Extending the coordination capabilities of tertiary phosphines and arsines: Preparation, molecular structure, and reactivity of dinuclear rhodium complexes with PR3 and AsR3 in a doubly bridging coordination mode

Article abstract of DOI:10.1039/b314734g

The reactions of [Rh₂(κ²-acac) ₂(μ-CPh₂)₂(μ-PR₃)] (PR ₃ = PMe₃ 4, PMe₂Ph 7, PEt₃ 8) with an equimolar amount of Me₃SiX (X = Cl, Br, I)

Full text of DOI:10.1039/b314734g

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Extending the coordination capabilities of tertiary phosphines and
arsines: preparation, molecular structure, and reactivity of
dinuclear rhodium complexes with PR3 and AsR3 in a doubly
bridging coordination mode
Thomas Pechmann, Carsten D. Brandt and Helmut Werner*
Institut fur Anorganische Chemie der Universität Würzburg, Am Hubland, D-97074 Würzburg,
Germany. E-mail: helmut.werner@mail.uni-wuerzburg.de
Received 14th November 2003, Accepted 26th January 2004
First published as an Advance Article on the web 12th February 2004
The reactions of [Rh2(κ2-acac)2(µ-CPh2)2(µ-PR3)] (PR3 = PMe3 4, PMe2Ph 7, PEt3 8) with an equimolar amount
of Me3SiX (X = Cl, Br, I) afforded the unsymmetrical complexes [Rh2X(κ2-acac)(µ-CPh2)2(µ-PR3)] 5, 9–12, which
contain the phosphine in a semi-bridging coordination mode. From 4 and excess Me3SiCl, the tetranuclear complex
[{Rh2Cl(µ-Cl)(µ-CPh2)2(µ-PMe3)}2] 6 was obtained. In contrast, the reaction of 4 with an excess of Me3SiX (X = Br,
I) yielded the dinuclear complexes [Rh2X2(µ-CPh2)2(µ-PMe3)] 13, 14 in which, as shown by the X-ray crystal structure
analysis of 14, the bridging phosphine is coordinated in a truly symmetrical bonding mode. While related compounds
with PEt3 and PMe2Ph as bridging ligands were prepared on a similar route, the complex [Rh2Cl2(µ-CPh2)2(µ-PiPr3)]
19 was obtained from the mixed-valence species [(PiPr3)Rh(µ-CPh2)2Rh(κ2-acac)2] 17 and HCl. The reaction of
[Rh2(κ2-acac)2(µ-CPh2)2(µ-SbiPr3)] 3 with AsMe3 gave the related Rh(µ-AsMe3)Rh compound 21. With Me3SiCl, the
acac ligands of 21 can be replaced stepwise by chloride to give [Rh2Cl(κ2-acac)(µ-CPh2)2(µ-AsMe3)] 23 and
[{Rh2Cl(µ-Cl)(µ-CPh2)2(µ-AsMe3)}2] 24, the latter being isomorphous to the phosphine-bridged dimer 6.
Tertiary phosphines PR3 with R = alkyl or aryl belong like CO
to the most well-known ligands in coordination chemistry.
While with regard to CO not only numerous metal complexes
with terminal but also with doubly bridging carbonyl ligands
have been described, the general knowledge is that tertiary
phosphines (and tertiary arsines AsR3 and stibines SbR3 as
well) behave exclusively as terminal coordinated ligands.1,2
In attempting to obtain a dinuclear palladium compound
with Pd(µ-PF3)Pd as the core unit, Balch et al. reported in 1990
the preparation and structural characterization of a cationic
Pd3 complex consisting of a nearly equilateral triangle of
palladium atoms bridged at the edges by diphenylphosphino-
methane ligands and capped by the triply bridging phosphorus
atom of PF3.3 More recently, we found in the context of our
studies on the reactivity of carbenerhodium() compounds,4
core. However, while according to the NMR data (the 31P NMR
spectrum of 4 shows a sharp triplet in the temperature range of
Ϫ60 ЊC to ϩ25 ЊC) there was no doubt, that the PMe3 ligand
should occupy a symmetrical doubly bridging position, the
X-ray crystal structure analysis of 4 revealed that in the solid
the distances between the phosphorus and the two rhodium
centers differ by 0.30 Å.6
In order to find out how strongly the anionic ligands influence
the binding of the bridging ligands, we attempted to reconvert
the Rh(κ2-acac) to RhX units (X = Cl, Br, I) and thus to reduce
both the coordination number and the steric crowding at the
metal centers. The present paper reports the synthesis and
structural characterization of dichloro-, dibromo- and diiodo-
rhodium complexes not only with PMe3 but also with PMe2Ph,
PEt3, PnBu3 and PiPr3 as bridging ligands, the conversion of
some Rh2(κ2-acac)2 to corresponding Rh2X(κ2-acac) derivatives
containing semi-bridging phosphines, and the preparation
of the first representatives with trimethylarsine in a doubly
bridging coordination mode. Some preliminary results of this
work have already been communicated.7
that the bis(stibine) derivatives trans-[RhCl(᎐CRRЈ)(SbiPr ) ]
᎐
3
2
generate upon heating dinuclear rhodium() complexes such as
[Rh2Cl2(µ-CPh2)2(µ-SbiPr3)] 1 with [Rh(µ-SbiPr3)Rh] as a build-
ing block.5 After initial attempts to substitute the triisopropyl-
stibine in 1 by a tertiary phosphine failed,5 we circumvented the
difficulties by the sequence of reactions shown in Scheme 1. The
crucial observation was that replacing the chloro ligands in 1 by
acetylacetonate (acac) changes the reactivity of the starting
material significantly and provides the opportunity to sub-
stitute SbiPr3 for PMe3 without fragmentation of the dinuclear
Results and discussion
A dimer with two [Rh(ꢀ-CPh2)2(ꢀ-PMe3)Rh] subunits
Taking the preference of Si–O bond formation into con-
sideration, we treated the bis(κ2-acac) compound 4 with an
excess of Me3SiCl in benzene at room temperature with the
aim to replace the acac by the chloro ligands. A slow reaction,
accompanied by a gradual change of color from brown to
red, takes place which gives a product 6, correctly analyzing
as [Rh2Cl2(CPh2)2(PMe3)], in 91% yield. The red solid is only
slightly air-sensitive, insoluble in pentane and diethyl ether,
and moderately soluble in benzene and dichloromethane. If the
reaction of 4 with Me3SiCl is carried out in the molar ratio of
1 : 1, the unsymmetrical complex 5 (Scheme 2) is obtained,
which has originally been prepared from 2 and PMe3 by bridge-
ligand exchange.6 According to the X-ray crystal structure
analysis, the trimethylphosphine occupies in 5 a semi-bridging
position, the phosphorus atom being 0.6 Å nearer to the rho-
dium center with the acac than to that with the chloro ligand.8
Scheme 1
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4
D a l t o n T r a n s . , 2 0 0 4 , 9 5 9 – 9 6 6
959

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nuclear compound is present (calc.: M 1370.5; found: M 1310),
the 31P NMR spectrum of 6 in C6D6 at room temperature is
concentration-dependent. The spectrum of a nearly saturated
solution (4 mmol lϪ1) exhibits a somewhat broadened triplet at
δ Ϫ24.6 (assigned to tetranuclear 6) which after lowering the
concentration to 0.1 mmol lϪ1 transforms into a sharp triplet
with a chemical shift of δ Ϫ20.4 (assigned to dinuclear 6Ј).
Since the data in CD2Cl2 are quite similar (a broadened triplet
being observed at δ Ϫ23.1 for a concentrated and a sharp triplet
at δ Ϫ15.7 for a diluted solution), we conclude that both in
benzene and dichloromethane a rapid equilibrium between the
Rh4 and the Rh2 species exists (see Scheme 2). At low concen-
trations the Rh2 species 6Ј dominates. At 193 K in toluene-d8,
the 31P NMR spectrum displays a doublet of doublets at
δ Ϫ30.4 (assigned to 6), which indicates that under these con-
ditions the conversion of 6 to 6Ј is slowed down on the NMR
time scale.
Scheme 2
While we anticipated, owing to the 1H and 13C NMR spectra,
that compound 6 possesses a structure analogously to the
stibine-bridged complex 1, the X-ray crystal structure analysis
revealed that in the lattice two dinuclear moieties are connected
via two bridging chlorides to give a Rh4 species with a chain-like
ClRh2Cl2Rh2Cl core (Fig. 1). Moreover, the midpoint of the
planar Rh(µ-Cl)2Rh fragment is a center of symmetry. While
the coordination geometry around Rh(1) is distorted tetra-
hedral, that around Rh(2) is best described as square-pyramidal
with the phosphorus atom P(1) in the apical position. The
structure of the fragment [ClRh(µ-CPh2)(µ-PMe3)RhCl2] is
thus similar to that of the unsymmetrical molecule 5.6 Besides
the Rh(1)–Rh(2) distance of 2.5054(2) Å (2.4993(2) Å for the
second independent molecule found in the asymmetric unit),
which differs only slightly to that of the stibine-bridged com-
pound 1 (2.5349(5) Å), the most important structural features
of 6 are the Rh–P bond lengths. They are 2.3625(6) and
2.4826(6) Å in the molecule shown in Fig. 1 and 2.3890(6) and
2.4173(6) Å in the second molecule (Table 1). Most noteworthy,
the difference between the two Rh–P distances is in each case
much less than for the bis(acac) complex 4, indicating that – at
least in the crystal lattice – the type of the anionic ligands
bonded to rhodium influences significantly the position of the
bridging phosphine unit. Due to the similarity of the Rh–Rh
and Rh–P distances, the bond angles of the Rh2P triangle are
nearly the same and deviate only marginally from the 60Њ value.
As expected, the bond length Rh(1)–Cl(1) is considerably
shorter than the distances Rh(2)–Cl(2) and Rh(2)–Cl(2A),
the latter differing in both independent molecules by less than
0.03 Å.
Tertiary phosphines in semi-bridging and truly symmetrical
doubly bridging positions
The reactivity of 4 toward Me3SiBr and Me3SiI is similarly to
that of the same starting material toward Me3SiCl. With equi-
molar amounts of the substrates, the unsymmetrical complexes
9 and 10 are formed (Scheme 3) and isolated as dark red (9)
or dark brown (10) solids in nearly quantitative yields. An
alternative procedure consists in the reaction of 5 with NaBr
or NaI in acetone which equally affords 9 and 10 in good to
excellent yields. Both 9 and 10 are thermally less stable than 5
but can be stored at room temperature under argon for weeks.
The same is true for the Rh2Cl(κ2-acac) compounds 11 and 12,
which are obtained from 7 or 8 and Me3SiCl in the molar ratio
of 1 : 1. As was already reported,6 the PEt3 derivative 12 is also
accessible from 2 and PEt3. The 31P NMR spectra of 9 and 10
display, similarly to the spectrum of 5, one doublet of doublets
with 103Rh–31P coupling constants that differ for 9 by 93.3 and
for 10 by 111.7 Hz (see, for comparison, 5: ∆J = 84.3 Hz). Since
for transition-metal complexes containing terminal phosphine
ligands, the size of the coupling constant 1J(Rh,P) can be corre-
lated to a first approximation to the Rh–P bond lengths,9 it
is conceivable that the difference in the distances between the
phosphorus atom of the semi-bridging trimethylphosphine
and the two rhodium centers is larger in 9 and particularly in
10 than in 5. It should be mentioned that for compound
[(κ2-acac)Rh(µ-CPh2)2(µ-CO)Rh(PMe3)(κ2-acac)], formed from
4 and CO, in which the PMe3 ligand occupies a terminal
position, the 103Rh–31P coupling constants are, respectively,
129.1 and 7.6 Hz.10
Although cryoscopic measurements with a saturated solution
of 6 in benzene confirm that under these conditions the tetra-
Fig. 1 An ORTEP plot of one of the centrosymmetric dimers of
rhodium complex 620 (symmetry operation used to generate equivalent
atoms: Ϫx, Ϫy ϩ 2, Ϫz).
Scheme 3
D a l t o n T r a n s . , 2 0 0 4 , 9 5 9 – 9 6 6
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Table 1 Selected bond lengths (Å) and angles (Њ) for compound 6 (there are two independent molecules 6a and 6b in the asymmetric unit; values
for 6b are in square brackets)
Rh(1)–Rh(2)
Rh(1)–P(1)
Rh(2)–P(1)
Rh(1)–C(1)
Rh(1)–C(2)
Rh(2)–C(1)
Rh(2)–C(2)
2.5054(2) [2.4993(2)]
2.4826(6) [2.4173(6)]
2.3625(6) [2.3890(6)]
1.988(2) [1.992(2)]
1.968(2) [1.978(2)]
2.044(2) [2.038(2)]
2.051(2) [2.046(2)]
Rh(1)–Cl(1)
Rh(2)–Cl(2)
Rh(2)–Cl(2A)
P(1)–C(3)
P(1)–C(4)
P(1)–C(5)
2.3088(6) [2.2979(6)]
2.4961(5) [2.4911(5)]
2.4636(5) [2.4685(6)]
1.815(3) [1.811(3)]
1.816(3) [1.814(3)]
1.839(3) [1.841(3)]
Rh(1)–P(1)–Rh(2)
62.217(15) [62.662(15)]
61.245(16) [59.225(15)]
56.538(14) [58.114(15)]
76.83(7) [76.65(7)]
C(1)–Rh(2)–Cl(2)
170.77(6) [173.90(6)]
159.02(6) [152.20(6)]
167.894(19) [172.29(2)]
77.733(19) [77.89(2)]
102.267(19) [102.11(2)]
P(1)–Rh(2)–Rh(1)
P(1)–Rh(1)–Rh(2)
Rh(1)–C(1)–Rh(2)
Rh(1)–C(2)–Rh(2)
C(2)–Rh(2)–Cl(2A)
Cl(1)–Rh(1)–Rh(2)
Cl(2A)–Rh(2)–Cl(2)
Rh(2A)–Cl(2)–Rh(2)
77.08(7) [76.77(7)]
The reactions of 4 with a twofold excess of Me3SiBr or
Me3SiI proceed quite slowly and afford in toluene after 24 h
(X = Br) or 2 h (X = I) at room temperature the dibromo and
diiodo complexes 13 and 14 in 90–92% isolated yield. In con-
trast to 6, the 31P NMR spectra of 13 and 14 are independent
of the concentration of the solution and display in each case
a sharp triplet at δ Ϫ18.3 (13) and δ Ϫ20.8 (14), respectively.
However, while the 31P NMR spectrum of 14 remains
unchanged by lowering the temperature to 193 K, the corre-
sponding spectrum of 13 changes and at 193 K shows together
with the triplet at δ Ϫ17.5 a doublet of doublets at δ Ϫ31.8.
If we take the data reported for 6 and 6Ј into account, the
appearance of these two signals indicate that in the case of
13 at low temperature apart from the dinuclear compound 13
alsothetetranuclearspecies[BrRh(µ-CPh2)2(µ-PMe3)Rh(µ-Br)2-
Rh(µ-CPh2)2(µ-PMe3)RhBr] is present. At 193 K, the equi-
librium between the monomer and the dimer is obviously very
slow on the NMR time scale and thus both the Rh2 and the Rh4
complexes can be observed.
The reactions of 7 and 8 with an excess of Me3SiCl also
lead to a displacement of the acac ligands by chloride. In both
cases, deeply colored solids 15 and 16 are obtained of which 15
is much better soluble in benzene than 16. The 31P NMR
spectrum of 15 displays (in C6D6) a sharp triplet at δ 6.0 which
does not change either at higher concentrations of the solution
or at lowering the temperature to 193 K. Therefore, we assume
that under the experimental conditions only the dinuclear
complex, probably having a similar structure as 14, exists. The
31P NMR spectrum of 16 shows (in CD2Cl2) at room temper-
ature a broadened triplet at δ Ϫ17.0 (with 1J(Rh,P) = 101.7 Hz)
which at 193 K becomes a doublet of doublets with 103Rh–31P
coupling constants of, respectively, 139.8 and 68.6 Hz. These
data indicate that in the case of the PMe2Ph-bridged complex
the tetranuclear compound 16 dominates at low temperatures
while at room temperature it is in equilibrium with the corre-
sponding Rh2 species. It should be mentioned that attempts
to prepare 15 and 16 from 1 and PEt3 or PMe2Ph by ligand
exchange failed.
The expectation that, owing to the 31P NMR data, the PMe3
ligand in 14 occupies, even in the solid, a truly symmetrical
bridging position was confirmed by an X-ray crystal structure
analysis. As shown in Fig. 2, the molecule contains a C2 axis
passing through the phosphorus atom and the midpoint of the
Rh–Rh distance. Therefore, the structure of 14 is quite similar
to that of the stibine-bridged compound 1.5 The I–Rh–Rh–I
axis in 14 is nearly linear with bond angles Rh(1)–Rh(1A)–
I(1A) = Rh(1A)–Rh(1)–I(1) = 174.37(2)Њ (Table 2). The Rh–P
bond lengths are 2.412(3) Å and thus lie between those of the
bis(acac) derivative 4 (2.2707(7) and 2.5700(8) Å).6 We note that
for the chain-like Rh4Cl4 complex 6 the Rh–P distances in each
of the dinuclear subunits differ by ca. 0.07 Å.
After we confirmed that PMe3, PMe2Ph and PEt3 can behave
as doubly bridging ligands, we finally also succeeded with trans-
ferring the more bulky PiPr3 from a terminal into a bridging
position. In our previous studies on the reactivity of 1 (the
precursor of compound 3) toward Lewis bases, we discovered
that this complex reacts with PiPr3 to give trans-[RhCl-
(᎐CPh )(PiPr ) ].5 In contrast, treatment of 3 with PiPr does
᎐
2
3
2
3
not lead to cleavage of the Rh(µ-CPh2)Rh bridges and affords
by migration of one acac ligand from one metal center to the
next the interesting mixed-valence compound 17 in excellent
yield.11 Since it is conceivable that this reaction proceeds via the
phosphine-bridged species [Rh2(κ2-acac)2(µ-CPh2)2(µ-PiPr3)],
we attempted to trap this intermediate by replacing the acac
ligands for chloride. Thus, the mixed-valence complex 17
(Scheme 4) was treated with an excess of Me3SiCl but on this
route a mixture of products was formed. A clean reaction
occurred, however, if a solution of HCl in benzene was dropped
under vigorous stirring to a solution of 17 in the same solvent
at room temperature. After removal of the volatile materials,
the required complex 19 was isolated as a red, moderately air-
stable solid in 91% yield. In the same way, the PPh3 counterpart
20 could be generated from 18. It contained, however, even after
recrystallization from acetone some impurities (ca. 10%) and
was thus characterized by spectroscopic techniques. The 13C
NMR spectra of 19 and 20 display a triplet for the CPh2 carbon
atoms at δ 180.7 (19) and δ 191.2 (20) with a 103Rh–13C coupling
Fig.
2
An ORTEP plot of rhodium complex 1420 (symmetry
operation used to generate equivalent atoms: Ϫx ϩ 2/3, Ϫx ϩ y ϩ 1/3,
Ϫz ϩ 5/6; there is a disorder of the methyl groups bonded to
phosphorus)
Scheme 4
D a l t o n T r a n s . , 2 0 0 4 , 9 5 9 – 9 6 6
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Table 2 Selected bond lengths (Å) and angles (Њ) for compound 14
Rh(1)–C(1)
2.004(9)
2.008(9)
2.004(9)
2.5042(14)
Rh(1)–P
Rh(1A)–P
Rh(1)–I(1)
2.413(3)
2.413(3)
2.5981(10)
Rh(1)–C(1A)
Rh(1A)–C(1)
Rh(1)–Rh(1A)
Rh(1)–C(1)–Rh(1A)
C(1)–Rh(1)–C(1A)
I(1)–Rh(1)–Rh(1A)
Rh(1)–P–Rh(1A)
C(1)–Rh(1)–P
77.2(3)
87.1(4)
174.37(2)
62.53(10)
89.5(3)
I(1)–Rh(1)–P
126.88(6)
126.1(2)
125.5(3)
51.3(2)
51.5(3)
58.74(5)
I(1)–Rh(1)–C(1)
I(1)–Rh(1)–C(1A)
C(1)–Rh(1)–Rh(1A)
C(1A)–Rh(1)–Rh(1A)
P–Rh(1)–Rh(1A)
C(1A)–Rh(1)–P
89.6(3)
constant of 28.6 Hz. Since the 31P NMR spectra of 19 and 20
also show a sharp triplet at, respectively, δ 14.9 (19) and δ 9.8
(20), there is no doubt that the more bulky phosphines PiPr3
and PPh3 can equally be linked to two rhodium centers in a
doubly bridging position.
phosphine-bridged complex 22 consists in treatment of a solu-
tion of 21 in toluene at Ϫ50 ЊC with one equivalent of PnBu3,
followed by warming to 0 ЊC and stirring the reaction mixture
at this temperature under reduced pressure for 20 min. Under
these conditions, the volatile trimethylarsine (bp 52 ЊC) is
removed and the product 22 isolated as a brown solid in 89%
yield. Due to the three C4 chains at the phosphorus atom,
22 is much better soluble in organic solvents than the PMe3
analogue. The 31P NMR spectrum of 22 (in C6D6) displays a
sharp triplet at δ Ϫ9.6 with a 1J(Rh,P) coupling constant that is
identical to that of 8. We note that both 7 and 8 are obtained
from 21 in much better yields than from 3 as the precursor.
Similarly to the PMe3 analogue 4, compound 21 reacts in
benzene with Me3SiCl in the molar ratio of 1 : 1.1 to give
the dinuclear complex 23 (Scheme 6), probably containing the
arsine in a semi-bridging coordination mode. As already
observed for the PMe3-counterparts 4 and 5, the unsymmetrical
species 23 (isolated as a brown solid in 88% yield) is signifi-
cantly more stable than 21 and does not decompose in benzene
even after storing for three days.
Trimethylarsine as a semi-bridging and a doubly bridging ligand
While the stibine-bridged compound 1 reacts smoothly with
PiPr3 and PPh3, it is inert toward the corresponding arsines
AsiPr3 and AsPh3 and does not form by bridge cleavage the
square-planar compounds trans-[RhCl(᎐CPh )(AsR ) ]. A slow
᎐
2
3 2
reaction takes place if 1 is treated with AsMe3 but in this case
a mixture of products results which could not be separated by
fractional crystallization or column chromatography.
The successful route to bind AsMe3 in a doubly bridging
position was similar to that found for PMe3.6 Treatment of the
bis(κ2-acac) derivative 3 with trimethylarsine in benzene leads
to an equilibrium between 3 and the AsMe3 counterpart, which
even in the presence of a large excess of the arsine could not be
completely shifted to the side of the required product. However,
in hexane as solvent (in which 21 is only sparingly soluble) the
arsine-bridged complex 21 precipitates and is obtained in 85%
isolated yield (Scheme 5). The light brown solid (the com-
position of which has been confirmed by elemental analysis and
a FAB mass spectrum) is thermally more stable than the PMe3
analogue 4 and can be stored under argon at low temperatures
for weeks. In benzene or acetone solution it decomposes
quite readily. The 1H and 13C NMR spectra of 21 indicate that
the two acac as well as the two CPh2 ligands are equivalent and
thus we assume that the trimethylarsine occupies a symmetrical
doubly bridging position.
Scheme 6
Replacing the remaining acac ligand of 23 by chloride is
more difficult and, even with a large excess of Me3SiCl, the
formation of 24 occurs only slowly at room temperature. After
removal of the volatiles, the dichloro derivative has been iso-
lated as a red–brown solid in 91% yield. As shown by the
X-ray crystal structure analysis, the AsMe3-bridged compound
24 is isomorphous to 6 and also possesses the midpoint of
the Rh(µ-Cl)2Rh unit as a center of symmetry (Fig. 3).
Analogously to 6, the half dimer structure is found twice in the
asymmetric unit. The Rh–Rh distance in each dinuclear subunit
is 2.5225(3) Å (2.5271(3) Å in the second independent
molecule) which is slightly larger than in 6. The two Rh–As
bond lengths in each subunit differ somewhat (0.05 Å in one
molecule and 0.10 Å in the other) reflecting the inequivalence
of the “outer” and “inner” metal centers of the ClRh2Cl2Rh2Cl
chain (Table 3). Not only the arsine but also the two diphenyl-
carbene ligands are linked to Rh(1) and Rh(2) in a slightly
unsymmetrical fashion. Therefore, no mirror plane exists along
the Cl(1)–Rh(1)–Rh(2) axis. As expected, the terminal distance
Rh(1)–Cl(1) is significantly shorter than the distances Rh(2)–
Cl(2) and Rh(2)–Cl(2A) in the centre of the molecule. Regard-
ing the unit cell of 24, it should be mentioned that there are no
Scheme 5
In contrast to 4, the AsMe3 counterpart 21 is rather labile
and reacts not only with SbiPr3 but also with tertiary phos-
phines such as PMe2Ph, PEt3 and PnBu3 by bridge-ligand
exchange. While compounds 7 and 8 could be prepared, simi-
larly to 4 (see Scheme 1), from 3 and PMe2Ph or PEt3, respec-
tively, attempts to obtain 22 in the same way (benzene, room
temperature) resulted in the formation of the mixed-valence
compound [(PnBu3)Rh(µ-CPh2)2Rh(κ2-acac)2].12 Although we
postulated that this molecule is generated via 22, we failed
to fully characterize this isomer. The procedure to isolate the
D a l t o n T r a n s . , 2 0 0 4 , 9 5 9 – 9 6 6
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Table 3 Selected bond lengths (Å) and angles (Њ) for compound 24 (there are two independent molecules 24a and 24b in the asymmetric unit; values
for 24b are in square brackets)
Rh(1)–Rh(2)
Rh(1)–As(1)
Rh(2)–As(1)
Rh(1)–C(1)
Rh(1)–C(14)
2.5225(3) [2.5271(3)]
2.5372(3) [2.5777(3)]
2.4899(3) [2.4757(3)]
1.989(2) [1.973(2)]
1.983(2) [1.994(2)]
Rh(1)–Cl(1)
Rh(2)–C(1)
Rh(2)–C(14)
Rh(2)–Cl(2)
Rh(2)–Cl(2A)
2.3006(7) [2.3108(6)]
2.048(2) [2.055(2)]
2.046(2) [2.047(2)]
2.4662(7) [2.4917(6)]
2.4895(7) [2.4669(6)]
Rh(1)–As(1)–Rh(2)
60.225(9) [59.970(9)]
58.958(8) [58.011(8)]
60.817(9) [62.018(8)]
77.32(8) [77.68(8)]
77.51(8) [77.39(8)]
91.66(9) [91.73(9)]
88.19(9) [87.92(9)]
Rh(1)–Rh(2)–Cl(2)
C(1)–Rh(2)–Cl(2)
C(14)–Rh(2)–Cl(2A)
Cl(1)–Rh(1)–Rh(2)
Cl(2A)–Rh(2)–Cl(2)
Rh(2A)–Cl(2)–Rh(2)
144.647(17) [138.145(16)]
96.89(7) [96.17(6)]
94.43(6) [95.11(6)]
173.55(2) [170.13(2)]
78.52(2) [78.43(2)]
101.48(2) [101.57(2)]
As(1)–Rh(1)–Rh(2)
As(1)–Rh(2)–Rh(1)
Rh(1)–C(1)–Rh(2)
Rh(1)–C(14)–Rh(2)
C(1)–Rh(1)–C(14)
C(1)–Rh(2)–C(14)
complexes such as 1–16 and 19–24 might only be the first step
into a new field. The recent discovery by Reau and co-workers16
that the phosphorus atom of substituted phospholes is able
to bridge two palladium centers in a symmetrical fashion
undoubtedly supports this prediction.
Experimental
All reactions were carried out under an atmosphere of argon by
Schlenk techniques. The starting materials 3,11 4,6 7,17 8,17 17,11
and 1811 were prepared as described in the literature. Melting
points were measured by differential thermal analysis (DTA).
NMR spectra were recorded (if not otherwise stated) at room
temperature on Bruker AC 200 and Bruker AMX 400 instru-
ments, IR spectra on a IFS 25 FT-IR infrared spectrometer,
and mass spectra on a Finnigan MAT 90 (70 eV) or on a
Hewlett-Packard G 1800 GCD instrument. Coupling constants
are given in hertz. Abbreviations used: s, singlet; d, doublet;
t, triplet; m, multiplet; br, broadened signal.
Fig. 3 An ORTEP plot of one of the centrosymmetric dimers of
rhodium complex 2420 (symmetry operation used to generate
equivalent atoms: Ϫx ϩ 1, Ϫy, Ϫz ϩ 1).
Preparations
[Rh2Cl(ꢁ2-acac)(ꢀ-CPh2)2(ꢀ-PMe3)] 5. A solution of 4 (54 mg,
0.07 mmol) in benzene (10 cm3) was treated with Me3SiCl
(0.008 cm3, 0.07 mmol) and stirred for 1 h at room temperature.
The solvent was evaporated in vacuo, the remaining red–brown
solid was washed twice with 3 cm3 portions of pentane–diethyl
ether (1 : 1) and dried; yield 47 mg (94%). The product was
characterized by comparison of the 1H and 31P NMR data with
those of an authentic sample.6
contacts either between the terminal chloride Cl(1) and the
corresponding Rh–Cl fragment of a neighbouring Rh2 subunit
or between Cl(1) and the benzene molecules incorporated in
the crystal. We also note that the 1H and the 13C NMR spectra
of 24 (in CD2Cl2) remain unchanged in the temperature range
between 193 and 333 K, indicating that under these conditions
probably no dissociation of the tetranuclear Rh4 to the di-
nuclear Rh2 species takes place.
[{Rh2Cl2(ꢀ-CPh2)2(ꢀ-PMe3)}2] 6. A solution of 4 (129 mg,
0.16 mmol) in benzene (20 cm3) was treated with Me3SiCl
(0.23 cm3, 1.81 mmol) and stirred for 24 h at room temperature.
The solvent was evaporated in vacuo, the remaining red solid
was washed twice with 5 cm3 portions of diethyl ether and
dried; yield 99 mg (91%). Alternatively, compound 6 can also be
prepared from 5 (518 mg, 0.69 mmol) and Me3SiCl (1.0 cm3,
7.88 mmol) in benzene (70 cm3); yield 436 mg (92%); mp 126 ЊC
(decomp.) (Found: C, 49.24; H, 6.18; mol. weight (benzene)
1310. C58H58Cl4P2Rh4 requires: C, 49.53; H, 6.24%; mol. weight
1370.5). NMR (C6D6): δH (400 MHz, 4 mmol lϪ1) 7.87, 7.66
(8 H, both m, ortho-H of C6H5), 6.83 (4 H, m, meta-H of C6H5),
6.74 (2 H, m, para-H of C6H5), 6.65 (6 H, m, meta-H and para-
H of C6H5), 0.88 [18 H, d, J(P,H) 10.6, PCH3]; δH (200 MHz,
0.1 mmol lϪ1) 7.62 (8 H, m, ortho-H of C6H5), 6.70 (12 H, m,
meta- and para-H of C6H5), 0.69 [18 H, d, J(P,H) 10.6, PCH3];
δC (100.6 MHz, 4 mmol lϪ1) 187.8 (m, CPh2), 153.5 [d, J(P,C)
3.8, ipso-C of C6H5], 152.3 (s, ipso-C of C6H5), 128.5, 127.6,
127.5, 127.0, 126.9, 124.8 (all s, C6H5), 23.5 [d, J(P,C) 40.0 Hz,
PCH3]; δP (81.0 MHz, 4 mmol lϪ1) Ϫ24.6 [br t, J(Rh,P) 109.3];
δP (81.0 MHz, 0.1 mmol lϪ1) Ϫ20.4 [t, J(Rh,P) 109.3]; NMR
(CD2Cl2): δP (81.0 MHz, 8 mmol lϪ1) Ϫ23.1 [br t, J(Rh,P)
109.4]; δP (81.0 MHz, 0.1 mmol lϪ1) Ϫ15.7 [t, J(Rh,P) 109.3];
NMR (toluene-d8): δP (81.0 MHz, 2 mmol lϪ1, 293 K): Ϫ21.3
Conclusion
The present investigation closes a gap in the field of co-
ordination chemistry. After it had been supposed for decades
that tertiary phosphines, arsines and stibines behave exclusively
as terminal ligands, it was only recently that this postulate
became weakened. The preparation of the Balch compound
with the triply bridging PF3 3 and the discovery of compound 1
(according to Caulton “the first outsider”)13 as well as its
SbMe3, SbEt3 and Sb(CH2Ph)3 analogues5,14 was followed by
the isolation and structural characterization of 4 and 5 and has
now culminated in the synthesis of the first arsine-bridged
species 21, 23 and 24. Apart from the preparation of these
new Rh(µ-AsMe3)Rh complexes, the most noteworthy feature
of this work is that with respect to tertiary phosphines not only
PMe3 but also the more bulky analogues PMe2Ph, PEt3, PnBu3,
PiPr3 and PPh3 can be forced to bind to two metal centers in
a doubly bridging position. That in general a bonding mode
such as M(µ-ER3)M with E = P, As, Sb should not be regarded
in principle as thermodynamically unfavorable, was recently
pointed out by Braunstein and Boag who took the isolobal
Ϫ
analogy between PR3 and SiR3 as well as the existence of
various silyl-bridged transition-metal compounds into con-
sideration.15 Thus, it is well possible that the preparation of
D a l t o n T r a n s . , 2 0 0 4 , 9 5 9 – 9 6 6
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[t, J(Rh,P) 111.9]; δP (81.0 MHz, 2 mmol lϪ1, 233 K) Ϫ26.0
(br m); δP (81.0 MHz, 2 mmol lϪ1, 193 K) Ϫ30.4 [dd, J(Rh,P)
128.4, J(RhЈ,P) 95.4].
J(P,C) 2.9, para-C of PC6H5], 129.1 [d, J(P,C) 9.5, ortho- or
meta-C of PC6H5], 128.6 [d, J(P,C) 10.5, ortho- or meta-C of
PC6H5], 127.8, 127.1, 126.6, 125.6, 125.5, 124.6 (all s, C6H5),
101.1 (s, CH of acac), 28.1 (s, CH3 of acac), 18.7 [d, J(P,C)
43.9, PCH3]; δP (162.0 MHz) Ϫ29.3 [dd, J(Rh,P) 152.6,
J(RhЈ,P) 69.8].
[Rh2Br(ꢁ2-acac)(ꢀ-CPh2)2(ꢀ-PMe3)] 9. (a) A solution of 4
(132 mg, 0.16 mmol) in toluene (20 cm3) was treated dropwise
at Ϫ50 ЊC with Me3SiBr (0.021 cm3, 0.16 mmol). After the solu-
tion was warmed to room temperature, it was stirred for 30 min.
The solvent was evaporated in vacuo, the remaining residue
was washed twice with 2 cm3 portions of diethyl ether, and then
recrystallized from acetone (5 cm3) at Ϫ60 ЊC. Dark red crystals
precipitated, which were separated from the mother-liquor
and dried; yield 118 mg (92%). (b) A solution of 5 (56 mg,
0.07 mmol) in acetone (25 cm3) was treated with NaBr (15 mg,
0.15 mmol) and stirred for 2 h at room temperature. The solvent
was evaporated in vacuo, and the residue was extracted twice
with 20 cm3 portions of pentane–dichloromethane (1 : 1). The
combined extracts were brought to dryness in vacuo, and the
remaining residue was recrystallized from acetone (3 cm3)
at Ϫ60 ЊC. Dark red crystals; yield 53 mg (89%); mp 144 ЊC
(decomp.) (Found: C, 51.07; H, 4.57. C34H36BrO2PRh2 requires:
C, 51.47; H, 4.57%). IR (KBr): (COacac) 1580, 1522 cmϪ1. NMR
(CD2Cl2): δH (400 MHz) 7.96 (4 H, m, ortho-H of C6H5), 7.20
(6 H, m, meta- and para-H of C6H5), 7.04 (4 H, m, ortho-H of
C6H5), 6.71 (6 H, m, meta-H and para-H of C6H5), 5.80 (1 H, s,
CH of acac), 2.23 (6 H, s, CH3 of acac), 0.92 [9 H, dd, J(P,H)
10.8, J(Rh,H) 0.6, PCH3]; δC (100.6 MHz) 189.3 (s, CO of
acac), 174.1 [ddd, J(Rh,C) 30.5, J(RhЈ,C) 21.0, J(P,C) 4.8,
CPh2], 153.3 [d, J(P,C) 1.9, ipso-C of C6H5], 153.2 (s, ipso-C of
C6H5), 127.6, 127.1, 126.6, 125.7, 125.5, 124.7 (all s, C6H5),
100.8 [d, J(Rh,C) 1.9, CH of acac], 28.2 (s, CH3 of acac), 22.0
[d, J(P,C) 40.0, PCH3]; δP (162.0 MHz) Ϫ27.3 [dd, J(Rh,P)
157.7, J(RhЈ,P) 64.4].
[Rh2Cl(ꢁ2-acac)(ꢀ-CPh2)2(ꢀ-PEt3)] 12. This compound was
prepared as described for 5, from 8 (60 mg, 0.07 mmol) and
Me3SiCl (0.01 cm3, 0.07 mmol) in benzene (10 cm3). A dark
brown solid was obtained; yield 50 mg (90%). The product was
characterized by comparison of the 1H and 31P NMR data with
those of an authentic sample.12
[Rh2Br2(ꢀ-CPh2)2(ꢀ-PMe3)] 13. A solution of 4 (100 mg, 0.12
mmol) in benzene (20 cm3) was treated with Me3SiBr (0.03 cm3,
0.25 mmol) and stirred for 24 h at room temperature. A gradual
change of color from red–brown to red occurred. The volatiles
were evaporated in vacuo, the residue was dissolved in acetone
(4 cm3) and the solution was stored for 12 h at Ϫ25 ЊC. Red
crystals precipitated, which were separated from the mother-
liquor and dried; yield 88 mg (92%); mp 63 ЊC (decomp.)
(Found: C, 44.44; H, 4.03. C29H29Br2PRh2 requires: C, 44.99; H,
3.78%). NMR (CD2Cl2): δH (400 MHz) 7.43 (4 H, m, ortho-H
of C6H5), 7.24, 6.80 (16 H, both m, ortho-, meta- and para-H of
C6H5), 1.08 [9 H, d, J(P,H) 10.6, PCH3]; δC (100.6 MHz) 188.6
(m, CPh2), 152.5 [d, J(P,C) 3.8, ipso-C of C6H5], 150.5 (s, ipso-C
of C6H5), 129.0, 127.8, 127.4, 126.9, 126.7, 123.2 (all s, C6H5),
22.9 [d, J(P,C) 40.0, PCH3]; δP (162.0 MHz, 293 K) Ϫ18.3 [t,
J(Rh,P) 108.5]; δP (81.0 MHz, 193 K) Ϫ17.5 [t, J(Rh,P) 108.0],
Ϫ31.8 [br dd, J(Rh,P) 132.2, J(RhЈ,P) 76.3].
[Rh2I2(ꢀ-CPh2)2(ꢀ-PMe3)] 14. This compound was prepared
as described for 13, from 4 (214 mg, 0.26 mmol) and Me3SiI
(0.067 cm3, 0.53 mmol) in benzene (20 cm3); reaction time 2 h.
A red microcrystalline solid was obtained; yield 205 mg (90%);
mp 80 ЊC (decomp.) (Found: C, 39.72; H, 3.52. C29H29I2PRh2
requires: C, 40.12; H, 3.37%). NMR (CD2Cl2): δH (300 MHz)
7.54 (4 H, m, ortho-H of C6H5), 7.24, 6.88, 6.75 (16 H, all m,
ortho-, meta- and para-H of C6H5), 1.02 [10.5, d, J(P,H) PCH3];
δC (75.5 MHz) 184.9 (m, CPh2), 152.2 [d, J(P,C) 4.0, ipso-C of
C6H5], 150.1 (s, ipso-C of C6H5), 128.8, 127.9, 127.4, 127.2,
126.9, 123.3 (all s, C6H5), 22.8 [d, J(P,C) 39.3, PCH3]; δP (81.0
MHz) Ϫ22.1 [t, J(Rh,P) 104.2].
[Rh2I(ꢁ2-acac)(ꢀ-CPh2)2(ꢀ-PMe3)] 10. (a) This compound
was prepared as described for 9, from 4 (99 mg, 0.12 mmol) and
Me3SiI (0.017 cm3, 0.12 mmol) in toluene (20 cm3). A dark
brown microcrystalline solid was obtained; yield 96 mg (94%).
(b) A solution of 5 (71 mg, 0.09 mmol) in acetone (25 cm3) was
treated with NaI (14 mg, 0.09 mmol) and stirred for 2 h at
room temperature. The solvent was evaporated in vacuo, and
the residue was extracted twice with 20 cm3 portions of pentane–
dichloromethane (1 : 1). The combined extracts were brought to
dryness in vacuo, and the remaining residue was recrystallized
from acetone (3 cm3) at Ϫ60 ЊC. Dark brown crystals; yield 59
mg (74%); mp 116 ЊC (decomp.) (Found: C, 49.01; H, 4.54.
C34H36IO2PRh2 requires: C, 48.60; H, 4.32%). NMR (CD2Cl2):
δH (400 MHz) 8.02 (4 H, m, ortho-H of C6H5), 7.17 (6 H, m,
meta- and para-H of C6H5), 7.10 (4 H, m, ortho-H of C6H5),
6.72 (6 H, m, meta- and para-H of C6H5), 5.79 (1 H, s, CH of
acac), 2.22 (6 H, s, CH3 of acac), 0.92 [9 H, d, J(P,H) 11.2,
PCH3]; δC (100.6 MHz) 189.4 (s, CO of acac), 171.5 [ddd,
J(Rh,C) 31.5, J(RhЈ,C) 20.0, J(P,C) 4.3, CPh2], 153.1 (s, ipso-C
of C6H5), 152.9 [d, J(P,C) 1.9, ipso-C of C6H5], 127.5, 127.1,
126.6, 125.8, 125.7, 125.3 (all s, C6H5), 100.7 (s, CH of acac),
28.2 (s, CH3 of acac), 21.2 [d, J(P,C) 41.0, PCH3]; δP (162.0
MHz) Ϫ22.3 [dd, J(Rh,P) 165.2, J(RhЈ,P) 53.5].
[Rh2Cl2(ꢀ-CPh2)2(ꢀ-PEt3)] 15. This compound was prepared
as described for 6, from 8 (145 mg, 0.17 mmol) and Me3SiCl
(5.0 cm3, 39.4 mmol) in benzene (20 cm3). A red micro-
crystalline solid was obtained; yield 103 mg (85%); mp 85 ЊC
(decomp.) (Found: C, 53.11; H, 5.09. C32H35Cl2PRh2 requires C,
52.84; H, 4.85%). NMR (C6D6): δH (400 MHz) 7.80, 7.57 (8 H,
both m, ortho-H of C6H5), 6.70 (10 H, m, meta- and para-H
of C6H5), 6.59 (2 H, m, para-H of C6H5), 0.77 (15 H, m,
PCH2CH3); δC (100.6 MHz) 186.7 (m, CPh2), 153.7 [d, J(P,C)
2.9, ipso-C of C6H5], 151.8 (s, ipso-C of C6H5), 128.6, 128.3,
127.7, 127.4, 127.0, 123.5 (all s, C6H5), 22.0 [d, J(P,C) 34.3,
PCH2], 9.3 [d, J(P,C) 4.8, PCH2CH3]; δP (162.0 MHz) 4.8 [t,
J(Rh,P) 102.5].
[Rh2Cl(ꢁ2-acac)(ꢀ-CPh2)2(ꢀ-PMe2Ph)] 11. This compound
was prepared as described for 5, from 7 (60 mg, 0.07 mmol) and
Me3SiCl (0.01 cm3, 0.07 mmol) in benzene (10 cm3). An orange
microcrystalline solid was obtained; yield 52 mg (93%); mp
63 ЊC (decomp.) (Found: C, 57.39; H, 5.08. C39H38ClO2PRh2
requires: C, 57.76; H, 4.72%). IR (KBr): ν(COacac) 1576, 1522
cmϪ1. NMR (CD2Cl2): δH (400 MHz) 7.93 (4 H, m, ortho-H of
C6H5), 7.37, 7.22, 7.13, 6.66, 6.58 (21 H, all m, C6H5), 5.81 (1 H,
s, CH of acac), 2.26 (6 H, s, CH3 of acac), 1.15 [6 H, d, J(P,H)
10.8, PCH3]; δC (100.6 MHz) 189.2 (s, CO of acac), 174.6 [ddd,
J(Rh,C) 30.5, J(RhЈ,C) 20.0, J(P,C) 3.8, CPh2], 154.0 (s, ipso-C
of CC6H5), 132.0 [d, J(P,C) 55.3, ipso-C of PC6H5], 129.8 [d,
[{Rh2Cl2(ꢀ-CPh2)2(ꢀ-PMe2Ph)}2] 16. This compound was
prepared as described for 6, from 7 (82 mg, 0.09 mmol) and
Me3SiCl (1.0 cm3, 7.88 mmol) in benzene (30 cm3). A red–
brown microcrystalline solid was obtained; yield 47 mg (67%);
mp 118 ЊC (decomp.) (Found: C, 54.30; H, 4.25. C34H31Cl2PRh2
requires C, 54.65; H, 4.18%). NMR (CD2Cl2): δH (400 MHz)
7.52, 7.38, 7.31, 7.23, 6.84, 6.74 (25 H, all m, C6H5), 1.38 [6 H,
d, J(P,C) 10.6, PCH3]; δC (100.6 MHz) 187.0 (m, CPh2), 153.2
[d, J(P,C) 3.8, ipso-C of CC6H5], 151.7 (s, ipso-C of CC6H5),
131.3 [d, J(P,C) 2.9, para-C of PC6H5], 129.8 [d, J(P,C) 9.5,
ortho- or meta-C of PC6H5], 129.4 [d, J(P,C) 10.5, ortho- or
D a l t o n T r a n s . , 2 0 0 4 , 9 5 9 – 9 6 6
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meta-C of PC6H5], 128.8, 127.7, 127.3, 126.7, 126.6, 123.6 (all s,
C6H5), 19.9 [d, J(P,C) 43.9, PCH3], signal of ipso-C atom of
PC6H5 could not be exactly located; δP (162.0 MHz, 293 K)
Ϫ17.0 [br t, J(Rh,P) 101.7]; δP (81.0 MHz, 193 K) Ϫ26.8 [dd,
J(Rh,P) 139.8, J(RhЈ,P) 68.6].
125.0, 124.1, 123.9 (all s, C6H5), 99.6 (s, CH of acac), 29.4 [d,
J(P,C) 31.5, PCH2], 28.3 (s, CH3 of acac), 26.7 [d, J(P,C) 4.8,
CH2 of PnBu3], 23.3 [d, J(P,C) 16.2, CH2 of PnBu3], 13.5 (s, CH3
of PnBu3); δP (162.0 MHz, 253 K) 13.6 [t, J(Rh,P) 103.6]; NMR
(C6D6): δP (81.0 MHz, 293 K) Ϫ9.6 [t, J(Rh,P) 104.3].
[Rh2(ꢁ2-acac)2(ꢀ-CPh2)2(ꢀ-PMe2Ph)] 7. This compound was
prepared as described for 22, from 21 (242 mg, 0.28 mmol) and
PMe2Ph (0.04 cm3, 0.28 mmol) in toluene (10 cm3). A red–
brown microcrystalline solid was obtained; yield 227 mg (92%).
[Rh2Cl2(ꢀ-CPh2)2(ꢀ-PiPr3)] 19. A solution of 17 (105 mg,
0.12 mmol) in benzene (100 cm3) was treated under vigorous
stirring with a 0.16 M solution of HCl in benzene (7.32 cm3,
1.17 mmol) at room temperature. A change of color from
greenish to red occurred. The solvent was evaporated in vacuo,
the remaining red–brown residue was washed three times with
3 cm3 portions of diethyl ether (0 ЊC) and dried; yield 82 mg
(91%); mp 119 ЊC (decomp.) (Found: C, 54.85; H, 5.40.
C35H41Cl2PRh2 requires C, 54.64; H, 5.37%). NMR (CD2Cl2):
δH (400 MHz, 263 K) 7.47, 7.22, 7.10, 6.73 (20 H, all m, C6H5),
1.45–1.10 (21 H, br m, PCHCH3); δC (100.6 MHz, 263 K) 180.7
[t, J(Rh,C) 28.6, CPh2], 152.4, 152.1 (both s, ipso-C of C6H5),
127.8, 127.3, 127.1, 127.0, 126.1, 122.9 (all s, C6H5), 28.4 (m,
PCHCH3), 20.7 (m, PCHCH3); δP (162.0 MHz, 263 K) 14.9 [t,
J(Rh,P) 92.6].
The product was characterized by comparison of the 1H and 31
NMR data with those of an authentic sample.12
P
[Rh2(ꢁ2-acac)2(ꢀ-CPh2)2(ꢀ-PEt3)] 8. This compound was pre-
pared as described for 22, from 21 (372 mg, 0.43 mmol) and
PEt3 (0.063 cm3, 0.43 mmol) in toluene (10 cm3). A red–brown
microcrystalline solid was obtained; yield 365 mg (98%). The
1
product was characterized by comparison of the H and 31P
NMR data with those of an authentic sample.12
[Rh2Cl(ꢁ2-acac)(ꢀ-CPh2)2(ꢀ-AsMe3)] 23. A solution of 21 (76
mg, 0.09 mmol) in benzene (10 cm3) was treated with Me3SiCl
(0.012 cm3, 0.10 mmol) and stirred for 1 h at room temperature.
The solvent was evaporated in vacuo, the remaining brown solid
was washed twice with 5 cm3 portions of pentane–diethyl ether
(10 : 1) and dried; yield 62 mg (88%); mp 105 ЊC (decomp.)
(Found: C, 51.12; H, 4.58. C34H36AsClO2Rh2 requires C, 51.51;
H, 4.58%). IR (KBr): (COacac) 1580, 1519 cmϪ1. NMR (C6D6):
δH (400 MHz) 8.36, 7.42 (8 H, both m, ortho-H of C6H5), 7.04
(4 H, m, meta-H of C6H5), 6.91 (2 H, m, para-H of C6H5),
6.63 (6 H, m, meta-H and para-H of C6H5), 5.41 (1 H, s, CH of
acac), 1.89 (6 H, s, CH3 of acac), 0.62 (9 H, s, AsCH3); δC (100.6
MHz) 189.1 (s, CO of acac), 178.2 [dd, J(Rh,C) 27.2, J(RhЈ,C)
20.0, CPh2], 154.7, 153.9 (both s, ipso-C of C6H5), 127.7, 127.3,
126.7, 126.4, 126.2, 124.9 (all s, C6H5), 101.0 (s, CH of acac),
28.0 (s, CH3 of acac), 18.6 (s, AsCH3).
Generation of [Rh2Cl2(ꢀ-CPh2)2(ꢀ-PPh3)] 20. A solution 18
(139 mg, 0.14 mmol) in benzene (100 cm3) was treated under
vigorous stirring with a 0.16 M solution of HCl in benzene
(8.70 cm3, 1.39 mmol) at room temperature. The solvent was
evaporated in vacuo, the remaining red–brown residue was
washed three times with 3 cm3 portions of diethyl ether (0 ЊC)
and dried. The NMR data revealed that a mixture of products
was formed with 20 as the dominating species (ca. 80%). Since
attempts to separate the by-products by fractional crystalliz-
ation or column chromatography failed, 20 was characterized
spectroscopically. Typical data: NMR (CD2Cl2): δC (100.6
MHz) 191.2 [t, J(Rh,C) 28.6, CPh2], 152.4, 151.6 (both s, ipso-C
of CC6H5; δP (162.0 MHz) 9.8 [t, J(Rh,P) 103.6].
[Rh2(ꢁ2-acac)2(ꢀ-CPh2)2(ꢀ-AsMe3)] 21. A suspension of 3
(553 mg, 0.56 mmol) in hexane (20 cm3) was treated with
AsMe3 (0.108 cm3, 1.00 mmol) and stirred for 6 h at room
temperature. A clear solution was formed, which was stored for
15 h at Ϫ78 ЊC. Light brown crystals precipitated, which were
separated from the mother-liquor, washed three times with 3
cm3 portions of hexane and dried; yield 408 mg (85%; mp
105 ЊC (decomp.) (Found: C, 54.44; H, 5.11. C39H43AsO4Rh2
requires C, 54.69; H, 5.06%). IR (KBr): (COacac) 1581, 1518
cmϪ1. NMR (C6D6): δH (200 MHz) 8.23, 7.32 (8 H, both m,
ortho-H of C6H5), 7.05, 6.73 (12 H, both m, meta- and para-H
of C6H5), 5.50 (2 H, s, CH of acac), 1.95 (12 H, s, CH3 of acac),
0.88 (9 H, s, AsCH3): δH (100.6 MHz, 233 K) 188.0 (s, CO of
acac), 172.7 [t, J(Rh,C) 22.4, CPh2], 155.1, 154.9 (both s, ipso-C
of C6H5), 126.9, 126.1, 125.8, 124.9, 124.4, 124.2 (all s, C6H5),
99.5 (s, CH of acac), 27.8 (s, CH3 of acac), 18.0 (s, AsCH3). MS
(FAB, 2-nitrophenyloctyl ether): m/z (relative intensity) 856
(12, Mϩ), 736 (100, Mϩ Ϫ AsMe3).
[{Rh2Cl2(ꢀ-CPh2)2(ꢀ-AsMe3)}2] 24. (a) A solution of 21 (82
mg, 0.10 mmol) in benzene (2 cm3) was treated with Me3SiCl
(2.0 cm3, 15.7 mmol) and stirred for 4 h at room temperature.
A small part of a red–brown solid precipitated. The volatile
materials were evaporated in vacuo, the remaining red–brown
solid was washed twice with 2 cm3 portions of benzene and
5 cm3 portions of diethyl ether and dried; yield 63 mg (90%).
(b) Under the same conditions as used for (a), the reaction of 23
(75 mg, 0.09 mmol) and Me3SiCl (2.0 cm3, 15.7 mmol) gave the
same product; yield 65 mg (93%); mp 120 ЊC (decomp.) (Found:
C, 51.76; H, 4.48. C58H58As2Cl4Rh2ؒ2C6H6 requires C, 52.07; H,
4.37%). NMR (CD2Cl2): δH (400 MHz) 7.75 (4 H, m, ortho-H
of C6H5), 7.24, 6.86, 6.76 (16 H, all m, C6H5), 1.19 (s, AsCH3);
δC (100.6 MHz) 153.2, 152.4 (both s, ipso-C of C6H5), 128.4,
128.2, 127.6, 127.2, 126.7, 126.4, 124.7 (all s, C6H5), 20.5 (s,
AsCH3); signal for the CPh2 carbon atoms could not be exactly
located.
[Rh2(ꢁ2-acac)2(ꢀ-CPh2)2(ꢀ-PnBu3)] 22. A solution of 21 (67
mg, 0.08 mmol) in toluene (10 cm3) was treated at Ϫ50 ЊC with
PnBu3 (0.02 cm3, 0.08 mmol) and after warming to 0 ЊC stirred
for 20 min under reduced pressure (ca. 20 mbar). All volatile
materials were evaporated in vacuo. The remaining brown solid
was washed twice with 2 cm3 portions of pentane (0 ЊC) and
dried; yield 65 mg (89%); mp 69 ЊC (decomp.) (Found: C, 54.44;
H, 5.11. C48H61O4PRh2 requires C, 54.69; H, 5.06%). IR (KBr):
(COacac) 1586, 1518 cmϪ1. NMR (CD2Cl2): δH (400 MHz, 253
K) 7.86 (4 H, m, ortho-H of C6H5), 7.09, 6.86, 6.64 (16 H, all m,
C6H5), 5.67 (2 H, s, CH of acac), 2.07 (12 H, s, CH3 of acac),
1.26, 1.00 (18 H, all m, CH2 of PnBu3), 0.74 (9 H, m, CH3 of
PnBu3); δC (100.6 MHz, 253 K) 187.9 (s, CO of acac), 174.0 [t,
J(Rh,C) 24.1, CPh2], 156.4 [d, J(P,C) 1.9, ipso-C of C6H5], 156.2
(s, ipso-C of C6H5), 132.5, 130.0, 128.3, 127.4, 126.3, 126.0,
Crystallography
Single crystals of 6 and 24 (both containing two independent
molecules of the complex and two molecules of benzene in the
asymmetric unit) were grown from benzene at room tem-
perature, and those of 14 (containing two thirds of a molecule
of acetone in the asymmetric unit) from a saturated solution
in acetone at Ϫ25 ЊC. Crystal data collection parameters are
summarized in Table 4. The data were collected on a Bruker
Smart Apex diffractometer with D8-Goniometer using
monochromated Mo-Kα radiation (λ = 0.71073 Å). Intensity
data were corrected by Lorentz and polarization effects, and
empirical absorption corrections were applied. The structures
of 6 and 14 were solved by the Patterson method, and the
structure of 24 was solved by direct methods (SHELXS-97).18
D a l t o n T r a n s . , 2 0 0 4 , 9 5 9 – 9 6 6
965

View Article Online
Table 4 Crystal data for complexes 6, 14 and 24
6
14
24
Formula
M
C58H58Cl4P2Rh4ؒ2C6H6
1526.64
C29H29I2PRh2ؒ2/3C3H6O
906.83
C58H58Cl4As2Rh4ؒ2C6H6
1614.54
Crystal size/mm
Crystal system
Space group
0.40 × 0.40 × 0.20
Monoclinic
P21/c (no. 14)
0.30 × 0.17 × 0.16
Trigonal
R3c (no. 167)
0.67 × 0.66 × 0.63
Monoclinic
P21/c (no. 14)
¯
Cell dimens. determn.
a/Å
b/Å
c/Å
8804 rflns., 2.303 < θ < 28.246Њ
9283 rflns., 2.431 < θ < 24.492Њ
8282 rflns., 2.292 < θ < 28.146Њ
22.2854(11)
15.1150(7)
19.2895(9)
90
22.9416(17)
22.9416(17)
31.230(3)
90
22.4012(13)
15.2521(9)
19.4838(11)
90
α/Њ
β/Њ
101.8780(10)
90
6358.4(5)
4
90
120
14235(2)
18
1.904
102.535(10)
90
6498.3(7)
4
γ/Њ
V/Å3
Z
Dc/g cmϪ3
1.595
1.650
T /K
173(2)
1.280
173(2)
3.066
173(2)
2.214
µ/mmϪ1
Scan method
2θ(max.)/Њ
ω
50.00
ω
50.28
ω
50.00
No. of reflections measured
No. of unique reflections (Rint
No. of observed reflections
No. of reflections used
for refinement
No. of parameters refined
Final R indices
60158
53403
35365
)
11183 (0.0228)
10525 (I > 2σ(I ))
3072
2818 (0.0787)
2780 (I > 2σ(I ))
2818
10588 (0.0154)
10117 (I > 2σ(I ))
3216
727
188
727
R1 = 0.0228, wR2 = 0.0541a
(I > 2σ(I ))
R1 = 0.0643, wR2 = 0.1368a
(I > 2σ(I ))
R1 = 0.0222, wR2 = 0.0531a
(I > 2σ(I ))
R indices (all data)
R1 = 0.0249, wR2 = 0.0551a
0.472/Ϫ0.351
R1 = 0.0649, wR2 = 0.1371a
1.526/Ϫ1.043
R1 = 0.0246, wR2 = 0.0540a
0.664/Ϫ0.356
Residual electron density/e ÅϪ3
a wϪ1 = [σ2Fo2 ϩ (0.0265P)2 ϩ 4.0856P] (6), wϪ1 = [σ2Fo2 ϩ (0.0186P)2 ϩ 644.7705P] (14), wϪ1 = [σ2Fo2 ϩ (0.0437P)2 ϩ 4.9135P] (24), where P = (Fo2 ϩ
2Fc2)/3.
105, 1498; P. Schwab, N. Mahr, J. Wolf and H. Werner, Angew.
Chem., Int. Ed. Engl., 1993, 32, 1480; H. Werner, P. Schwab,
E. Bleuel, N. Mahr, P. Steinert and J. Wolf, Chem. Eur. J., 1997,
3, 1375.
5 P. Schwab, N. Mahr, J. Wolf and H. Werner, Angew. Chem., 1994,
106, 82; P. Schwab, N. Mahr, J. Wolf and H. Werner, Angew. Chem.,
Int. Ed. Engl., 1994, 33, 97; P. Schwab, J. Wolf, N. Mahr, P. Steinert,
U. Herber and H. Werner, Chem. Eur. J., 2000, 6, 4471.
6 T. Pechmann, C. D. Brandt and H. Werner, Angew. Chem., 2000,
112, 4069; T. Pechmann, C. D. Brandt and H. Werner, Angew.
Chem., Int. Ed., 2000, 39, 3909.
Atomic coordinates and anisotropic thermal parameters of
non-hydrogen atoms were refined by full-matrix least squares
on F 2 (SHELXL-97).19 The positions of all hydrogen atoms
were calculated according to ideal geometry and refined using
the riding method.
CCDC reference numbers 178267 (6), 202931 (14) and
178268 (24).
See http://www.rsc.org/suppdata/dt/b3/b314734g/ for crystal-
lographic data in CIF or other electronic format.
7 T. Pechmann, C. D. Brandt, C. Röger and H. Werner, Angew. Chem.,
2002, 114, 2398; T. Pechmann, C. D. Brandt, C. Röger and
H. Werner, Angew. Chem., Int. Ed., 2002, 41, 2301; T. Pechmann,
C. D. Brandt and H. Werner, Chem. Commun., 2003, 1136.
8 For definition of semi-bridging, see: F. A. Cotton, Prog. Inorg.
Chem., 1976, 21, 1.
Acknowledgements
We thank the Deutsche Forschungsgemeinschaft (Grant SFB
347) and the Fonds der Chemischen Industrie for financial
support. Moreover, we gratefully acknowledge support by Mrs
R. Schedl and Mr C. P. Kneis (elemental analyses and DTA),
Mrs M.-L. Schäfer and Dr W. Bertermann (NMR measure-
ments), Dr G. Lange and Mr F. Dadrich (mass spectra), and
BASF AG for gifts of chemicals.
9 P. S. Pregosin and R. W. Kunz, 31P and 13C NMR of Transition
Metal Phosphine Complexes, Springer, New York, 1979, p. 25;
D. W. Meek and T. J. Mazanek, Acc. Chem. Res., 1981, 14, 266.
10 T. Pechmann, Dissertation, Universität Würzburg, 2003.
11 U. Herber, T. Pechmann, B. Weberndörfer, K. Ilg and H. Werner,
Chem. Eur. J., 2002, 8, 309.
12 U. Herber, Dissertation, Universität Würzburg, 2000.
13 K. G. Caulton, Chemtracks: Inorg. Chem., 1999, 592.
14 T. Pechmann, C. D. Brandt and H. Werner, Dalton Trans., 2003,
1495.
15 P. Braunstein and N. M. Boag, Angew. Chem., 2001, 113, 2493;
P. Braunstein and N. M. Boag, Angew. Chem., Int. Ed., 2001, 40,
2427.
16 M. Sauthier, B. Le Guennic, V. Deborde, L. Toupet, J.-F. Halet and
R. Reau, Angew. Chem., 2001, 113, 234; M. Sauthier, B. Le Guennic,
V. Deborde, L. Toupet, J.-F. Halet and R. Reau, Angew. Chem., Int.
Ed., 2001, 40, 228; F. Leca, M. Sauthier, V. Deborde, L. Toupet and
R. Reau, Chem. Eur. J., 2003, 9, 3785.
17 T. Pechmann, C. D. Brandt and H. Werner, Chem. Eur. J., 2004, 10,
728.
18 G. M. Sheldrick, Acta Crystallogr., Sect. A, 1990, 46, 467;
G. M. Sheldrick, SHELXS-97, Universität Göttingen, 1997.
19 G. M. Sheldrick, SHELXL-97, Programm zur Strukturver-
feinerung, Universität Göttingen, Göttingen, Germany, 1997.
20 M. N. Burnett and C. K. Johnson ORTEP3, Report ORNL-6895,
Oak Ridge National Laboratory, Oak Ridge, TN, 1996.
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D a l t o n T r a n s . , 2 0 0 4 , 9 5 9 – 9 6 6
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