D(DG°) of about 50 kJ mol−1. Not only
substitution of electron withdrawing aryl for electron
donating alkyl groups on the phosphorus centres ac-
count for this relatively large shift. The first redox
potential of the complex [Rh(troppcyc)2]+, in which all
Pꢀphenyl groups in the tropp ligand have been replaced
by cyclohexyl groups, is only shifted by 290 mV
[D(DG°)=28 kJ] to more negative potentials. If we
assume that a shift of about 200 mV to more negative
potentials is due to an electronic effect caused by
replacing one phenyl group for one alkyl group in each
tropp unit, there still remains a difference of about 300
mV. Hence, relative to 1 and 7, the redox couple
[15]+/[15]0 is at least 30 kJ mol−1 higher in energy
which we attribute to strain in the ligand system. Note,
that the potential difference DE=E11/2−E12/2between
both redox steps does not change very much for 15, 1,
7 and [Rh(troppcyc)2]+ [min. D(DG°)=33 kJ mol−1 for
[Rh(troppcyc)2]0/[Rh(troppcyc)2]−1; max. D(DG°)=40 kJ
mol−1 for [7]0/[7]−1]. Therefore, we assume that this
extra-energy of ]30 kJ mol−1 is build up when the
cation [Rh{R,S-bis(troppPh)}]+ is reduced to the
rhodium(0) complex. The irreversibility of the reduc-
tions in acetonitrile or CH2Cl2 as solvent, indicate a
high reactivity of 15 which is under current
investigation.
ppm). 31P chemical shifts are calibrated against 85%
H3PO4 as external standard.
5.4. X-ray crystallographic analyses
Single crystals of 15 or 16 were obtained by slow
diffusion of n-hexane into a concentrated methylene
chloride solution. The crystallographic data for 15 and
16 were collected on a Siemens SMART PLATFORM
with CCD Detector and are listed in Table 4.
The structures of 15 and 16 were solved by using
direct methods. The data sets were refined against the
full matrix (vs. F2) with SHELXTL (Version 5.0) [27].
Non-hydrogen atoms were treated anisotropically, hy-
drogen atoms were refined on calculated positions using
the riding model. For the structure 15 the PF6 anion
was refined isotropically as rigid group. The phenyl-
rings at the phosphorus atoms were partially disor-
dered. Crystals of 16 contained two non-coordinating
dichloromethane solvent molecules, which were as the
CF3SO3 counter anion refined with anisotropic temper-
ature factors. An empirical absorption-correction was
performed using SADABS. The plots shown in Figs. 1
and 2 were produced by the PLUTO program.
5.5. Syntheses
1,2-bis(phenylphosphano)ethane was prepared fol-
lowing a literature known procedure [15].
5.5.1. Synthesis of the 1,2-[(5H-dibenzo[a,d]-
cyclohepten-5-yl)phenylphosphano]ethanes R,S-11 and
R,R-/S,S-12, [bis(troppPh)]
A solution of 1,2-bis(phenylphosphano)ethane (9, 1.2
g, 5 mmol) in toluene (50 ml) was added to a solution
of 5-chloro-5H-dibenzo[a,d]cycloheptene (10, 3.5 g, 10
5. Experimental
5.1. General considerations
All manipulations were performed under dry argon

C. Laporte et al. / Journal of Organometallic Chemistry 641 (2002) 227–234
233
mmol) in toluene (50 ml) at room temperature (r.t.),
leading to the precipitation of the hydrophosphonium
salt [{(trop)PhHPCH2}2]2+ 2Cl−, as a white solid. Af-
ter stirring for 1 h at r.t., the reaction mixture was
refluxed for 30 min. After cooling to r.t., a 1 M solution
of potassium carbonate was added and the reaction
mixture was refluxed until the organic phase became
yellow. The organic layer was then separated and dried
over CaCO3. The colourless meso compound R,S-11
crystallised from the toluene phase upon cooling to
−20 °C (1.23 g, 67%). The racemic mixture of R,R-/
S,S-12 was subsequently crystallised from a mixture of
toluene–acetonitrile (0.61 g, 33%) and contained about
5–8% R,S-11. Total yield (1.84 g, 60%).
R,S-11: melting point (m.p.) 254 °C. MS: m/z=
191.2 [trop, 100%], 435.2 [M-trop, 14%]. 31P-NMR
1
(CDCl3): l= −16.52, s. H-NMR (CDCl3): l=1.16
(m, PCH2CH2P, 4H), 3.94 (m, CHP, 2H), 6.34 (d,
3JHH=7.5 Hz, ꢁCH, 2H), 6.85–7.26 (m, ꢁCH, 2H, and
CHar, 26H). 13C-NMR (CDCl3): l=25.1 (m,
PCH2CH2P), 61.1 (m, 1JPC+2JPC=18.5 Hz, CHP),
127.7 (m, ꢁCH), 127.9, 128.6, 129.6, 129.7, 129.9, 130.6,
131.1, 131.3, 131.6, 131.8, 132.9, 133.3, 134.1, 134.7 (m,
ꢁCH, 1C, and CHar,13C), 136.7 (m, Cquat), 136.9 (m,
Table 4
Crystallographic data for 15 and 16
15
16
Empirical formula
Formula weight
Colour
Temperature (K)
Wavelength (A)
C55H52Cl4F3IrO3P2S C44H36F6P3Rh
1245.97
Red
874.55
Red
293(2) K
0.71073
Tetragonal
P4(1)2(1)2
1
Cquat), 139.5 (m, JPC+4JPC=20.9 Hz, PCquat), 139.8
233(2)
(m, Cquat), 140.3 (m, Cquat).
,
R,R-/S,S-12: m.p. 220 °C. 31P-NMR (CDCl3): l=
Crystal system
Space group
Orthorhombic
P212121
−18.41, s. 1H-NMR (CDCl3): l=1.27 (m,
3
PCH2CH2P, 4H), 3.90 (m, CHP, 2H), 6.27 (d, JHH
=
Unit cell dimensions
,
a (A)
10.3764(16)
19.993(3)
24.368(4)
5055.4(13)
4
1.637
Empirical
(SADABS)
13.42820(10)
13.42820(10)
43.3344(2)
7813.91(9)
8
7.5 Hz, ꢁCH, 2H), 6.65–7.25 (m, ꢁCH, 2H, and CHar,
,
b (A)
26H). 13C-NMR (CDCl3): l=22.2 (m, PCH2CH2P),
,
c (A)
1
59.4 (m, JPC+2JPC=16.7 Hz, CHP), 125.8–132.9 (m,
3
,
V (A )
ꢁCH, 2C, and CHar, 13C), 133.9 (m, Cquat), 136.6 (m,
Cquat), 137.7 (m, Cquat), 139.1 (m, Cquat).
Z
Dcalc (mg m−3
Absorbtion
correction
)
1.487
5.5.2. Synthesis of {R,S-1,2-[(5H-dibenzo-
[a,d]cyclohepten-5-yl)phenylphosphano]ethane}rhodium
hexafluorophosphate (15)
Data collection
Siemens SMART PLATFORM with CCD
detector graphite monochromator
Omega-scans
Method
Solution by
direct methods
Equimolar amounts of R,S-1,2-[(5H-dibenzo[a,d]-
cyclohepten-5-yl)phenylphosphano]ethane, R,S-11, (254
mg, 0.4 mmol), bis[(chloro)(1,5-cyclooctadiene)-
rhodium], 13, (100 mg, 0.2 mmol) and tallium hex-
afluorophosphate (141 mg, 0.4 mmol) were dissolved in
tetrahydrofuran (20 ml). The resulting reaction mixture
was stirred 2 h at r.t. and a red solution and some
precipitate were formed. The solvent was evaporated,
the residue was re-dissolved in methylene chloride (20
ml), and filtered over Celite. The methylene chloride
phase was concentred to 10% of its volume and hexane
was added to precipitate the pure complex 15 as red
micro-crystalline powder; yield (135 mg, 94%).
Absorption
3.013
0.620
coefficient (mm−1
F(000)
)
2496
0.62×0.58×0.40
1.32–30.54
3552
0.42×0.34×0.31
1.59–23.25
Crystal size (mm)
q range for data
collection (°)
Limiting indices
−145h514,
−285k528,
−345l534
59 852/15 257
[Rint=0.0684]
30.54 (%) 99.9
−145h514,
−85k514,
−485l547
37 949/5616
[Rint=0.0667]
23.25 (%) 100.0
Reflections
collected/unique
Completeness to
q=
Max. and min.
transmission
0.3787 and 0.2567
0.8310 and 0.7807
Refinement method Full-matrix
31P-NMR (CD2Cl2): l=107.64 (d, 1JPRh=176.8
least-squares on F2
1
Hz). H-NMR (CD2Cl2): l=1.52 (m, CH2, 2H), 2.40
Data/restraints/param 15 257/0/622
eters
5616/0/413
2
(m, CH2, 2H), 4.88 (m, JPH+4JPH=15.4 Hz, CHP,
2H), 6.67–7.63 (m, ꢁCH, 2H, and CHar, 26H). 13C-
Goodness-of-fit on
1.044
1.143
F2
1
NMR (CD2Cl2): l=23.5 (m, JPC+2JPH=40.05 Hz,
Final R indices
[I\2|(I)]
R indices (all data)
R1=0.0403,
wR2=0.0890
R1=0.0582,
wR2=0.0969
1.505 and −1.543
R1=0.0688,
wR2=0.1831
R1=0.0744,
wR2=0.1890
1.014 and −1.222
PCH2CH2P), 53.1 (m, 1JPC+3JPC=17.3 Hz, CHP),
97.5 (m, ꢁCH), 103.1 (m, ꢁCH), 127.7 (m, CHar), 128.1
(m, CHar), 128.7 (m, CHar), 129.1 (m, CHar), 129.4 (m,
CHar), 129.7 (m, CHar), 130.5 (m, CHar), 132.1 (m,
CHar), 133.2 (m, Cquat), 134.4 (m, Cquat), 135.8 (m,
Cquat), 135.2 (m, Cquat).
Largest difference
peak and hole
−3
,
(e A
)

234
C. Laporte et al. / Journal of Organometallic Chemistry 641 (2002) 227–234
[6] See for example: C.O. Dietrich-Buchecker, J. Guilhem, J.-M.
Kern, C. Pascard, J.P. Sauvage, Inorg. Chem. 33 (1994) 3498
and literature cited therein.
[7] See for example: S.A. Jackson, O. Eisenstein, J.D. Martin, A.C.
Albeniz, R.H. Crabtree, Organometallics 10 (1991) 3062.
[8] The influence of the distortion of the electronically preferred
coordination sphere of a metal centre by the ligand is most
prominently exemplified in copper enzymes (entatic state ap-
proach). See for example:
[9] G. Pilloni, M. Vecchi, M. Martelli, J. Electroanal. Chem. Interf.
Electrochem. 45 (1973) 483.
[10] G. Pilloni, G. Zotti, M. Martelli, Inorg. Chem. 21 (1982) 1284
and literature cited therein.
5.5.3. Synthesis of {R,S-1,2-[(5H-dibenzo[a,d]-
cyclohepten-5-yl)phenylphosphano]ethane} cyclo
octadiene iridium trifluoromethanesulfonate (16)
Equimolar amounts of R,S-1,2-[(5H-dibenzo[a,d]-
cyclohepten-5-yl)phenylphosphano]ethane, R,S-11, (50
mg, 0.08 mmol) and of bis(1,5-cyclooctadien)iridium(I)
triflate (44 mg, 0.08 mmol) were dissolved in methylene
chloride (20 ml). The resulting orange reaction mixture
was stirred 2 h at r.t. The methylene chloride phase was
concentred to 10% of its volume and hexane was added
to precipitate the pure complex 16 as slightly yellow
micro-crystalline powered; yield (57 mg, 88%).
31P-NMR (CD2Cl2): l=12.72 (s), 66.58 (s).
[11] B.K. Teo, A.P. Ginsberg, K.C. Calabrese, J. Am. Chem. Soc. 98
(1976) 3027.
[12] A.J. Kunin, E.J. Nanni, R. Eisenberg, Inorg. Chem. 24 (1985)
1852.
[13] K.T. Mueller, A.J. Kunin, S. Greiner, T. Henderson, R.W.
Kreilick, R. Eisenberg, J. Am. Chem. Soc. 109 (1987) 6313 and
literature cited therein.
6. Supplementary material
[14] (a) B. Bogdanovic´, W. Leitner, C. Six, U. Wilczok, K.
Wittmann, Angew. Chem. 109 (1997) 518;
(b) B. Bogdanovic´, W. Leitner, C. Six, U. Wilczok, K.
Wittmann, Angew. Chem. Int. Ed. Engl. 36 (1997) 500.
[15] J. Dogan, J.B. Schulte, G.F. Swiegers, S.B. Wild, J. Org. Chem.
65 (2000) 951.
[16] If the literature reported diastereomer ratio for bis(phosphane) 9
(30% meso and 70% racemic) is correct [15], the reaction with
tropylidenyl chloride 10 seems to be quite stereoselective, since
this ratio is reversed.
Crystallographic data (excluding structure factors)
for the structures reported in this paper have been
deposited with the Cambridge Data Centre as supple-
mentary publication nos. CCDC-171219 (15) and
CCDC-171218 (16). Copies of the data can be obtained
free of charge on application to CCDC, 12 Union
Road, Cambridge CB21EZ, UK [fax: +44-1223-
336033; e-mail: deposit@ccdc.cam.ac.uk or www: http:/
/www.ccdc.cam.ac.uk).
[17] The trans-isomers of rhodium tropp complexes show 13C reso-
nances for the co-ordinated olefins which are shifted to lower
frequencies (Dl :10–20) than in their corresponding cis-iso-
mers: S. Deblon, ETH-Dissertation Nr. 13920 (2000).
[18] A.G. Orpen, L. Brammer, F.H. Allen, O. Kennard, D.G. Wat-
son, R. Taylor, J. Chem. Soc. (1989) S1.
Acknowledgements
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H. Gru¨tzmacher, Helv. Chim. Acta 84 (2001) 3127.
[20] H.H. Brintzinger, D. Fischer, R. Mu¨lhaupt, B. Rieger, R.M.
Waymouth, Angew. Chem. Int. Ed. Engl. 34 (1995) 1143.
[21] For a recent reference on the ansa-effect on CꢀH and CꢀC bond
activation see: D. Churchill, J.H. Shin, T. Hascall, J.M. Hahn,
B.M. Bridgewater, G. Parkin, Organometallics 18 (1999) 2403;
and literature cited therein.
[22] I. Manners, J. Chem. Soc., Chem. Commun. (1999) 857.
[23] P.W.N.M. van Leeuwen, P.C.J. Kamer, J.N.H. Reek, P.
Dierkes, Chem. Rev. 100 (2000) 2741.
[24] For a recent example see: H. Urtel, C. Meier, F. Eisentra¨ger, F.
Rominger, J.P. Joschek, P. Hofmann, Angew. Chem. Int. Ed.
Engl. 40 (2001) 781, and literature cited therein.
This work was supported by the National Science
Foundation of Switzerland.
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