The influence of steric pressure on the structure of homodimetallic rhodium(I) and iridium(I) complexes containing a bis(bidentate) phosphane ligand

Article abstract of DOI:10.1002/ejic.200601210

Fifteen novel homodimetallic Rh^I and Ir^I complexes containing the bis(bidentate) phosphane ligand cis,trans,cis-1,2,3,4- tetrakis(diphenylphosphanyl)cyclobutane (dppcb) were prepared and characterized by NMR spectroscopy (¹H, ¹³C{¹H}, ³¹P{¹H}), mass spectrometry, IR spectroscopy, elemental analyses and melting points. Furthermore, the solid-state structures of seven of these new compounds were fully determined by single-crystal X-ray diffraction analyses to study the influence of steric pressure. The precursor complex [Rh₂(η⁴-cod)₂(dppcb)]X₂ (1), X^- = BF₄^-, PF₆^-, SbF ₆^-, completely characterized by its X-ray structure, smoothly reacts with mono- or bidentate ligands containing phosphorus or nitrogen donor atoms. Thus, monophosphanes and monophosphites produce compounds of the structure type [Rh₂L₄(dppcb)](SbF₆) ₂ [L = PMe₂Ph, 2, PMePh₂, 3; P(OMe) ₃, 5; P(OPh)₃, 6]. The X-ray structures of 3 and 6 show that PMePh₂ and P(OPh)₃ are capable of compensating steric interactions. The treatment of 1 with diphosphanes leads to the structure type [Rh₂L₂(dppcb)](SbF₆)₂ [L = bis(diphosphanyl)-methane, dppm, 7; bis(diphenylphosphanyl)amine, dppam, 8; 1,2-bis(diphenylphosphanyl)ethane, dppe, 9; cis-1,2-bis(diphenylphosphanyl) ethene, cis-dppen, 10]. The X-ray structures of 8, 9 and 10 clearly indicate the onset of steric pressure as a consequence of mechanical coupling, which results in two different coordination moieties for homodimetallic species 9 and 10. Under steric pressure, the quadrant effects of dppcb and dppe become comparable to Rh¹ complexes. The use of mono- or bidentate ligands containing nitrogen donor atoms leads to the compounds [Rh₂(pyridine) ₄-(dppcb)](SbF₆)₂ (11), [Rh₂(2, 2′-bipyridine)₂(dppcb)](SbF₆)₂ (12) and [Rh₂(1,10-phenanthroline)₂(dppcb)](SbF₆) ₂ (13). The crystal structures of 1 and the novel complex [Ir ₂(η⁴-cod)₂-(dppcb)]X₂ (14), X^- = BF₄^-, SbF₆^-, are isomorphous. However, it was only possible to produce restricted examples of derivatives of 14 owing to the reluctance of 14 to release cod. Thus, monophosphanes lead to the five-coordinate species [Ir₂(η ⁴-cod)₂(PMe₂Ph)₂(dppcb)](BF ₄)₂ (15), whereas the four-coordinate compound [Ir ₂(PMePh₂)₄(dppcb)](BF₄)₂ (16) is formed by using the sterically more demanding ligand PMePh₂ instead of PMe₂Ph. Wiley-VCH Verlag GmbH & Co. KGaA, 2007.

Full text of DOI:10.1002/ejic.200601210

FULL PAPER
DOI: 10.1002/ejic.200601210
The Influence of Steric Pressure on the Structure of Homodimetallic
Rhodium(I) and Iridium(I) Complexes Containing a Bis(bidentate) Phosphane
Ligand
Christian Bachmann,[a] Rene Gutmann,[a] Georg Czermak,[a] Alexander Dumfort,[a]
Sylvia Eller,[a] Markus Fessler,[a] Holger Kopacka,[a] Karl-Hans Ongania,[b] and
Peter Brüggeller*[a]
Keywords: X-ray diffraction / Chelates / Phosphane ligands / Iridium / Rhodium
Fifteen novel homodimetallic RhI and IrI complexes contain-
ing the bis(bidentate) phosphane ligand cis,trans,cis-1,2,3,4-
tetrakis(diphenylphosphanyl)cyclobutane (dppcb) were pre-
pared and characterized by NMR spectroscopy (1H, 13C{1H},
31P{1H}), mass spectrometry, IR spectroscopy, elemental
analyses and melting points. Furthermore, the solid-state
structures of seven of these new compounds were fully deter-
mined by single-crystal X-ray diffraction analyses to study
the influence of steric pressure. The precursor complex
bis(diphenylphosphanyl)ethene, cis-dppen, 10]. The X-ray
structures of 8, 9 and 10 clearly indicate the onset of steric
pressure as a consequence of mechanical coupling, which re-
sults in two different coordination moieties for homodi-
metallic species 9 and 10. Under steric pressure, the quad-
rant effects of dppcb and dppe become comparable to RhI
complexes. The use of mono- or bidentate ligands containing
nitrogen donor atoms leads to the compounds [Rh2(pyridine)4-
(dppcb)](SbF6)2 (11), [Rh2(2,2Ј-bipyridine)2(dppcb)](SbF6)2
[Rh2(η4-cod)2(dppcb)]X2 (1), X– = BF4–, PF6–, SbF6–, com- (12) and [Rh2(1,10-phenanthroline)2(dppcb)](SbF6)2 (13). The
pletely characterized by its X-ray structure, smoothly reacts
with mono- or bidentate ligands containing phosphorus or
crystal structures of 1 and the novel complex [Ir2(η4-cod)2-
(dppcb)]X2 (14), X– = BF4–, SbF6–, are isomorphous. However,
nitrogen donor atoms. Thus, monophosphanes and mono- it was only possible to produce restricted examples of deriva-
phosphites produce compounds of the structure type
[Rh2L4(dppcb)](SbF6)2 [L = PMe2Ph, 2; PMePh2, 3; P(OMe)3,
5; P(OPh)3, 6]. The X-ray structures of 3 and 6 show that
PMePh2 and P(OPh)3 are capable of compensating steric in-
teractions. The treatment of 1 with diphosphanes leads to the
structure type [Rh2L2(dppcb)](SbF6)2 [L = bis(diphosphanyl)-
methane, dppm, 7; bis(diphenylphosphanyl)amine, dppam,
8; 1,2-bis(diphenylphosphanyl)ethane, dppe, 9; cis-1,2-
tives of 14 owing to the reluctance of 14 to release cod. Thus,
monophosphanes lead to the five-coordinate species [Ir2(η4-
cod)2(PMe2Ph)2(dppcb)](BF4)2 (15), whereas the four-coordi-
nate compound [Ir2(PMePh2)4(dppcb)](BF4)2 (16) is formed
by using the sterically more demanding ligand PMePh2 in-
stead of PMe2Ph.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2007)
contrast, it is well-known that in the case of hydrofor-
mylation the catalytically active species is a trigonal bipy-
ramidal hydrido rhodium complex, which usually contains
two phosphorus donor ligands.[4] Because the whole cata-
lytic cycle for the hydroformylation reaction consists of five-
coordinate species,[4] the enlargement in the coordination
number relative to the above square planar catalysts cer-
tainly increases the steric pressure.[5]
In this context, steric pressure is defined as the sum of
intramolecular steric interactions keeping the coordination
geometry and/or conformation of a certain complex vir-
tually intact.[5] This means that the onset of steric pressure
enforcing intramolecular contact approaches also changes
the quadrant effect[1–3] in square planar species because the
quadrant diagrams usually depend on the spatial distribu-
tions of the phenyl groups present in bidentate or bis(bi-
dentate) phosphanes (see below, Figure 3). A certain intra-
molecular steric pressure only allows distinct spatial distri-
Introduction
Only recently was it shown that the efficiency of catalysts
containing bidentate or bis(bidentate) phosphanes is
strongly determined by the flexibility of their back-
bones.[1–3] This influence of the backbone can be seen in
different catalytic applications like the copolymerization of
carbon monoxide and ethene,[1] the oxidative carbonylation
of styrene[2] and the Schulz–Flory oligomerization of eth-
ene.[3] The common feature of these different kinds of
homogeneous catalysis is that square-planar complexes are
thought to be responsible for the catalytic activity.[1–3] By
[a] Institut für Allgemeine, Anorganische und Theoretische
Chemie, Universität Innsbruck,
Innrain 52a, 6020 Innsbruck, Austria
E-mail: Peter.Brueggeller@uibk.ac.at
[b] Institut für Organische Chemie, Universität Innsbruck,
Innrain 52a, 6020 Innsbruck, Austria
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P. Br üggeller et al.
FULL PAPER
butions of these phenyl rings and hence controls the quad- on the steric demands of the involved ligands is typical for
rant effect. Thus, steric pressure is important to determine this kind of reaction.[13] However, the use of the sterically
the structure of the resultant complexes and steers their congested monophosphane PPh3 and 1 leads to a fast equi-
catalytic activity by the quadrant effect.[1–3]
librium between the different compounds as evidenced from
Therefore, in this work, the onset of steric pressure is the resulting 31P{1H} NMR spectrum; no product could be
studied in a series of X-ray structures of novel square isolated.
planar homodimetallic RhI species containing the bis(bi-
dentate) phosphane cis,trans,cis-1,2,3,4-tetrakis(diphenyl-
phosphanyl)cyclobutane (dppcb) prepared in our labora-
tory.[6] It seemed to us especially interesting to see if the
rigidity of the backbone of dppcb is enough to provide the
same usual quadrant effect,[1–3] which shows one axial and
one equatorial phenyl group at each phosphorus centre,
even under “steric pressure”. In the case of dppcb, the onset
of intramolecular steric interactions leads to excellent pho-
tochemical and photophysical properties of six-coordinate
homodimetallic RuII compounds.[7,8] However, in general,
sterically congested five-coordinate complexes of RhI are
rare and/or unstable towards the loss of the fifth ligand.[9]
In contrast, a typical situation for carbonylation catalysis is
that starting square planar compounds lead to five-coordi-
nate species.[9,10] Therefore, a subtle choice of the steric
requirements of further ligands should make it possible to
observe a change in the usual conformation of the cyclobu-
tane backbone of dppcb also in square planar RhI com-
plexes, which would result in a different quadrant effect.
This study should shed light on the question of whether
there is sufficient flexibility in the ligand backbone of dppcb
to allow easy access to all intermediates in the catalytic car-
bonylation cycle.[11]
Results and Discussion
Synthesis of Complexes
–
–
–
[Rh2(η4-cod)2(dppcb)]X2 (1), X– = BF4 , PF6 , SbF6
(Figure 4), was prepared by a reported procedure for the
in situ generation of a coordinatively unsaturated species
[Rh(η4-cod)(acetone)2]X from the chlorido-bridged dirho-
dium complex [RhCl(η4-cod)]2.[12] Treatment of [Rh(η4-
cod)(acetone)2]X with dppcb leads to 1 in excellent yield.
Figure 1. Structure types observed in Rh and Ir complexes.
Phosphanes can be sterically and electronically modi-
–
The use of different anions was necessary, as only the BF4
salt yielded single crystals suitable for X-ray structure
–
analysis, PF6 is the only innocent counterion under the fied.[14] Because the propensity for incremental tuning is
planned catalytic hydroformylation conditions, and only the crucially important in catalysis,[14] we successfully extended
–
SbF6 form of 1 produced further derivatives in excellent the reaction sequence above to phosphites, diphosphanes
yield. The exchange of these anions was carried out by uti- and mono- and bidentate nitrogen ligands (see complexes
lizing the corresponding silver salt.[12]
5–13 in Figures 1 and 2). Only this synthetic effort made it
The apparent simple cod substitution from 1 makes it a possible to obtain a series of X-ray structures containing
suitable precursor complex.[13] Thus, 1 smoothly reacts with increasingly sterically more-demanding ligands and to study
the monophosphane PMe2Ph to give [Rh2(PMe2Ph)4- their fine tuning of the conformation of dppcb. It was espe-
(dppcb)](SbF6)2 (2, see Figure 1). However, in the case of cially hoped that the onset of steric pressure could lead to
the sterically more demanding monophosphane PMePh2, different coordination units within these homodimetallic
not only the main product (Figure 1) [Rh2(PMePh2)4- species.
(dppcb)](SbF6)2 (3) was formed, but also the known homo-
In the case of IrI, the analogous reaction sequence lead-
leptic complex[12b] [Rh(PMePh2)4](SbF6) (4), which was ing to 1 was unsuccessful. However, as observed pre-
produced in a side reaction and isolated in a pure form viously[15] for a comparable synthesis, it was possible to per-
(Figure 1). The formation of homoleptic species depending form the direct reaction of dppcb with the chlorido-bridged
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Steric Pressure and the Structure of Homodimetallic RhI and IrI Complexes
FULL PAPER
clean synthesis of 1 or 14, though this lability is consistent
with predictions that are based on hard–soft acid–base
theory where the soft olefin ligand is replaced preferentially
by a soft triorganophosphane donor.[18] However, as a con-
sequence of the intrinsic problem of pentacoordination in
the IrI case it was not possible to obtain the same series of
complexes as for those of RhI, which reflected increasing
steric pressure.
Comparison of the Quadrant Effects in Complexes of dppe
and dppcb
Conformations and quadrant diagrams for M(dppe) and
M2(dppcb) complexes are shown in Figure 3.[19] The con-
formation adopted by the phenyl groups in dppe complexes
is noticeably different from that for the dppcb ligand. In
terms of the quadrant effect, the steric crowding provided
by the phenyl groups is diagonally disposed with respect to
the coordination plane in the dppe derivatives. By contrast,
this steric crowding is usually concentrated on one side of
the coordination plane in the dppcb complexes owing to
spatial distributions of the phenyl substituents on the phos-
phorus donors so that the two axial phenyl rings are located
on the same side of the coordination plane.[2,19] However, it
is clear that the conformations shown in Figure 3 are only
the borderline cases.
Figure 2. Structure types observed in Rh and Ir complexes.
diiridium complex [IrCl(η4-cod)]2 and [Ir2Cl2(η4-cod)2-
(dppcb)] resulted in excellent yield. Subsequent treatment
with AgX produced [Ir2(η4-cod)2(dppcb)]X2 (14), X–
=
–
–
BF4 , SbF6 (see Figure 1).[15]
–
Analogous to 1, only the BF4 salt of 14 yielded single
crystals that were suitable for X-ray structure analysis; the
crystal structures of 1 and 14 are isomorphous. However,
synthetic difficulties appeared when 14 was used as a pre-
–
cursor complex for further derivatives in its BF4 as well as
–
SbF6 forms. Similar observations were made when other
chelating ligands were treated with iridium precursor com-
plexes and even under harsh conditions it was not possible
to displace the remaining cod ligand; decomposition oc-
curred instead.[11] This is in agreement with the higher de-
gree of metal-to-cod bonding and back-bonding in the iri-
dium complexes than in the rhodium complexes and hence
with the greater stability of the Ir–cod compounds.[16] Be-
cause the ligand exchange reactions observed in RhI and IrI
systems are likely to proceed by associative mechanisms,[13]
14 smoothly reacts with the monophosphane PMe2Ph to
give the five-coordinate species [Ir2(η4-cod)2(PMe2Ph)2-
(dppcb)](BF4)2 (15, see Figure 2). Pentacoordinated struc-
Figure 3. Quadrant effects in complexes of dppe and dppcb.
Especially for M2(dppcb), a centrosymmetric structure is
tures containing cod are well-known for IrI;[17] they are depicted where each coordination unit contains two axial
much more stable than the RhI complexes (compare analo- and two equatorial phenyl groups. This conformation is
gous complexes 2 and 15 in Figures 1 and 2).
typical for homodimetallic species of dppcb without steric
However, the greater steric pressure of PMePh2 leads to pressure.[1–3,6,20] The distances of the ipso carbon atoms of
the displacement of cod in 14 and the four-coordinate com- the phenyl substituents from the coordination planes clearly
pound [Ir2(PMePh2)4(dppcb)](BF4)2 (16, Figure 1) is define axial, equatorial and diagonal dispositions (see Fig-
formed.
ure 3). Thus, the influence of the onset of steric pressure on
Interestingly, the substitutional lability of 1,5-cod versus the idealized M2(dppcb) conformation in Figure 3 can be
triorganophosphane ligands does not play any role in the directly measured.
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Isomorphous X-ray Structures of [M2(η4-cod)2(dppcb)]-
(BF4)2 [M = Rh (1), Ir (14)]
and a dihedral angle of 22.5(1)° with the Cg1–Rh(1)–Cg2
plane occurs. In 1, the distortion from an ideal square-
planar coordination is larger than that in [Rh(η4-
cod){CH2P(C6H4-SiMe2CH2CH2C6F13-p)2}2](BPh4) with a
corresponding dihedral angle of 13.1°,[21] though the steric
crowding of the latter complex is considerable. A further
indication of steric pressure in 1 is the significantly different
Rh–C distances,[21,23] where selected bond lengths and
angles of 1 and 14 are given in Table 1.
It is well-known that the time-averaged preferred confor-
mations of the five-membered metallarings in complexes of
type 1 (see Figure 1) are dictated by the nature of the biden-
tate or bis(bidentate) phosphane ligand.[2] The X-ray struc-
ture of 1 (see Figure 4) clearly reveals that only the phenyl
groups attached to P(2) show axial and equatorial spatial
distributions with respect to the coordination plane.
Table 1. Selected bond lengths [Å] and angles [°] for 1 and 14.
Compound 1
Compound 14
Rh(1)–P(1)
Rh(1)–P(2)
Rh(1)–C(3)
Rh(1)–C(4)
Rh(1)–C(7)
Rh(1)–C(8)
C(3)–C(4)
C(7)–C(8)
2.2797(7)
2.2671(7)
2.264(3)
2.227(3)
2.276(3)
2.234(3)
1.368(5)
1.361(6)
Ir(1)–P(1)
Ir(1)–P(2)
Ir(1)–C(3)
Ir(1)–C(4)
Ir(1)–C(7)
Ir(1)–C(8)
C(3)–C(4)
C(7)–C(8)
P(1)–Ir(1)–P(2)
2.2775(13)
2.2815(13)
2.192(6)
2.242(6)
2.200(5)
2.225(6)
1.360(11)
1.396(9)
84.45(4)
P(1)–Rh(1)–P(2) 83.65(2)
However, these Rh–C distances are significantly shorter
than in cis-[Rh(η4-cod){Me2Si(µ-NtBu)2PCH2}2](BF4) with
an average Rh–C distance of 2.305(6) Å.[14] The latter long
rhodium–carbon bonds were interpreted in terms of an in-
creased steric repulsion between the bis(phosphane) and the
cyclooctadiene moieties.[14] Obviously, in 1 the steric pres-
sure is efficiently released by a suitable distortion of the
Figure 4. View of the cation of [Rh2(η4-cod)2(dppcb)](BF4)2 (1).
The atom labelling scheme is shown.
The diagonal disposition of the phenyl substituents at square-planar coordination together with a suitable spatial
P(1) is characteristic of dppe.[2,19,21] This difference is shown distribution of the phenyl substituents leading to an un-
by the deviations of the four ipso carbon atoms C(11), usual quadrant effect for homodimetallic species of dppcb.
C(21), C(31) and C(41) of –1.177(4), 1.650(4), 0.187(5) and
The structures of 1 and 14 fulfill the criteria of the same
–2.079(5) Å, respectively, from the coordination plane de- ligands, the same coordination number and geometry, iso-
fined by the atoms Rh(1), P(1) and P(2) and the centres of morphous crystal lattices and equal experimental condi-
gravity of the cod double bonds (see Figure 4).[22] The tions, and, therefore, allow an exact comparison of their
largest deviations for C(21) of 1.650(4) Å and for C(41) of geometric features.[24] The quadrant effect in 14, given by
–2.079(5) Å indicate that the diagonal quadrants are more the deviations of the four ipso carbon atoms C(11), C(21),
hindered (see Figures 3 and 4). Obviously the steric pressure C(31) and C(41) of –1.169(5), 1.647(5), 0.217(6) and
introduced by cod is already enough to change the usual –2.080(5) Å, respectively, from the coordination plane de-
quadrant effect of M2(dppcb) in Figure 3. The atoms and fined as above, is nearly identical to 1. The same is true for
centres of gravity (Cg) defining the coordination plane are the tetrahedral distortion of the coordination plane, as
tetrahedrally distorted, as shown by the deviations from this shown by the deviations from this plane: Ir(1) 0.032(2) Å,
plane: Rh(1) 0.038(2) Å, P(1) 0.267(2) Å, P(2) –0.289(2) Å, P(1) 0.272(2) Å, P(2) –0.289(2) Å, Cg1 0.297(3) Å, Cg2
Cg1 0.284(3) Å, Cg2 –0.301(3) Å.
–0.311(3) Å and the resulting dihedral angle defined as
It was shown that short intramolecular contacts due to above of 23.0(1)°. Also, the steric pressure of 14 is compar-
steric pressure in homodimetallic complexes of dppcb able to 1 (see above): C(22)···H(46B) 2.982 Å, H(3A)···
change their photochemical and photophysical proper- H(42A) 2.908 Å, H(4A)···H(36A) 2.417 Å, H(7A)···H(22A)
ties.[7,8] In the case of 1, this steric pressure is clearly respon- 2.729 Å, H(8A)···H(12A) 2.602 Å. However, the mean value
sible for the observed unusual quadrant effect. Thus, in 1 of the significantly shorter Ir–C distances (see Table 1) of
the shortest intramolecular contact between the phenyl 2.196(4) Å in 14 is significantly shorter than the corre-
rings along a trans axis of the cyclobutane ring is C(22)··· sponding parameter of 2.231(2) Å in 1. Analogously, the
H(46B) of 2.988 Å. Furthermore, the hydrogen atoms at- mean value of the significantly longer Ir–C distances of
tached to the double bonds of cod (see Figure 4) also show 2.234(4) Å is significantly shorter than the corresponding
pronounced intramolecular interactions with adjacent parameter of 2.270(2) Å in 1.
phenyl groups: H(3A)···H(42A) 2.980 Å, H(4A)···H(36A)
Olefins such as cod are generally considered to be rather
2.475 Å, H(7A)···H(22A) 2.764 Å, H(8A)···H(12A) 2.558 Å. weakly binding ligands.[11] However, the possibility of a pre-
As a consequence, the C=C bonds of the cod ring are not organization of the metal binding site leads to the forma-
coordinated perpendicular to the P(1)–Rh(1)–P(2) plane tion of especially stable complexes and the olefin is not dis-
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Steric Pressure and the Structure of Homodimetallic RhI and IrI Complexes
FULL PAPER
placed in the catalytically active species.[11] It was shown
above in the cases of 1 and 14 that a reorganization of
dppcb, which results in the coordination of cod, produces
a suitable quadrant effect and a tetrahedrally distorted co-
ordination sphere that are responsible for the tighter bind-
ing of cod. As a consequence, the C=C bond lengths of cod
are elongated in these complexes, which is normal for diene
complexes of this type.[16] The mean values of the cod
double bond lengths are 1.365(4) Å in 1 and 1.378(7) Å in
14. Thus, the bond lengthening in the iridium complex is
slightly more pronounced, which is in agreement with its
higher degree of metal-to-ligand bonding and back-bond-
ing.[16] This is in line with the significantly longer mean
value of the M–centroid distances of 2.145(2) Å in 1 relative
to the corresponding parameter of 2.105(4) Å in 14.
Though the M–cod trans influence is smaller than the M–
Figure 5. View of the cation of [Rh2(PMePh2)4(dppcb)](SbF6)2 (3).
The atom labelling scheme is shown. Hydrogen atoms attached to
the phenyl rings are omitted for clarity.
C trans influence of organic groups,[25] the significantly
longer M–centroid mean value in 1 produces a significantly
shorter mean value of the M–P bond lengths of 2.2734(5) Å
in 1 versus 2.2795(9) Å in 14. The use of weakly bonded
olefins as leaving groups is wide-spread for rhodium cata- largest deviations for C(21) of 1.534(5) Å and for C(41) of
lysts.[26] The M–C distances in 1 and 14 are located within –2.022(6) Å again indicate that the diagonal quadrants are
the typical range of 2.1–2.3 Å.[15,17,18,27] However, the sig- more hindered. Also, in 3 the coordination plane is tetrahe-
nificantly longer mean value of the M–centroid distances in drally distorted, as shown by the deviations from this plane:
1 than in 14 nicely corresponds to the fact that cod is easily Rh(1) 0.008(1) Å, P(1) 0.303(1) Å, P(2) –0.305(1) Å, P(3)
displaced in the case of 1, whereas difficulties appear using 0.261(1) Å, P(4) –0.267(1) Å.
14 as a precursor complex as a result of the five-coordinate
species that still contains cod.
Although alternative ligand conformations are undoubt-
edly accessible in solution, the solid-state structures pre-
sented in this work exhibit trends which are consistent with
steric effects exerting an important influence on reactiv-
ity.[19] In the case of 3, the occurrence of steric pressure
leads to isolable side product 4. The evidence of steric
crowding in 3, which is comparable to 1 and 14, again stems
from short intramolecular contact approaches. Thus, the
X-ray Structures of [Rh2(PMePh2)4(dppcb)](SbF6)2 (3),
[Rh(PMePh2)4](SbF6) (4) and [Rh2{P(OPh)3}4(dppcb)]-
(SbF6)2 (6)
To confirm the observation that steric pressure changes shortest intramolecular contact between the phenyl rings
the quadrant effect in homodimetallic species of dppcb such along a trans axis of the cyclobutane ring is C(33)···H(22B)
as 1 and 14, a series of X-ray structures showing increasing of 3.080 Å. The shortest intramolecular contact between
steric pressure were analyzed. Figure 5 clearly shows that in the dppcb and PMePh2 moieties is H(46A)···H(86A) of
3 the phenyl rings at P(1) are diagonally disposed with re- 2.449 Å and the shortest intramolecular contact between
spect to the coordination plane, whereas the phenyl groups the PMePh2 moieties is H(4B)···C(56) of 2.554 Å. As in the
attached to P(2) show axial and equatorial spatial distribu- cases of 1 and 14, the steric pressure in 3 is partly released
tions. This means that the quadrant effect in 3 is similar to by a dihedral angle of 20.7(1)° between the P(1)–Rh(1)–P(2)
1 and 14 (compare with Figure 4). In all cases, this influence and P(3)–Rh(1)–P(4) planes.
on the quadrant effect by conformational restraints can be
The formation of homoleptic complexes such as 4 is typi-
referred to as mechanical coupling in analogy to the con- cal for the reaction type leading to 3.[13] In general, the
cept of molecular mechanics strain energy.[28] As in the field amount of tetrahedral distortion in cations [(phosphane)4-
of molecular mechanics, mechanical coupling refers to the Rh]+ can be rationalized as a compromise between the pre-
energy terms associated with bond stretches, bond angle ferred square planar arrangement of a d8 transition-metal
bends, torsional deformations, van der Waals and electro- centre and the steric repulsion of the four PR2 groups.[13]
static interactions.[28]
In Figure 6 this tetrahedral distortion can be clearly seen
Obviously, the steric pressure imposed by cod in 1 and for 4.
14 is comparable to two PMePh2 moieties in 3. Thus, the
The crystal structure of 4 consists of two slightly dif-
deviations of the four ipso carbon atoms C(11), C(21), ferent conformations and shows dihedral angles defined as
C(31) and C(41) from the coordination plane defined by the above between cis P–Rh–P planes of 34.5(1)° and 31.2(1)°,
atoms Rh(1), P(1), P(2), P(3) and P(4) are –1.175(6), respectively. The Rh–P bond lengths in the two conforma-
1.534(5), 0.237(5) and –2.022(6) Å, respectively (see Fig- tions of 4 are significantly shorter than the Rh–PMePh2
ure 5). These deviations are almost identical to the corre- distances in 3 (see Table 2). This is in line with a better
sponding parameters of 1 and 14 (see above), where the release of steric pressure in 4 than in 3, where also the elec-
Eur. J. Inorg. Chem. 2007, 3227–3239
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3231

P. Br üggeller et al.
FULL PAPER
Figure 6. View of the cation of one conformation of [Rh(PMePh2)4]-
(SbF6) (4). The atom labelling scheme is shown. Hydrogen atoms
attached to the phenyl rings are omitted for clarity.
Figure 7. View of the cation of [Rh2{P(OPh)3}4(dppcb)](SbF6)2 (6).
The atom labelling scheme is shown. Hydrogen atoms attached to
the phenyl rings are omitted for clarity.
tronic bite angle effect of dppcb could play a part.[4] The
Rh–P bond lengths of 4 are in between the average of the
equatorial Rh–P bond lengths of 2.308 Å and the corre-
sponding axial parameter of 2.337(2) Å in the distorted tri-
gonal bipyramidal [RhH(PMePh2)4].[29] Distortions of the
square planar structure like in 4 for example, also in the
case of chlorotris(triphenylphosphane)rhodium(I), are well-
known.[18]
For comparable (diphosphane)rhodium compounds it
was found that structures determined by X-ray methods in
the solid state and structures characterizing the ensembles
of molecules in solution are congruent within the limits of
error.[30] This means that a change in the quadrant effect
observed in the solid state is important for active catalysts
derived from precatalysts such as 1. Also, in 6 short intra-
molecular contact approaches are responsible for the ob-
served conformation leading to a change in the quadrant
effect compared with 1, 3 and 14. The shortest intramolecu-
lar contact between the phenyl rings along a trans axis of
the cyclobutane ring is C(34)···H(26B) of 2.910 Å. The
shortest intramolecular contact between the dppcb and
P(OPh)3 moieties is H(16A)···H(6BA) of 2.591 Å and the
shortest intramolecular contact between the P(OPh)3 moie-
ties is H(1BA)···H(4BA) of 2.604 Å. However, the dihedral
angle of 12.4(1)° between the P(1)–Rh(1)–P(2) and P(3)–
Rh(1)–P(4) planes in 6 is significantly smaller than the cor-
responding parameters in 1, 3 and 14. This means that the
steric pressure in 6 is better released by a change in the
quadrant effect of dppcb than by a strong tetrahedral dis-
tortion of the coordination plane.
It was recognized that the quadrant effect in 1, 3 and 14
is nearly identical and shows axial and equatorial phenyl
rings attached to one phosphorus atom and diagonally dis-
posed phenyl groups attached to the other. However, fur-
ther enhancement in the steric pressure as a result of the
introduction of P(OPh)3 leads to a change in the quadrant
effect in 6. Thus, in 6 the deviations of the four ipso carbon
atoms C(11), C(21), C(31) and C(41) from the coordination
plane defined by the atoms Rh(1), P(1), P(2), P(3) and P(4)
are 0.620(5), –1.945(5), 1.480(5) and –1.394(6) Å, respec-
tively (see Figure 7). This means that in contrast to 1, 3 and
14, no typical equatorial phenyl ring is present in 6 and,
rather, the spatial distributions of the phenyl substituents
resemble pseudoequatorial, pseudoaxial and diagonal dis-
positions, respectively.[22] However, the largest deviations
for C(21) of –1.945(5) Å and for C(31) of 1.480(5) Å are
still in line with more hindered diagonal quadrants as in the
cases 1, 3 and 14 (see Figures 3 and 7). In 6, the coordina-
tion plane is again tetrahedrally distorted, where the
deviations from this plane are: Rh(1) 0.028(1) Å, P(1)
–0.180(1) Å, P(2) 0.163(1) Å, P(3) –0.171(1) Å, P(4)
0.160(1) Å.
X-ray Structures of [Rh2(dppam)2(dppcb)](SbF6)2 (8),
[Rh2(dppe)2(dppcb)](SbF6)2 (9) and [Rh2(cis-dppen)2-
(dppcb)](SbF6)2 (10)
To the best of our knowledge, the X-ray structures of 8–
10 are the first X-ray structural characterizations of rhodi-
Table 2. Selected bond lengths [Å] and angles [°] for 3, 4 and 6.
Compound 3
Compound 6
Compound 4
Rh(1)–P(1)
Rh(1)–P(2)
Rh(1)–P(3)
Rh(1)–P(4)
P(1)–Rh(1)–P(2)
P(3)–Rh(1)–P(4)
2.2838(14)
2.3021(14)
2.3576(14)
2.3454(13)
81.18(5)
Rh(1)–P(1)
Rh(1)–P(2)
Rh(1)–P(3)
Rh(1)–P(4)
P(1)–Rh(1)–P(2)
P(3)–Rh(1)–P(4)
2.3259(10)
2.3349(9)
2.2674(11)
2.2550(10)
83.83(3)
Rh(1)–P(1)
Rh(2)–P(2)
P(1)–Rh(1)–P(1A)
P(2)–Rh(2)–P(2A)
2.3278(7)
2.3325(7)
92.631(7)
92.145(6)
92.67(5)
91.30(4)
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Steric Pressure and the Structure of Homodimetallic RhI and IrI Complexes
FULL PAPER
um(I) complexes with two different chelating phosphanes.
Of course, the difficulty in preparing and crystallizing these
compounds stems from a further enhancement of steric
pressure. As a consequence, dppcb could show the ability
to form different coordination sites within homodimetallic
species. This may have implications for their potential use as
dimetallic catalysts, comparable to the ability of dimetallic
systems with M2P4 units containing bis[{(diphenylphos-
phanyl)ethyl}phenylphosphanyl]methane to move between
open and closed bridged forms.[25] Indeed, Figures 8, 9 and
10 clearly show that the stronger steric pressure in 9 and
10, which contain 5–5–4–5–5-membered ring systems com-
pared with 8 that contains a 4–5–4–5–4-membered ring sys-
tem, produces two different coordination sites in 9 and 10,
respectively. As all X-ray structures presented in this study
show no strong intermolecular contacts or no obvious
packing effect, we may assume as a first approximation that
the forces on the molecule in solution are of the same order
of magnitude as those in the solid.[31] Though the crystallo-
graphic restraints are strong for structures 1, 3, 6, 8 and 14
because of the presence of centres of symmetry in the mid-
dle of the cyclobutane rings, this means that centrosymmet-
ric conformations are also available in solution for these
structures. By contrast, it seems likely that 9 and 10 crys-
tallizing in different space groups not only show noncentro-
symmetric structures in the solid state, but also in solution.
Interestingly, the quadrant effect of dppcb in 8 is similar
to 1, 3 and 14. Thus, the phenyl groups attached to one
phosphorus atom show axial and equatorial spatial distri-
butions, whereas a diagonal disposition occurs for the
phenyl substituents of the other phosphorus atom (see Fig-
ure 8).
Figure 8. View of the cation of [Rh2(dppam)2(dppcb)](SbF6)2 (8).
The atom labelling scheme is shown. Hydrogen atoms attached to
the phenyl rings are omitted for clarity.
This is clearly reflected in the deviations of the four ipso
carbon atoms C(11), C(21), C(31) and C(41) of –0.164(5),
1.912(5), 1.368(5) and –1.253(5) Å, respectively, from the
coordination plane defined by the atoms Rh(1), P(1), P(2),
P(3) and P(4). However, the largest deviations for C(21) of
1.912(5) Å and for C(31) of 1.368(5) Å concentrate the ste-
ric crowding now on one side of the coordination plane (see
Figures 3 and 8). The atoms defining this least-squares
Figure 9. View of the cation of [Rh2(dppe)2(dppcb)](SbF6)2 (9). The
atom labelling scheme is shown. Hydrogen atoms attached to the
phenyl rings are omitted for clarity.
plane are again tetrahedrally distorted, as shown by the de- resulting dihedral angle defined as above of 16.9(1)°. Also,
viations from this plane: Rh(1) 0.025(1) Å, P(1) 0.185(1) Å, in this case short intramolecular contacts indicate steric
P(2) –0.196(1) Å, P(3) 0.213(1) Å, P(4) –0.226(1) Å and the crowding. The shortest contacts are 3.199 Å for H(22A)···
Table 3. Selected bond lengths [Å] and angles [°] for 8, 9 and 10.
Compound 8
Compound 9
Compound 10
Rh(1)–P(1)
Rh(1)–P(2)
Rh(1)–P(3)
Rh(1)–P(4)
N(1)–H(3)
P(1)–Rh(1)–P(2)
P(3)–Rh(1)–P(4)
2.2860(10)
2.2909(10)
2.3124(10)
2.2937(10)
0.74(4)
82.42(4)
69.90(4)
Rh(1)–P(1)
Rh(1)–P(2)
Rh(1)–P(3)
Rh(1)–P(4)
Rh(2)–P(5)
Rh(2)–P(6)
Rh(2)–P(7)
Rh(2)–P(8)
P(1)–Rh(1)–P(2)
P(3)–Rh(1)–P(4)
P(5)–Rh(2)–P(6)
P(7)–Rh(2)–P(8)
2.314(4)
2.289(4)
2.303(4)
2.317(4)
2.323(4)
2.314(4)
2.289(4)
2.287(4)
83.64(16)
81.12(13)
82.56(15)
86.63(13)
Rh(1)–P(1)
Rh(1)–P(2)
Rh(1)–P(5)
Rh(1)–P(6)
Rh(2)–P(3)
Rh(2)–P(4)
Rh(2)–P(7)
Rh(2)–P(8)
P(1)–Rh(1)–P(2)
P(5)–Rh(1)–P(6)
P(3)–Rh(2)–P(4)
P(7)–Rh(2)–P(8)
2.305(3)
2.302(3)
2.287(3)
2.293(3)
2.294(3)
2.282(3)
2.303(3)
2.310(3)
80.97(10)
83.70(13)
86.35(10)
83.48(11)
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3233

P. Br üggeller et al.
FULL PAPER
Indeed, the quadrant effects of dppcb become completely
different for the two unequal coordination units of 9 and
10, respectively (see Figures 9 and 10). In the cases of 9 and
10, the phenyl rings attached to the phosphorus atoms of
dppcb coordinating to the Rh(1) atoms show pseudoaxial
and pseudoequatorial spatial distributions, as indicated by
the deviations of the four ipso carbon atoms from the coor-
dination planes defined as above: 9, C(3A) –0.541(5) Å,
C(3G) 1.653(5) Å, C(4A) –0.514(5) Å, C(4G) 1.670(5) Å;
10, C(1A) –0.755(4) Å, C(1G) 1.619(4) Å, C(2A)
–0.359(5) Å, C(2G) 1.786(5) Å. In both cases the steric
crowding is concentrated on one side of the coordination
plane as a result of the largest deviations for C(3G) of
1.653(5) Å and for C(4G) of 1.670(5) Å in the case of 9 and
for C(1G) of 1.619(4) Å and for C(2G) of 1.786(5) Å in the
case of 10 (see Figures 3, 9 and 10). In contrast, the phenyl
substituents attached to the phosphorus atoms of dppcb
coordinating to the Rh(2) centres unequivocally show
diagonal dispositions, where the corresponding parameters
are: 9, C(7A) 1.599(5) Å, C(7G) –1.270(5) Å, C(8A)
–1.373(5) Å, C(8G) 1.511(5) Å; 10, C(3A) –1.116(5) Å,
C(3G) 1.667(5) Å, C(4A) –1.446(4) Å, C(4G) 1.402(4) Å.
Figure 10. View of the cation of [Rh2(cis-dppen)2(dppcb)](SbF6)2
(10). The atom labelling scheme is shown. Hydrogen atoms at-
tached to the phenyl rings are omitted for clarity.
H(46B) along a trans axis of the cyclobutane ring and The largest deviations corresponding to the Rh(2) centre in
2.540 Å for H(36A)···H(62A) between the dppcb and 9 for C(7A) of 1.599(5) Å and for C(8G) of 1.511(5) Å
dppam moieties. However, the small four-membered chelate again concentrate the steric crowding on one side of the
ring angle P(3)–Rh(1)–P(4) of only 69.90(4)° (see Table 3) coordination plane (see Figures 3 and 9).
moves the phenyl groups of dppam away from the rhodium
Interestingly, the analogous largest deviations in 10 for
centre, which reduces the overall steric pressure of 8. In con- C(3G) of 1.667(5) Å and for C(4A) of –1.446(4) Å are in
trast to 9 and 10, this could still allow a centrosymmetric line with more hindered diagonal quadrants (see Figures 3
conformation for 8. Compound 8 shows only very weak and 10). However, in 9 and 10 all deviations from the least-
hydrogen bonds between the located N–H hydrogen atoms squares coordination planes are below 0.08 Å, as also indi-
–
of dppam and the SbF6 counterions. This is nicely con- cated by the resulting small dihedral angles defined as
firmed by the shorter N–H bond length of 0.74(4) Å (see above: 9, at Rh(1) 3.3(2)°, at Rh(2) 1.8(2)°; 10, at Rh(1)
Table 3) than in comparable complexes of dppam with 7.4(2)°, at Rh(2) 5.0(2)°. Again, short intramolecular con-
strong hydrogen bonds (0.78–0.79 Å).[32] Furthermore, it is tacts are responsible for the observed unusual conforma-
also in agreement with the values of 8 for ν(N–H) and δ(N– tions. The shortest intramolecular contacts along a trans
H) of 3330 and 1266 cm–1, respectively.[33] It is well-known axis of the cyclobutane rings are 2.778 Å for C(4A)···
that π–π stacking of phenyl rings might play a role in tuning H(7FA) in 9 and 2.916 Å for C(2A)···H(3HA) in 10.
organometallic reactivity.[19] Though no crystallographic re- Furthermore, the shortest intramolecular contacts are
straints influence the X-ray structures of 9 and 10, these 2.310 Å for H(2HA)···H(3LA) between the dppcb and dppe
alignments of the phenyl groups can be seen in several cases moieties and 2.635 Å for H(6HA)···H(2FA) between the
in Figures 9 and 10.
dppcb and cis-dppen moieties.
Obviously, the steric pressure is so enormous in 9 and 10
Obviously, the significantly larger five-membered chelate
that it is necessary to minimize steric tensions through this ring angles of dppe and cis-dppen in 9 and 10 relative to
effect. This has dramatic consequences on the conforma- dppam in 8 (see Table 3) move the phenyl groups of dppe
tions of 9 and 10. The envelope-foldings, defined as the and cis-dppen towards the rhodium centres. This leads to
angles between the least-squares planes through the chelat- overall steric pressures in 9 and 10, which cannot be re-
ing phosphorus and backbone carbon atoms of dppcb and leased within centrosymmetric conformations. Interestingly,
through the chelating phosphorus atoms of dppcb and the though no crystallographic restraints are present, the cyclo-
rhodium atoms, are usually orientated towards the cyclobu- butane rings of 9 and 10 are completely planar within statis-
tane rings: 20.6(1)°, 1; 30.7(1)°, 3; 16.5(1)°, 6; 31.8(1)°, 8; tical significance, which further confirms the similarity of
20.4(1)°, 14. The same effect is present in 9 and 10 with the solid state to the solution structures, as discussed above.
regard to the Rh(1) atom: 38.1(2)°, 9; 36.8(2)°, 10. However, Both 9 and 10 show significantly different chelate angles of
the analogous envelope-foldings for the Rh(2) centres in 9 dppcb for the two unequal coordination units: 9, 81.12(13)°
and 10 are orientated away from the cyclobutane rings: and 86.63(13)°; 10, 80.97(10)° and 86.35(10)° (see Table 3).
10.0(2)°, 9; 11.8(2)°, 10. This inverse envelope-folding is This means that under steric pressure the chelate angles
rare for homodimetallic species of dppcb and indicative of may also change, which is, beside the quadrant effect, an-
strong steric pressure.[7]
other important feature for catalytic cycles.[19]
3234
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Steric Pressure and the Structure of Homodimetallic RhI and IrI Complexes
FULL PAPER
Investigation of 1–3 and 5–16 in the
Solution State
Summary
The series of X-ray structures presented in this work indi-
cates that steric pressure changes the quadrant effect. It
clearly proves that the rigid cyclobutane backbone of dppcb
does not play any role in determining the quadrant effect,
if homodimetallic RhI complexes of dppcb are under steric
pressure. Depending on the kind and magnitude of steric
pressure, both borderline cases typical for the M(dppe) and
M2(dppcb) quadrant effects in Figure 3 are observed. In
consequence of which, dppcb and dppe show no differentia-
tion between their distributions of equatorial–equatorial
and equatorial–axial geometric isomers for the five-coordi-
nate intermediates in the case of catalytic hydrofor-
mylation.[36] However, the regioselectivity of hydrofor-
mylation is governed by a complex web of electronic and
steric effects that have so far defied unravelling.[19] Never-
theless, excellent progress was made in realizing when and
how regioselectivity and enantioselectivity are controlled
within a catalytic hydroformylation cycle.[37] Our work
shows that it is not enough to study the quadrant effects of
precatalysts such as 1,[38] but it is also necessary to realize
how the steric pressure varies during the catalytic cycle. Pre-
dictions solely based on X-ray structures of precatalysts
such as 1 could be misleading. The occurrence of two dif-
ferent coordination units in 9 and 10 also seems to be of
some importance for rhodium-catalysed tandem reac-
tions.[39]
All compounds 1–3 and 5–16 show sharp doublets in the
rhodium cases that are due to JRh,P couplings and sharp
1
single peaks in the iridium cases for the dppcb resonances
in the 31P{1H} NMR spectra at ambient temperature. This
indicates equivalent phosphorus atoms of dppcb for all
complexes. Cooling the sample down to 178 K only leads
to broadened signals. This behaviour is typical for dppcb
conformations with planar cyclobutane rings,[34] in contrast
to folded cyclobutane ring conformations.[6] Because planar
cyclobutane rings were also observed in the X-ray struc-
tures of 1, 3, 6, 8, 9, 10 and 14 independent of crystallo-
graphic constraints, this nicely confirms the fact that the
solid-state and solution structures are identical. The struc-
ture types shown in Figures 1 and 2 are not only deter-
mined by these X-ray structures, but also their 1JRh,P values
of dppcb are consistent with the proposed trans influence
series for RhI species.[12a] Typically, nitrogen-containing li-
gands such as those in 11–13 exert a weak trans influence,
1
which leads to JRh,P parameters of 162–180 Hz. In 1 cod
1
shows a stronger trans influence with JRh,P of 154 Hz,
which is characteristic of cod.[12b] In this series, the two li-
gand types with the highest trans influence, phosphanes and
1
phosphites,[12a] produce JRh,P values for dppcb of 129–
137 Hz. A similar range of 121–135 Hz was observed for
1
the corresponding JRh,P parameters of the mono- and di-
phosphanes in 2, 3 and 7–10. The phosphites in 5 and 6
1
show larger JRh,P values of 199 Hz and 234 Hz,
Experimental Section
respectively, which is also in agreement with earlier re-
2
sults.[12a] The range of JP,P parameters of 234–267 Hz is
Reagents and General Procedures: The ligand dppcb was prepared
as described earlier.[6] [RhCl(η4-cod)]2, [IrCl(η4-cod)]2, AgX (X– =
trans
2
typical for phosphanes, where also in this case the JP,P
trans
–
–
–
BF4 , PF6 , SbF6 ) and dry solvents of purissimum grade quality
were obtained from Fluka. Mono- and diphosphanes, phosphites
and nitrogen-containing ligands were purchased from Strem. A
Schlenk apparatus and oxygen-free, dry Ar were used in the synthe-
ses of all complexes. Solvents were degassed by several freeze–
pump–thaw cycles prior to use. All reactions were carried out at
ambient temperature.
values for phosphites of 364 Hz in 5 and 380 Hz in 6 are
larger.[12a]
In 9 and 10, the two different coordination units inter-
change with one another, with the asymmetric isomer being
the more stable one. This leads to a coalescence of the
31P{1H} NMR signals for dppcb as well as for those of
dppe and cis-dppen, which cannot be resolved at low tem-
peratures. This effect is comparable to the conformational
isomerisation of [(κ-PMes2CH2CH(OH)CH2-κ-PPh2]-
Rh(η4-cod)]+, where δ and λ isomers interchange with one
another, with the λ isomer being the more stable one.[30] In
this complex, the mesityl groups at the PMes2 entity behave
as coupled rotors, their rotation is only possible in the
transformation between the δ and λ forms.[30] In an analo-
gous manner, the phenyl groups at the PPh2 entities in 9
and 10 behave as coupled rotors, their rotation is only pos-
sible in the transformation between pseudoaxial and
pseudoequatorial spatial distributions and diagonal disposi-
tions of the phenyl rings of dppcb (see Figures 9 and 10).
This is clearly confirmed by the observed π–π stacking
interactions in 9 and 10 and could be regarded as a
“mechanical molecular machine”.[30] These confor-
mational properties will control the stereochemical
result of reactions mediated by chelate compounds of
dppcb.[35]
Instrumentation: Fourier-mode 1H, 13C{1H} and 31P{1H} NMR
spectra were obtained with a Bruker DPX-300 spectrometer (in-
ternal deuterium lock) at 298 K. Positive chemical shifts are down-
1
field from the standards: TMS for the H and 13C{1H} resonances
and 85% H3PO4 for the 31P{1H} resonances. Owing to the restric-
ted solubility of compounds 1–16, the 13C{1H} NMR spectra in
CD2Cl2 are rather insensitive to structural variations, where only
the 13C{1H} NMR resonances of the phenyl rings are clearly re-
solved in the range 128–136 ppm as multiplets.
[Rh2(η4-cod)2(dppcb)]X2 (1), X– = BF4 , PF6 , SbF6 : [RhCl(η4-
cod)]2 (40 mg, 0.081 mmol) was dissolved in acetone (5 mL) and
AgBF4 (32 mg, 0.162 mmol) also dissolved in acetone (1 mL) was
added with vigorous stirring. The reaction mixture was stirred for
3 h, and the formed AgCl was filtered off. To this solution, dppcb
(64 mg, 0.081 mmol) was added with vigorous stirring. The dark
orange solution was stirred for 15 min, and the solvent was then
completely removed. The orange residue was washed with diethyl
ether (5 mL) and dried in vacuo. Red crystals were recrystallized
–
–
–
1
from CH2Cl2. Yield: 0.125 g (91%). M.p. 255 °C (dec.). H NMR
(300 MHz, CD2Cl2, 25 °C): δ = 6.5–8.0 (m, 40 H, Ph), 4.92 [s, 8 H,
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3235

P. Br üggeller et al.
FULL PAPER
CH (cod)], 4.30 [br. s, 4 H, CH (cyclobutane)], 2.41 [s, 16 H, CH2
a CH2Cl2 solution of 6 under an Ar atmosphere at ambient tem-
perature.
(cod)] ppm. 31P{1H} NMR (121.495 MHz, CH2Cl2, 25 °C):
1
δ = 72.9 (d, JRh,P = 154 Hz) ppm. MS (FAB+): m/z (m/zcalcd.) =
[Rh2(dppm)2(dppcb)](SbF6)2 (7): [Rh2(η4-cod)2(dppcb)](SbF6)2
(100 mg, 0.0593 mmol) was dissolved in CH2Cl2 (10 mL) and dppm
(46 mg, 0.119 mmol) was added in solid form with vigorous stir-
ring. The reaction mixture was stirred for 1 h and a clear orange
solution was obtained. The solvent was completely removed, the
yellow residue was washed with diethyl ether (5 mL) and dried in
vacuo. A yellow powder was recrystallized from CH2Cl2. Yield:
0.106 g (80%). M.p. 250–253 °C. 1H NMR (300 MHz, CD2Cl2,
25 °C): δ = 6.5–8.0 (m, 80 H, Ph), 4.30 [br. s, 4 H, CH (cyclobu-
1302.0 (1301.7) [M
– – .
BF4]+, 607.7 (607.5) [M BF4]2+
C68H68B2F8P4Rh2·3.68CH2Cl2 (1701.07): calcd. C 50.61, H 4.47;
found C 50.72, H 4.53. Single crystals suitable for X-ray structure
analysis with the composition [Rh2(η4-cod)2(dppcb)](BF4)2·
3.68CH2Cl2 were obtained by slow evaporation of a CH2Cl2 solu-
tion of the orange residue under a dry atmosphere of Ar at ambient
temperature. [Rh2(η4-cod)2(dppcb)]X2, X– = PF6 , SbF6 , were pre-
pared in an analogous manner showing yields of 82 and 90%,
respectively.
–
–
tane)], 2.22 (br. m,
4
H, CH2P) ppm. 31P{1H} NMR
1
[Rh2(PMe2Ph)4(dppcb)](SbF6)2 (2): [Rh2(η4-cod)2(dppcb)](SbF6)2
(160 mg, 0.0950 mmol) was dissolved in CH2Cl2 (8 mL) and
PMe2Ph (53 mg, 0.380 mmol) was added by syringe with vigorous
stirring. The reaction mixture was stirred for 1 h. In the meantime,
the colour of the solution changed from orange into golden brown.
The solvent was completely removed, the yellow residue was
washed with diethyl ether (5 mL) and dried in vacuo. A yellow
powder was recrystallized from CH2Cl2. Yield: 0.163 g (85%). M.p.
198 °C (dec.). 1H NMR (300 MHz, CD2Cl2, 25 °C): δ = 6.4–8.1
(m, 60 H, Ph), 4.31 [br. s, 4 H, CH (cyclobutane)], 1.08 (br. m, 24
H, CH3) ppm. 31P{1H} NMR (121.495 MHz, CH2Cl2, 25 °C): δ =
(121.495 MHz, CH2Cl2, 25 °C): δ = 79.7 (dd, JRh,P = 137 Hz,
2JP,P
= 259 Hz, dppcb), –18.5 (dd, JRh,P = 121 Hz, JP,P
=
1
2
trans
trans
259 Hz, dppm) ppm. MS (FAB+): m/z (m/zcalcd.) = 2238.1 (2237.8)
[M – H]+, 2003.4 (2003.1) [M – SbF6]+. C102H88F12P8Rh2Sb2
(2238.84): calcd. C 54.72, H 3.96; found C 54.83, H 4.03.
[Rh2(dppam)2(dppcb)](SbF6)2 (8): Procedure as per that of 7. Yield:
1
87%. M.p. 275–278 °C. H NMR (300 MHz, CD2Cl2, 25 °C): δ =
6.0–8.3 (m, 80 H, Ph), 4.29 [br. s, 4 H, CH (cyclobutane)] ppm.
31P{1H} NMR (121.495 MHz, CH2Cl2, 25 °C): δ = 78.5 (dd, 1JRh,P
2
1
= 137 Hz, JP,P
= 267 Hz, dppcb), 45.9 (dd, JRh,P = 121 Hz,
trans
2JP,P
= 267 Hz, dppam) ppm. IR (KBr): ν = 3330 (m) (N–H),
˜
trans
1
2
trans
1266 (m) (N–H) cm–1. MS (FAB+): m/z (m/zcalcd.) = 2240.2 (2239.9)
[M – H]+, 2005.1 (2005.1) [M – SbF6]+. C100H86F12N2P8Rh2Sb2
(2240.81): calcd. C 53.60, H 3.87, N 1.25; found C 53.73, H 3.93,
N 1.15. Single crystals suitable for X-ray structure analysis with the
composition [Rh2(dppam)2(dppcb)](SbF6)2 were obtained by slow
evaporation of a CH2Cl2 solution of 8 under an Ar atmosphere at
277 K.
76.5 (dd, JRh,P = 129 Hz, JP,P
= 234 Hz, dppcb), –6.9 (dd,
1JRh,P = 129 Hz, JP,P
= 234 Hz, PMe2Ph) ppm. MS (FAB+):
2
trans
m/z (m/zcalcd.) = 1786.9 (1786.9) [M – SbF6]+, 775.5 (775.6) [M –
SbF6]2+. C84H88F12P8Rh2Sb2 (2022.644): calcd. C 49.88, H 4.39;
found C 49.97, H 4.46.
[Rh2(PMePh2)4(dppcb)](SbF6)2 (3): Procedure as per that of 2.
Yield: 85%. M.p. 151–154 °C. 1H NMR (300 MHz, CD2Cl2,
25 °C): δ = 6.3–8.2 (m, 80 H, Ph), 4.28 [br. s, 4 H, CH (cyclobu-
tane)], 1.10 (br. m, 12 H, CH3) ppm. 31P{1H} NMR (121.495 MHz,
[Rh2(dppe)2(dppcb)](SbF6)2 (9): Procedure as per that of 7. Yield:
83%. M.p. 140 °C (dec.). 1H NMR (300 MHz, CD2Cl2, 25 °C): δ
= 6.5–8.1 (m, 80 H, Ph), 4.31 [br. s, 4 H, CH (cyclobutane)], 2.25
(br. m, 8 H, CH2P) ppm. 31P{1H} NMR (121.495 MHz, CH2Cl2,
1
2
CH2Cl2, 25 °C): δ = 75.1 (dd, JRh,P = 137 Hz, J
= 234 Hz,
P,Ptrans
dppcb), 6.6 (dd, 1JRh,P = 129 Hz, 2J
= 234 Hz, PMePh2) ppm.
P,Ptrans
MS (FAB+): m/z (m/zcalcd.) = 2035.1 (2035.1) [M – SbF6]+, 899.8
(899.7) [M – SbF6]2+. C104H96F12P8Rh2Sb2·2CH2Cl2 (2440.76):
calcd. C 52.16, H 4.13; found C 52.29, H 4.25. Single crystals suit-
able for X-ray structure analysis with the composition
[Rh2(PMePh2)4(dppcb)](SbF6)2·2CH2Cl2 were obtained by slow
evaporation of a CH2Cl2 solution of 3 under an Ar atmosphere at
ambient temperature.
1
2
25 °C): δ = 81.7 (dd, JRh,P = 137 Hz, JP,P
= 241 Hz, dppcb),
1
2
54.3 (dd, JRh,P = 131 Hz, JP,P
= 241trHansz, dppe) ppm. MS
(FAB+): m/z (m/zcalcd.) = 2267.2tra(n2s 266.9) [M]+, 2031.5 (2031.2)
[M – SbF6]+. C104H92F12P8Rh2Sb2·3.85CH2Cl2 (2593.845): calcd.
C 49.94, H 3.87; found C 49.99, H 3.95. Single crystals suitable for
X-ray structure analysis with the composition [Rh2(dppe)2-
(dppcb)](SbF6)2·3.85CH2Cl2 were obtained by slow evaporation of
a CH2Cl2 solution of 9 under an Ar atmosphere at ambient tem-
perature.
[Rh2{P(OMe)3}4(dppcb)](SbF6)2 (5): Procedure as per that of 2.
Yield: 82%. M.p. 215 °C (dec.). 1H NMR (300 MHz, CD2Cl2,
25 °C): δ = 6.5–8.0 (m, 40 H, Ph), 4.30 [br. s, 4 H, CH (cyclobu-
tane)], 3.20–3.50 (m, 36 H, OCH3) ppm. 31P{1H} NMR
[Rh2(cis-dppen)2(dppcb)](SbF6)2 (10): Procedure as per that of 7.
Yield: 78%. M.p. 205–207 °C. 1H NMR (300 MHz, CD2Cl2,
25 °C): δ = 6.5–8.1 (m, 80 H, Ph), 6.30 (br. m, 4 H, PCHCHP), 4.32
[br. s, 4 H, CH (cyclobutane)] ppm. 31P{1H} NMR (121.495 MHz,
1
(121.495 MHz, CH2Cl2, 25 °C): δ = 78.2 (dd, JRh,P = 129 Hz,
1
2
2JP,P
= 364 Hz, dppcb), 117.0 [dd, JRh,P = 199 Hz, JP,P
=
trans
trans
1
2
364 Hz, P(OMe)3] ppm. MS (FAB+): m/z (m/zcalcd.) = 1730.8
CH2Cl2, 25 °C): δ = 76.8 (dd, JRh,P = 129 Hz, JP,P
= 259 Hz,
trans
SbF6]+, 747.4 (747.5) [M SbF6]2+
–
1
2
(1730.7) [M .
–
dppcb), 66.2 (dd, JRh,P = 135 Hz, JP,P
= 259 Hz, cis-dppen)
trans
ppm. MS (FAB+): m/z (m/zcalcd.) = 2262.0 (2261.8) [M – H]+,
2027.5 (2027.1) [M – SbF6]+. C104H88F12P8Rh2Sb2·5.16CH2Cl2
(2701.066): calcd. C 48.54, H 3.67; found C 48.64, H 3.73. Single
crystals suitable for X-ray structure analysis with the composition
[Rh2(cis-dppen)2(dppcb)](SbF6)2·5.16CH2Cl2 were obtained by
slow evaporation of a CH2Cl2 solution of 10 under an Ar atmo-
sphere at ambient temperature.
C64H80F12O12P8Rh2Sb2 (1966.43): calcd. C 39.09, H 4.10, O 9.76;
found C 39.15, H 4.20, O 9.53.
[Rh2{P(OPh)3}4(dppcb)](SbF6)2 (6): Procedure as per that of 2.
1
Yield: 82%. M.p. 95–98 °C. H NMR (300 MHz, CD2Cl2, 25 °C):
δ = 6.1–8.5 (m, 100 H, Ph), 4.33 [br. s, 4 H, CH (cyclobutane)]
ppm. 31P{1H} NMR (121.495 MHz, CH2Cl2, 25 °C): δ = 76.1 (dd,
2
1
1JRh,P = 129 Hz, JP,P
= 380 Hz, dppcb), 112.0 [dd, JRh,P =
trans
2
234 Hz, JP,P
=
380 Hz, P(OPh)3] ppm. MS (FAB+):
2475.2 (2475.4) [M
C124H104F12O12P8Rh2Sb2·4.26CH2Cl2 (3072.97): calcd. C 50.13, H idine (19 mg, 0.237 mmol) was added by syringe with vigorous stir-
[Rh2(pyridine)4(dppcb)](SbF6)2 (11): [Rh2(η4-cod)2(dppcb)](SbF6)2
trans
m/z
(m/zcalcd.
)
=
–
SbF6]+. (100 mg, 0.0593 mmol) was dissolved in CH2Cl2 (7.5 mL) and pyr-
3.69, O 6.25; found C 50.25, H 3.85, O 6.10. Single crystals suitable
for X-ray structure analysis with the composition [Rh2{P(OPh)3}4-
(dppcb)](SbF6)2·4.26CH2Cl2 were obtained by slow evaporation of
ring. The reaction mixture was stirred for 10 min and a clear, yellow
solution was obtained. The solvent was then completely removed,
and the yellow residue was washed with diethyl ether (5 mL) and
3236
www.eurjic.org
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2007, 3227–3239

Steric Pressure and the Structure of Homodimetallic RhI and IrI Complexes
FULL PAPER
dried in vacuo. A yellow powder was recrystallized from CH2Cl2.
Yield: 0.083 g (78%). M.p. 240–243 °C. 1H NMR (300 MHz,
CD2Cl2, 25 °C): δ = 8.2–9.4 (m, 20 H, pyridine), 6.6–8.1 (m, 40
H, Ph), 4.30 [br. s, 4 H, CH (cyclobutane)] ppm. 31P{1H} NMR
resulting dark red solution was completely removed and the red–
brown residue was dried in vacuo. Red–brown crystals were recrys-
tallized from CH2Cl2. Yield: 0.210 g (75%). M.p. 225 °C (dec.). H
1
NMR (300 MHz, CD2Cl2, 25 °C): δ = 6.4–8.1 (m, 40 H, Ph), 4.90
(121.495 MHz, CH2Cl2, 25 °C): δ = 91.3 (d, 1JRh,P = 178 Hz) ppm. [s, 8 H, CH (cod)], 4.25 [br. s, 4 H, CH (cyclobutane)], 2.43 [s, 16 H,
MS (FAB+): m/z (m/zcalcd.) = 656.2 (656.5) [M – H – SbF6]2+
.
CH2 (cod)] ppm. 31P{1H} NMR (121.495 MHz, CH2Cl2, 25 °C): δ
= 63.5 (s) ppm. MS (FAB+): m/z (m/zcalcd.) = 1480.3 (1480.3) [M –
BF4]+. C68H68B2F8Ir2P4·3.68CH2Cl2 (1879.69): calcd. C 45.80, H
4.04; found C 45.95, H 4.10. Single crystals suitable for X-ray struc-
ture analysis with the composition [Ir2(η4-cod)2(dppcb)](BF4)2·
3.68CH2Cl2 were obtained by slow evaporation of a CH2Cl2 solu-
tion of the red–brown residue under a dry Ar atmosphere at ambi-
ent temperature. [Ir2(η4-cod)2(dppcb)](SbF6)2 was prepared in an
analogous manner with a yield of 67%.
C72H64F12N4P4Rh2Sb2 (1786.538): calcd. C 48.41, H 3.61, N 3.14;
found C 48.53, H 3.73, N 3.07.
[Rh2(2,2Ј-bipyridine)2(dppcb)](SbF6)2 (12): Procedure as per that of
11 with a reaction time of 2 h. The dark red powder was recrys-
tallized from CH2Cl2. Yield: 77%. M.p. 270 °C (dec.). 1H NMR
(300 MHz, CD2Cl2, 25 °C): δ = 8.1–9.3 (m, 16 H, bipyridine), 6.5–
8.2 (m, 40 H, Ph), 4.35 [br. s, 4 H, CH (cyclobutane)] ppm. 31P{1H}
NMR (121.495 MHz, CH2Cl2, 25 °C): δ = 95.4 (d, 1JRh,P = 162 Hz)
ppm. MS (FAB+): m/z (m/zcalcd.) = 1547.1 (1546.8) [M – SbF6]+,
1391.0 (1390.6) [M – bipyridine – SbF6]+, 655.2 (655.5) [M –
SbF6]2+. C72H60F12N4P4Rh2Sb2 (1782.506): calcd. C 48.52, H 3.39,
N 3.14; found C 48.61, H 3.47, N 3.05.
[Ir2(η4-cod)2(PMe2Ph)2(dppcb)](BF4)2 (15): Procedure as per that of
2 with 2 equiv. of the monophosphane. Yield: 67%. M.p. 168–
170 °C. 1H NMR (300 MHz, CD2Cl2, 25 °C): δ = 6.4–8.1 (m, 50
H, Ph), 4.80 [s, 8 H, CH (cod)], 4.26 [br. s, 4 H, CH (cyclobutane)],
2.35 [s, 16 H, CH2 (cod)], 0.95 (br. s, 12 H, CH3) ppm. 31P{1H}
NMR (121.495 MHz, CH2Cl2, 25 °C): δ = 36.3 (s, dppcb), –52.4
(s, PMe2Ph) ppm. MS (FAB+): m/z (m/zcalcd.) = 1843.1 (1843.4)
[M]+, 1479.4 (1479.3) [M – H – BF4 – 2PMe2Ph]+. C84H90B2F8Ir2P6
(1843.44): calcd. C 54.73, H 4.92; found C 54.85, H 4.98.
[Rh2(1,10-phenanthroline)2(dppcb)](SbF6)2 (13): Procedure as per
that of 11 with a reaction time of 40 min. The dark red powder
was recrystallized from CH2Cl2.Yield: 81%. M.p. 260 °C (dec.). 1H
NMR (300 MHz, CD2Cl2, 25 °C): δ = 8.3–9.5 (m, 16 H, phenan-
throline), 6.4–8.3 (m, 40 H, Ph), 4.45 [br. s, 4 H, CH (cyclobutane)]
ppm. 31P{1H} NMR (121.495 MHz, CH2Cl2, 25 °C): δ = 95.9 (d, [Ir2(PMePh2)4(dppcb)](BF4)2 (16): Procedure as per that of 2. Yield:
1JRh,P = 180 Hz) ppm. MS (FAB+): m/z (m/zcalcd.) = 1594.0
68%. M.p. 145–147 °C. H NMR (300 MHz, CD2Cl2, 25 °C): δ =
6.3–8.2 (m, 80 H, Ph), 4.29 [br. s, 4 H, CH (cyclobutane)], 1.05 (br.
s, 12 H, CH3) ppm. 31P{1H} NMR (121.495 MHz, CH2Cl2, 25 °C):
1
(1593.8) [M – H – SbF6]+, 678.9 (679.0) [M – H – 2SbF6]2+
.
C76H60F12N4P4Rh2Sb2 (1830.55): calcd. C 49.87, H 3.30, N 3.06;
found C 49.95, H 3.41, N 3.01.
2
2
δ = 63.0 (d, JP,P
= 255 Hz, dppcb), –3.1 (d, JP,P
= 255 Hz,
trans
trans
PMePh2) ppm. MS (FAB+): m/z (m/zcalcd.) = 2065.6 (2065.9) [M +
–
–
[Ir2(η4-cod)2(dppcb)]X2 (14), X– = BF4 , SbF6 : [IrCl(η4-cod)]2
(100 mg, 0.149 mmol) was dissolved in toluene (7 mL) and dppcb
(118 mg, 0.149 mmol) was added with vigorous stirring. The reac-
tion mixture was stirred for 6 h, and the formed yellowish precipi-
tate was filtered off. The precipitate was washed with toluene
(10 mL) and dried in vacuo. The obtained yellowish powder was
suspended in CH2Cl2/DMF (5:2, 20 mL). To this suspension was
slowly added AgBF4 (58 mg, 0.298 mmol) dissolved in CH2Cl2/
H
– BF4]+, 1865.4 (1865.7) [M + H – BF4 PMePh2]+.
–
C104H96B2F8Ir2P8 (2151.74): calcd. C 58.05, H 4.50; found C 58.19,
H 4.61.
X-ray Crystallography: Details of the crystals and data collections
are summarized in Tables 4 and 5. In the cases of 4 and 8, the data
collections were performed with a Nonius Kappa CCD dif-
fractometer with the use of combined φ–ω scans. Cell refinement,
DMF (5:2, 5 mL). Immediately an intense red solution formed. data reduction and the empirical absorption correction were done
This solution was stirred for several minutes, the solvent was com-
pletely removed and the residue dried in vacuo. The residue was
then suspended in CH2Cl2 (20 mL) and the suspension was stirred
for 45 min. The formed AgCl was filtered off, the solvent of the
by Denzo and Scalepack programs.[40] In the cases of 1, 3, 6, 9, 10
and 14, all data were collected with a Siemens P4 diffractometer
by using ω scans. Cell refinement and data reduction were done by
the software of the Siemens P4 diffractometer,[41] and the empirical
Table 4. Crystallographic data for 1, 14, 3 and 4.
Compound
1
14
3
4
Empirical formula
C68H68B2F8P4Rh2·
3.68CH2Cl2
1701.07
C68H68B2F8Ir2P4·
3.68CH2Cl2
1879.69
C104H96F12P8Rh2Sb2·
2CH2Cl2
2436.73
C52H52F6P4RhSb
Formula mass
Crystal system
Space group
1139.49
tetragonal
I41/a
triclinic
triclinic
triclinic
¯
¯
¯
P1
P1
P1
a [Å]
b [Å]
c [Å]
α [°]
13.068(2)
13.442(2)
13.686(2)
108.23(1)
96.30(1)
118.95(1)
1896.9(6)
1
13.054(4)
13.422(3)
13.713(4)
108.13(1)
96.66(1)
118.80(1)
1896.2(10)
1
12.590(4)
14.504(3)
16.947(2)
69.94(1)
71.86(1)
68.72(1)
2646.2(11)
1
16.0493(4)
38.0415(6)
β [°]
γ [°]
V [Å]3
9798.7(4)
8
Z
T [K]
298(2)
9056
7978
298(2)
8364
7357
298(2)
10399
7566
298(2)
5621
5378
Measured reflections
Independent reflections
Final R1, wR2 [IϾ3σ(I) 1, 0.0392
14, 3; IϾ2σ(I) 4] 0.1037
0.0367
0.0931
0.0436
0.1013
0.0344
0.0747
Eur. J. Inorg. Chem. 2007, 3227–3239
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
3237

P. Br üggeller et al.
FULL PAPER
Table 5. Crystallographic data for 6, 8, 9 and 10.
Compound
6
8
9
10
Empirical formula
C124H104F12O12P8Rh2Sb2· C100H86F12N2P8Rh2Sb2
C104H92F12P8Rh2Sb2·
3.85CH2Cl2
2593.845
monoclinic
P21
C104H88F12P8Rh2Sb2·
5.16CH2Cl2
2691.41
monoclinic
P21/n
4.26CH2Cl2
Formula mass
Crystal system
Space group
3063.67
triclinic
2240.81
triclinic
P1
¯
P1
¯
a [Å]
b [Å]
c [Å]
α [°]
13.496(3)
16.922(2)
18.541(2)
64.06(1)
85.44(1)
69.93(1)
3564.0(11)
1
13.1051(3)
14.1135(3)
14.9161(3)
73.281(1)
68.345(1)
66.887(1)
2325.18(9)
1
13.9880(10)
30.376(4)
14.342(3)
17.148(3)
31.557(5)
22.126(4)
β [°]
107.80(1)
92.16(1)
γ [°]
V [Å]3
5802.2(15)
2
11965(4)
4
Z
T [K]
298(2)
16436
14420
218(2)
18256
9347
0.0316
298(2)
14111
12251
0.0531
0.1269
298(2)
21099
15536
0.0617
0.1514
Measured reflections
Independent reflections
Final R1, wR2 [IϾ3σ(I) 6, 0.0437
9, 10; IϾ2σ(I) 8] 0.1192
0.0736
[7]
[8]
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Kopacka, K.-H. Ongania, P. Brüggeller, Inorg. Chem. 2001, 40,
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absorption corrections were based on ψ scans of nine reflections,
respectively (χ = 78 to 102°, 360° scans in 10° steps in ψ).[42] All
structure determination calculations were carried out with
SHELXTL NT 5.10 including SHELXS-97[43] and SHELXL-97.[44]
Final refinements on F2 were carried out with anisotropic thermal
parameters for all non-hydrogen atoms in all cases. For 8, the pro-
tons attached to the nitrogen atom of dppam and the cyclobutane
ring were located and isotropically refined with fixed U. All other
hydrogen atoms were included using a riding model with isotropic
U values depending on the Ueq of the adjacent carbon atoms. The
CH2Cl2 solvent molecules in 1, 3, 6, 9, 10 and 14 are disordered.
Tables 1, 2 and 3 contain selected bond lengths and bond angles
of all eight structures.
[9]
[10]
[11]
[12]
CCDC-628190 to -628197 contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/datarequest/cif.
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Acknowledgments
R. G., G. C. and A. D., and S. E., and M. F. thank the University
of Innsbruck for post doctoral, PhD and diploma grants, respec-
tively. This research was also financially supported by the Fonds
zur Förderung der wissenschaftlichen Forschung, Vienna, Austria
and the Tiroler Wissenschaftsfonds, Innsbruck, Austria. P. B.
thanks Claudio Bianchini and Werner Oberhauser for helpful dis-
cussions.
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Received: December 19, 2006
Published Online: May 23, 2007
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