In situ synthesis of neutral dinuclear rhodium diphosphine complexes [{Rh(diphosphine)(μ2-X)}2]: Systematic investigations

Article abstract of DOI:10.1002/cplu.201402213

As the workhorses for many applications, neutral dimeric μ₂-X-bridged diphosphine rhodium complexes of the type [{Rh(diphosphine)(μ₂-X)}₂] (X = Cl, OH) are usually prepared in situ by the addition of diphosphine ligands to

Full text of DOI:10.1002/cplu.201402213

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DOI: 10.1002/cplu.201402213
In Situ Synthesis of Neutral Dinuclear Rhodium
Diphosphine Complexes [{Rh(diphosphine)(m2-X)}2]:
Systematic Investigations
Antje Meißner, Angelika Preetz, Hans-Joachim Drexler, Wolfgang Baumann,
Anke Spannenberg, Anja Kçnig, and Detlef Heller*[a]
As the workhorses for many applications, neutral dimeric m2-X-
bridged diphosphine rhodium complexes of the type
[{Rh(diphosphine)(m2-X)}2] (X=Cl, OH) are usually prepared in
situ by the addition of diphosphine ligands to the rhodium
complex [{Rh(diolefin)(m2-X)}2] (diolefin=cyclooctadiene (cod)
or norbornadiene (nbd)) or [{Rh(monoolefin)2(m2-Cl)}2] (monoo-
lefin=cyclooctene (coe) or ethylene (C2H4)). The in situ proce-
dure has been investigated for the diphosphines 2,2’-bis(diphe-
nylphosphino)-1,1’-binaphthyl (BINAP), 5,5’-bis(diphenylphos-
phino)-4,4’-bi-1,3-benzodioxole (SEGPHOS), 5,5’-bis[di(3,5-xylyl)-
ethane (DPPE), 1,2-bis(o-methoxyphenylphosphino)ethane
(DIPAMP), 4,5-bis(diphenylphosphinomethyl)-2,2-dimethyl-1,3-
dioxalane (DIOP), 1,2-bis(2,5-dimethylphospholano)benzene
(Me-DuPHOS), 1,4-bis(diphenylphosphino)butane (DPPB), and
1,3-bis(diphenylphosphino)propane (DPPP); the resulting com-
plexes have been characterized by 31P NMR spectroscopy
and, in most cases, also by X-ray analysis. Depending on
the diphosphine ligand, the solvent, the temperature, and
the rhodium precursor, species other than the desired
one [{Rh(diphosphine)(m2-X)}2] are formed, for example,
phosphino]-4,4’-bi-1,3-benzodioxole
(DM-SEGPHOS),
5,5’-
[(diolefin)Rh(m2-Cl)2Rh(diphosphine)],
[Rh(diphosphine)-
bis[di(3,5-di-tert-butyl-4-methoxyphenyl)phosphino]-4,4’-bi-1,3-
benzodioxole (DTBM-SEGPHOS), 2,2’-bis(diphenylphosphino)-
1,1’-dicyclopentane (BICP), 1-[2-(diphenylphosphino)ferroceny-
l]ethyldi-tert-butylphosphine (PPF-PtBu2), 1,1’-bis(diisopropyl-
(diolefin)]+, [Rh(diphosphine)2]+, and [Rh(diphosphine)-
(diolefin)(Cl)]. The results clearly show that the in situ method
commonly applied for precatalyst preparation cannot be re-
garded as an optimal strategy for the formation of such neu-
tral [{Rh(diphosphine)(m2-X)}2] complexes.
phosphino)ferrocene
(DiPPF),
1,2-bis(diphenylphosphino)-
Introduction
The choice of precatalyst in homogeneous catalysis has a cru-
cial impact on the course of the reaction. This has been dem-
onstrated in several cases, including asymmetric hydrogena-
tion.[1]
It is generally accepted that treatment of complexes [{Rh-
(cod)(m2-Cl)}2] (1; cod=cyclooctadiene), [{Rh(nbd)(m2-Cl)}2] (2;
nbd=norbornadiene), [{Rh(coe)2(m2-Cl)}2] (3; coe=cyclooc-
tene), [{Rh(cod)(m2-OH)}2] (4), and [{Rh(C2H4)2(m2-Cl)}2] (5) with
diphosphine ligands smoothly affords neutral m2-bridged dinu-
clear rhodium complexes of the type [{Rh(diphosphine)(m2-X)}2]
(X=Cl or OH; Scheme 1). Hence, being considered straightfor-
ward, such in situ precatalyst preparation is applied almost ex-
clusively in the catalyses mentioned above.
Neutral m2-bridged dinuclear rhodium complexes of the type
[{Rh(diphosphine)(m2-X)}2] (X=Cl, or more rarely, OH) are
known to promote (stereoselective) catalytic processes, for ex-
ample, the hydrogenation of prochiral olefins,[2] ketones,[3]
CO2,[4] and polynuclear heteroaromatic compounds;[5] the ring
opening of oxa- and azabicyclic alkenes,[6] the hydrogen-medi-
ated formation of CÀC bonds;[7] the addition of carboxylic
acids to alkynes;[8] CO-gas-free hydroformylation[9] and carbon-
ylations;[10] Pauson–Khand-type reactions;[11] olefin isomeriza-
tion;[12] cycloadditions;[13] the coupling of aldehydes and al-
lenes;[14] and 1,4-addition of organoboronic acids,[15] to men-
tion only a few.
Scheme 1. Conventional synthesis of [{Rh(diphosphine)(m2-X)}2] (PP=diphos-
phine, X=Cl or OH).
In the present study, systematic investigations into the syn-
thesis of [{Rh(diphosphine)(m2-X)}2] complexes according to
Scheme 1 were carried out with precursors 1–5. Diphosphine
ligands such as 2,2’-bis(diphenylphosphino)-1,1’-binaphthyl
(BINAP), 5,5’-bis(diphenylphosphino)-4,4’-bi-1,3-benzodioxole
[a] A. Meißner, Dr. A. Preetz, Dr. H.-J. Drexler, Priv.-Doz. Dr. W. Baumann,
Dr. A. Spannenberg, A. Kçnig, Prof. Dr. D. Heller
Leibniz-Institut fꢀr Katalyse e.V. an der Universitꢁt Rostock
Albert-Einstein-Strasse 29a, 18059 Rostock (Germany)
Fax: (+49)381-1281-51183
(SEGPHOS),
5,5’-bis[di(3,5-xylyl)phosphino]-4,4’-bi-1,3-benzo-
E-mail: detlef.heller@catalysis.de
dioxole (DM-SEGPHOS), 5,5’-bis[di(3,5-di-tert-butyl-4-methoxy-
phenyl)phosphino]-4,4’-bi-1,3-benzodioxole (DTBM-SEGPHOS),
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cplu.201402213.
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2,2’-bis(diphenylphosphino)-1,1’-dicyclopentane (BICP), 1-[2-(di-
phenylphosphino)ferrocenyl]ethyldi-tert-butylphosphine (PPF-
PtBu2), 1,1’-bis(diisopropylphosphino)ferrocene (DiPPF), 1,2-bis(-
diphenylphosphino)ethane (DPPE), 1,2-bis(o-methoxyphenyl-
phosphino)ethane (DIPAMP), 4,5-bis(diphenylphosphinometh-
yl)-2,2-dimethyl-1,3-dioxalane (DIOP), 1,2-bis(2,5-dimethylphos-
pholano)benzene (Me-DuPHOS), 1,4-bis(diphenylphosphino)bu-
tane (DPPB), and 1,3-bis(diphenylphosphino)propane (DPPP)
were applied.
plexes range between 184 and 206 Hz. Similar chemical shifts
and coupling constants were determined for the intermediate
[(diolefin)Rh(m2-Cl)2Rh(diphosphine)].
The values of the pentacoordinated neutral complexes
[Rh(diphosphine)(diolefin)(Cl)] are similar to those of the corre-
sponding cationic complexes [Rh(diphosphine)(diolefin)]BF4, al-
though the coupling constants J(P,Rh) of the neutral com-
plexes are smaller than those of the cationic ones.
BINAP
The synthesis of [{Rh(BINAP)(m2-Cl)}2] has been published re-
cently;[15,16] however, it was included in this study only for com-
parison. At room temperature the in situ formation of the de-
sired dinuclear rhodium complex [{Rh(BINAP)(m2-Cl)}2] from the
cod precursor 1 and two molar equivalents of BINAP (1/BINAP
1:2) in tetrahydrofuran (THF) as well as in toluene[16] is practi-
cally quantitative (>98% conversion) as proven by NMR spec-
troscopic measurements.[17,18] In dichloromethane (CH2Cl2), too,
quantitative yields of [{Rh(BINAP)(m2-Cl)}2] are observed at
room temperature, with no oxidative addition of the solvent to
rhodium taking place as a side reaction as previously described
when using DPPE as the ligand.[19]
If BINAP is added in substoichiometric amounts to 1 (1/
BINAP=1:1) both in THF as well as in CH2Cl2 at room tempera-
ture, a mixture of two species is obtained as shown by the
31P NMR spectrum. Surprisingly, the target compound [{Rh-
(BINAP)(m2-Cl)}2] is detectable as a byproduct in addition to the
main species; this is probably the intermediate [(cod)Rh(m2-
1
Cl)2Rh(BINAP)]. According to the H NMR spectrum in CH2Cl2,
traces of unreacted 1 are also present. Single crystals suitable
for X-ray analysis were isolated from the reaction mixture. The
corresponding molecular structure is reported in Figure 1.
Figure 1 shows the expected intermediate, [(cod)Rh(m2-
Cl)2Rh(BINAP)], that arises from the stepwise exchange of the
diolefin ligands (see Scheme 1).[20] To the best of our knowl-
edge, this is the first case in which the structure of this kind of
complex has been unambiguously assigned by X-ray analy-
sis.[21] If the crystals are redissolved in THF at room tempera-
Special attention was devoted to assess, depending on sol-
vent and temperature, the completeness of the conversion
and the formation of intermediates and possible byproducts.
In the case of BINAP, the rate of in situ precatalyst formation
was also investigated.
Results and Discussion
31P NMR spectroscopic data of
the depicted complexes
The 31P NMR spectroscopic data
of the complexes described in
the manuscript as a result of our
investigations are summarized in
Table 1. It is evident that the
phosphorous signals of the neu-
tral complexes of the type
[{Rh(diphosphine)(m2-Cl)}2]
are
quite low-field-shifted relative to
those of the corresponding cat-
ionic complexes and of the pen-
tacoordinated complexes. The
typical J(P,Rh) coupling constants
Figure 1. 31P NMR spectrum of the solution obtained by dissolving single crystals of [(cod)Rh(m2-Cl)2Rh(BINAP)]
(yellow) in [D8]THF at room temperature and the molecular structure of [(cod)Rh(m2-Cl)2Rh(BINAP)]; ORTEP, 30%
probability ellipsoids. Hydrogen atoms are omitted for clarity. See below for a short discussion of the structural
of such dinuclear neutral com- data. Selected bond lengths and angles are listed in Table S2 of the Supporting Information.
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placed very rapidly. The kinetically uniform reaction is
in fact complete within seven seconds (Figure 2, left).
The exchange of the second diolefin ligand (reac-
tion of crystals of the intermediate [(cod)Rh(m2-Cl)2Rh-
(BINAP)] with BINAP) is much slower under the
chosen experimental conditions (Figure 2, right). The
overall reaction time of approximately 30 minutes for
the formation of [{Rh(BINAP)(m2-Cl)}2] by in situ reac-
tion of 1 with two equivalents of BINAP is thus due
to the much slower second reaction step.[24]
Table 1. 31P NMR spectroscopic data relative to all complexes discussed in this study.
Ligand [PP]
BINAP
Type of complex
Solvent
31P NMR spectroscopic data
d [ppm]
J(P,Rh) [Hz]
[{Rh(PP)(m2-Cl)}2]
[(cod)Rh(m2-Cl)2Rh(PP)]
[{Rh(PP)(m2-OH)}2]
[Rh(PP)(nbd)]+
THF
THF
THF
MeOH
CH2Cl2
49.1
194.8
199.6
185.5
156.8
141.2
49.9
54.5
26.1
25.0
[Rh(PP)(nbd)(Cl)]
SEGPHOS
[{Rh(PP)(m2-Cl)}2]
[(cod)Rh(m2-Cl)2Rh(PP)]
[Rh(PP)2]+
[Rh(PP)(cod)]+
THF
THF
CH2Cl2
MeOH
46.4
47.6
20.7
25.1
194.8
199.6
141.4
146.7
If the nbd complex [{Rh(nbd)(m2-Cl)}2] (2) is used in-
stead of 1 under otherwise identical reaction condi-
tions, the m2-Cl-bridged dimer complex is formed in
only 80% yield. An unknown species and traces of
the oxidized ligand are also detected (Figure 3a).[25] In
CH2Cl2 the yield of the target compound [{Rh(BINAP)-
(m2-Cl)}2] drops to 22%, the main complex (78%)
being an unexpected pentacoordinated neutral com-
plex [Rh(BINAP)(nbd)(Cl)] as described in the litera-
ture[26] (Figure 3b).
DM-SEGPHOS
BICP
[{Rh(PP)(m2-Cl)}2]
THF
MeOH
47.6
25.6
194.4
155.5
[Rh(PP)(nbd)]+
[{Rh(PP)(m2-Cl)}2]
[Rh(PP)(cod)]+
THF
MeOH
46.9
26.0
186.3
142.5
PPF-PtBu2 (A)
[{Rh(PP)(m2-Cl)}2]
THF
41.2
111.9
23.9
199.1 (44.0)[a]
206.0 (44.0)[a]
155.5 (29.8)[a]
151.6 (28.8)[a]
[Rh(PP)(nbd)]+
MeOH
Unexpectedly, in MeOH the ligand exchange pro-
vides no [{Rh(BINAP)(m2-Cl)}2] but rather the cationic
complex [Rh(BINAP)(nbd)]+,[27] which, according to
the 31P NMR spectrum, is formed quantitatively, with
chloride most likely being the counteranion (Fig-
ure 3c).[28]
74.6
DiPPF
[{Rh(PP)(m2-Cl)}2]
THF
63.1
205.2
DPPE
[{Rh(PP)(m2-Cl)}2]
[(cod)Rh(m2-Cl)2Rh(PP)]
[Rh(PP)2]+
THF
THF
CH2Cl2
72.9
76.9
57.9
198.3
200.9
133.5
When the precursor [{Rh(coe)2(m2-Cl)}2] (3) is used,
the desired dinuclear complex [{Rh(BINAP)(m2-Cl)}2] is
formed nearly quantitatively in THF at room tempera-
ture.
DIPAMP
[{Rh(PP)(m2-Cl)}2]
[(cod)Rh(m2-Cl)2Rh(PP)]
[Rh(PP)2]+
[Rh(PP)(cod)]+
THF
THF
CH2Cl2
MeOH
74.3
79.0
54.9
51.2
199.6
202.2
137.4
150.3
The yield drops to 45%, as shown by 31P NMR
spectroscopy, if the complex is prepared in situ at
room temperature from [{Rh(cod)(m2-OH)}2] (4) and
BINAP either in THF or toluene.[29] Recently, it has
been reported that rhodium–hydride species can be
formed from 4.[30]
DIOP
[{Rh(PP)(m2-Cl)}2]
[(cod)Rh(m2-Cl)2Rh(PP)]
[Rh(PP)(cod)]+
THF
THF
MeOH
33.5
36.3
12.8
190.5
194.8
143.8
Me-DuPHOS
[{Rh(PP)(m2-Cl)}2]
[(cod)Rh(m2-Cl)2Rh(PP)]
[Rh(PP)2]+
THF
THF
THF
96.5
98.4
76.6
199.1
199.6
130.9
DPPB
DPPP
[{Rh(PP)(m2-Cl)}2]
[Rh(DPPB)(nbd)(Cl)]
THF
THF
44.8
28.2
190.5
132.2
SEGPHOS
Like BINAP, the cod precursor 1 and SEGPHOS (1:2)
react in THF at room temperature to afford exclusive-
ly the desired complex [{Rh(SEGPHOS)(m2-Cl)}2]. Its
molecular structure is shown in Figure 4. If CH2Cl2 is
used, surprisingly, only 68% of the target compound
is formed. The byproduct is most likely [Rh-
(SEGPHOS)2]+. This complex can also be formed through an
excess amount of ligand (the ratio of 1 and SEGPHOS is 1:4).
Changing the ratio of 1 to SEGPHOS to 1:1 leads to 96% of
the expected intermediate [(cod)Rh(m2-Cl)2Rh(SEGPHOS)]. This
[{Rh(PP)(m2-Cl)}2]
[Rh(DPPP)(cod)(Cl)]
[Rh(DPPP)(nbd)(Cl)]
THF
THF
THF
31.9
13.0
18.7
184.0
116.6
128.3
[a] J (P,P) [Hz]
ture, [{Rh(BINAP)(m2-Cl)}2] and [{Rh(cod)(m2-Cl)}2] are formed,
each of which accounts for approximately 4% of the rhodium
content (according to 31P NMR (Figure 1) and H NMR spectros-
1
copy).
This unexpected result clearly suggests that the equilibrium
depicted in Scheme 2, undescribed thus far, is established in
solution. A detailed discussion of such equilibrium and the rate
constants are to be found in the literature.[22]
Exploratory UV/Vis spectroscopic investigations (stopped-
flow/diode array)[23] in THF confirm that the diolefin ligand ex-
change is a stepwise process with the first diolefin being re-
Scheme 2. Reaction sequence of the equilibrium between the intermediate
[(diolefin)Rh(m2-X)2Rh(diphosphine)] on one side and the precursor [{Rh-
(diolefin)(m2-X)}2] and the desired dinuclear complex [{Rh(diphosphine)(m2-
X)}2] on the other.
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only in 76% yield. The remaining
26% rhodium content is shared
between the target complex
[{Rh(SEGPHOS)(m2-Cl)}2] and the
intermediate [(cod)Rh(m2-Cl)2Rh-
(SEGPHOS)]. The molecular struc-
ture as well as the 31P NMR spec-
trum of the cationic complex
[Rh(SEGPHOS)(cod)]BF4
shown in Figure 5.
are
If the nbd precursor 2 is used,
only 80% of the target com-
pound of [{Rh(SEGPHOS)(m2-Cl)}2]
is formed in THF, together with
6% of the intermediate and
a third unidentified species. In
CH2Cl2 the yield drops to 35%.
The main species (55%) as evi-
Figure 2. Left: UV/Vis reaction spectra of the first step, the reaction of 0.593 mmolLÀ1 [{Rh(cod)(m2-Cl)}2] and
0.594 mmolLÀ1 BINAP in THF at 258C (spectra recorded every 7 ms, overall time 8 s, layer thickness 0.15 cm).
Right: UV/Vis reaction spectra of the second step, the reaction of 0.310 mmolLÀ1 [(cod)Rh(m2-Cl)2Rh(BINAP)] and
0.316 mmolLÀ1 BINAP in THF at 258C, cycle time 2.1 min, overall time 30 min, layer thickness 0.5 cm.
1
denced by H NMR spectroscopy
is probably a pentacoordinated
monomeric rhodium complex
[Rh(SEGPHOS)(nbd)(Cl)].
Starting with 4 and SEGPHOS
in a ratio of 1:2 in toluene leads
to only 52% of [{Rh(SEGPHOS)-
(m2-OH)}2], the structure of which
could be assigned by compari-
son with the 31P NMR spectro-
scopic data of [{Rh(BINAP)(m2-
OH)}2].[15]
DM-SEGPHOS
As with BINAP and SEGPHOS,
the addition of two equivalents
of DM-SEGPHOS to 1 at room
temperature quantitatively leads
to the desired dinuclear complex
[{Rh(DM-SEGPHOS)(m2-Cl)}2]
in
THF.
If 2 is used as the rhodium
source,
[{Rh(DM-SEGPHOS)(m2-
Cl)}2] is formed in just 30% yield
together with an as yet unidenti-
fied species. In CH2Cl2 the yield
of the target compound is only
10%. Like for BINAP and SEG-
Figure 3. 31P NMR spectra of the reaction solution of the in situ ligand exchange of [{Rh(nbd)(m2-Cl)}2] (2) and
BINAP (1:2) at room temperature in a) THF, b) CH2Cl2, and c) MeOH.
species can be safely assigned on the basis of the chemical
shifts of the olefinic hydrogen atoms of cod, which are easily
distinguished from the corresponding ones in the starting
complex 1. In addition, to about 4% of [{Rh(SEGPHOS)(m2-Cl)}2]
almost 6% of the starting complex 1 is present in solution (ac-
cording to 1H NMR spectroscopy), which is once again diagnos-
tic of the presence of an equilibrium similar to the one already
discussed for BINAP (Scheme 2).
PHOS, the main product formed in situ is probably the penta-
coordinated rhodium complex [Rh(DM-SEGPHOS)(nbd)(Cl)].[26]
In MeOH, DM-SEGPHOS reacts with 2 to give the cationic rho-
dium species [Rh(DM-SEGPHOS)(nbd)]+ in quantitative yield.
The above results show that at room temperature the dinu-
clear complexes of the general formula [{Rh(diphosphine)(m2-
Cl)}2] can be quantitatively prepared in situ starting from the
cod complex 1 when using the three ligands BINAP, SEGPHOS,
and DM-SEGPHOS either in CH2Cl2 or THF. Recently we report-
ed comparable results for the structurally similar ligand 6,6’-
Analogously to BINAP, a cationic complex is formed if MeOH
is used instead of THF (the ratio of 1 to ligand is 1:2), although
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No yield increase could be achieved by using 2, which af-
forded a reactivity trend analogous to that observed with 1. In
CH2Cl2 the cationic complex [Rh(DTBM-SEGPHOS)(nbd)]+ was
formed in nearly quantitative yield (97%).
BICP
[{Rh(BICP)(m2-Cl)}2] is quantitatively formed from 1 and two
equivalents of BICP at room temperature in THF as the
31P NMR spectrum of the reaction solution in Figure 6 shows.
Figure 6 also shows the molecular structure of the target com-
plex.
As reported in the literature,[33] such a precatalyst was used
for enyne cyclizations in toluene. In this solvent, we observed
that the in situ preparation effectively affords the catalyst in
quantitative yield (>99%). However, if methanol is added to
the toluene reaction mixture, the formation of the cationic
species is observed (54%), as already noted for BINAP and SEG-
PHOS. If the reaction is carried out in methanol as the sole sol-
vent, the yield in the cationic complex [Rh(BICP)(cod)]+ is prac-
tically quantitative (98%). The molecular structure of this com-
plex with BF4 as counterion has been described previously.[34]
Figure 4. Molecular structure of [{Rh(SEGPHOS)(m2-Cl)}2]; ORTEP, 30% proba-
bility ellipsoids. Hydrogen atoms are omitted for clarity. See below for
a short discussion of the structural data. Selected bond lengths and angles
are listed in Table S1 of the Supporting Information.
bis(diphenylphosphino)-2,2’,3,3’-tetrahydro-5,5’-bi-1,4-benzo-
dioxin (SYNPHOS).[31]
In contrast, in methanol and under otherwise identical con-
ditions, the cationic complexes [Rh(diphosphine)(cod)]+ are
formed, with full conversion of 1 in the case of BINAP.
When using the nbd precursor 2, the yield in the desired di-
nuclear complexes never ex-
ceeded 80%. Hence, 2 should
not be considered the precursor
of choice, and 4 is even less suit-
able.
DTBM-SEGPHOS
Preliminary experiments on the
structurally related ligand DTBM-
SEGPHOS show that in THF the
reaction of 1 with two equiva-
lents of the ligand only provides
Figure 5. 31P NMR spectrum in CD2Cl2 and molecular structure of the cation in [Rh(cod)(SEGPHOS)]BF4; ORTEP,
30% probability ellipsoids. Hydrogen atoms are omitted for clarity. Selected distances [ꢁ] and angles [8]: RhÀP
2.306–2.319(2), RhÀC 2.219–2.308(4); P-Rh-P 90.41–90.68(4).
about 18% of [{Rh(DTBM-
SEGPHOS)(m2-Cl)}2]. The structure
of this complex has been as-
signed based on the similarity of
its spectroscopic features (chem-
ical shifts and J(P,Rh) coupling
constants) with those of the ana-
logue complexes with SEGPHOS
and DM-SEGPHOS. The yield of
the target compound could not
be improved by changing the
solvent to CH2Cl2, although the
cationic complex [Rh(DTBM-
SEGPHOS)(cod)]+ is formed in
50% yield. To confirm its struc-
À
ture, the BF4 analogue was in-
dependently prepared by the re-
action of [Rh(diolefin)(acac)]
Figure 6. 31P NMR spectrum of the solution resulting from the in situ conversion of [{Rh(cod)(m2-Cl)}2] (1) in the
presence of BICP (1:2) at room temperature in [D8]THF and molecular structure of the product [{Rh(BICP)(m2-Cl)}2];
ORTEP, 30% probability ellipsoids. Hydrogen atoms are omitted for clarity. See below for a short discussion of the
structural data. Selected bond lengths and angles are listed in Table S1 of the Supporting Information.
(acac=acetylacetonate)
and
DTBM-SEGPHOS followed by the
addition of HBF4.[32]
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Although m2-Cl-bridged dinuclear rhodium complexes with
either BINAP or SEGPHOS can be conveniently prepared in
CH2Cl2, the latter is unsuitable for [{Rh(BICP)(m2-Cl)}2] which, in
this solvent, rearranges into an unidentified, unsymmetrical
species.
When single crystals of A are dissolved in CH2Cl2, both spe-
cies A and B are again detectable by 31P NMR spectroscopy.
Apparently, the isomers A and B are in equilibrium; NMR spec-
tra recorded at variable low temperatures in THF can be found
in the Supporting Information.
Only very poor yields (ca. 5%) of [{Rh(BICP)(m2-Cl)}2] are ob-
tained if the nbd precursor 2 is used, whether in THF or tolu-
ene.
At variance with what has been observed with the ligands
mentioned so far, which in no case led to a quantitative forma-
tion of the respective neutral dinuclear complexes when using
the nbd precursor 2, complex [{Rh(PPF-PtBu2)(m2-Cl)}2] was
formed quantitatively in THF at room temperature as a mixture
of the two isomers A and B in a ratio of 73:27 (compare also
Scheme 3).
PPF-PtBu2
In situ prepared neutral complexes that contain the Josiphos
derivative PPF-PtBu2 have been used in the ring-opening reac-
tion of oxa- and azabicyclic alkenes.[6] The Merck Group applies
this kind of in situ prepared complex in the industrial hydroge-
nation of an unprotected dehydro b-amino acid.[2c,35,36]
Like the ligands discussed above, PPF-PtBu2 reacts with 2 in
MeOH to give a cationic rhodium species [Rh(PPF-PtBu2)-
(nbd)]+,[36] the molecular structure of which has been previous-
ly reported in the literature.[6b]
NMR spectroscopic analysis of a solution that contains 1 and
two equivalents of PPF-PtBu2 in THF at room temperature re-
vealed the presence of two species in a ratio of 73:27. The two
species A and B (Scheme 3), in which the two diphosphine li-
DiPPF
The molecular structure of the complex that contains the achi-
ral ferrocene ligand DiPPF ([{Rh(DiPPF)(m2-Cl)}2]), prepared in
situ from 1 in THF as described above (97% yield), is shown in
Figure 8.
Scheme 3. Solution equilibrium between (A) trans- and (B) cis-[{Rh(PPF-
PtBu2)(m2-Cl)}2].
gands have either the same or opposite orientation (cis/trans),
such as has been described for a P,N ligand in the literature,[37]
1
exhibit very similar NMR spectroscopic data in the H, 31P, as
well as the 103Rh NMR spectrum.
Figure 8. Molecular structure of [{Rh(DiPPF)(m2-Cl)}2]; ORTEP, 30% probability
ellipsoids. Hydrogen atoms are omitted for clarity. See below for a short dis-
cussion of the structural data. Selected bond lengths and angles are listed in
Table S1 of the Supporting Information.
Recrystallization in THF/MeOH of the precipitate that result-
ed upon removal of the solvent from a solution of [{Rh(cod)-
(m2-Cl)}2] and PPF-PtBu2 (1:2) in THF gave single crystals suitable
for X-ray analysis. The molecular structure shown in Figure 7
indicates that the dimer A [{Rh(PPF-PtBu2)(m2-Cl)}2], in which
equivalent phosphorus are trans to each other, was isolated.
DPPE
As reported by Bosnich et al., the in situ formation of [{Rh-
(DPPE)(m2-Cl)}2] from 1 in toluene can only be achieved at ele-
vated temperatures.[38] The hitherto unpublished molecular
structure of [{Rh(DPPE)(m2-Cl)}2], which is used as precatalyst
for the aforementioned coupling of aldehydes and allenes,[14] is
shown in Figure 9, right.
If the reaction is carried out at room temperature in THF,
a precipitate is formed that accounts for more than 95% of
the rhodium content. The precipitate could be identified as
the known complex [Rh(DPPE)2][Rh(cod)(Cl)2].[39] Its molecular
structure, in addition to the one reported in the literature,[39a]
can be found in the Supporting Information. The 31P NMR
spectrum of the supernatant solution shows a doublet that
could be assigned to the complex [(cod)Rh(m2-Cl)2Rh(DPPE)].
The desired complex [{Rh(DPPE)(m2-Cl)}2] was not formed at all.
Figure 7. Molecular structure of [{Rh(PPF-PtBu2)(m2-Cl)}2]; ORTEP, 30% proba-
bility ellipsoids. Hydrogen atoms are omitted for clarity. See below for
a short discussion of the structural data. Selected bond lengths and angles
are listed in Table S1 of the Supporting Information.
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Figure 9. Molecular structure of [(cod)Rh(m2-Cl)2Rh(DPPE)] (left); molecular
structure of [{Rh(DPPE)(m2-Cl)}2] (right); ORTEP, 30% probability ellipsoids. Hy-
drogen atoms are omitted for clarity. See below for a short discussion of the
structural data. Selected bond lengths and angles are listed in Table S1 and
S2 of the Supporting Information).
Figure 10. Molecular structure of [Rh(DIPAMP)2][Rh(C2H4)2(Cl)2]; ORTEP, 30%
probability ellipsoids. Hydrogen atoms are omitted for clarity. See below for
a short discussion of the structural data. Selected bond lengths and angles
are listed in Table S3 of the Supporting Information.
Single crystals suitable for X-ray analysis could be isolated from
the solution (Figure 9, left).
By the use of precursors 2, 3, and the m2-OH-bridged com-
plex 4 and two equivalents of DIPAMP in THF, similar results
were obtained. The target compound is formed in very poor
yield (10% maximum), whereas, according to the 31P NMR spec-
tra, the main species obtained from the in situ synthesis (up to
94% yield) is the complex that contains the cation [Rh-
(DIPAMP)2]+.
When the crystals are redissolved, an equilibrium analogous
to the one described above for BINAP and SEGPHOS is estab-
lished in solution (Scheme 2).
If the nbd precursor 2 is used instead of 1 under otherwise
identical reaction conditions, a precipitate is formed as well, al-
though in much lower amount, which was identified as the an-
alogue complex [Rh(DPPE)2][Rh(nbd)(Cl)2] due to identical
Regardless of the solvent used (CH2Cl2, THF, iPrOH, MeOH,
acetone), the ionic complex [Rh(DIPAMP)2][Rh(C2H4)2(Cl)2] is
mostly formed when using the ethylene complex [{Rh(C2H4)2-
(m2-Cl)}2] (5). Its molecular structure is shown in Figure 10.[42]
Although the cationic moiety of the complex is known, to
the best of our knowledge the anion has not yet been report-
ed. NMR spectra recorded at variable low temperatures in
CH2Cl2 as well as 103Rh NMR spectra of [Rh(DIPAMP)2][Rh-
(C2H4)2(Cl)2] can be found in the Supporting Information.
Besides those with DPPE[39a] and 1,2-bis(dimethylphosphino)-
ethane (DMPE),[43] the aforementioned complexes are the only
examples reported so far that comprise a square-planar coordi-
nated cation [Rh(PP)2]+ and anion [Rh(olefin)2(Cl)2]À. A compar-
ison of their structural features can be found in the Supporting
Information.
1
31P NMR spectrum and coordinated nbd found in the H NMR.
The hydrogen atoms of nbd are distinguisheable from the cor-
responding ones in the starting complex 2.
The desired complex [{Rh(DPPE)(m2-Cl)}2], which is soluble in
THF, was obtained in approximately 89% yield according to
the 31P NMR spectrum of the homogeneous phase.[40] The in
situ formation starting from 3 leads to 84% of the target com-
plex [{Rh(DPPE)(m2-Cl)}2] without the formation of a precipitate,
which suggests that the coe complex 3 is a better precursor
than the cod complex 1 and the nbd complex 2, at least at
room temperature.
The m2-OH-bridged precursor 4 seems to be unsuitable for
the in situ preparation of [{Rh(DPPE)(m2-OH)}2], which is formed
in only 4% yield in THF at room temperature.[41]
DIOP
DIPAMP
The in situ reaction of 1 with two equivalents of DIOP in THF
at room temperature is not selective in affording the target
compound [{Rh(DIOP)(m2-Cl)}2] (36%). [(cod)Rh(m2-Cl)2Rh(DIOP)]
(2%) and other unidentified species are observed by NMR
spectroscopic analysis of the resulting solution (see Figure 11).
Neither CH2Cl2 nor acetone afforded better yields.[44]
Upon addition of two equivalents of DIPAMP to 1 in THF at
room temperature, a precipitate is formed that, analogously to
DPPE, very likely is the ionic complex [Rh(DIPAMP)2][Rh-
(cod)(Cl)2]. The complex is much more soluble than the DPPE
analogue. Two other species in addition to the ionic complex
were detected in solution. As suggested by the comparison
with the analogous complexes of DPPE, these are most proba-
bly the intermediate [(cod)Rh(m2-Cl)2Rh(DIPAMP)] and the
target compound [{Rh(DIPAMP)(m2-Cl)}2], which, however, rep-
resents only 5% of the total rhodium content. The yield of
[{Rh(DIPAMP)(m2-Cl)}2] could be increased to 83% by very slow
dropwise addition of the ligand to 1 at À908C.
However, we were able to show that by very slow addition
of a solution of the ligand to 1 the yield of the target com-
pound can be increased to about 80%. If the in situ synthesis
is carried out in toluene at 1258C, following the procedure de-
scribed by Bosnich et al.,[38] the yield further improves to reach
90% (see the Supporting Information).[45]
In MeOH, the cationic complex [Rh(DIOP)(cod)]+—probably
with ClÀ as the counterion—is formed in 87% yield, similarly
to what observed with BINAP, SEGPHOS, BICP, and PPF-PtBu2.
The identity of the compound could be established by com-
As already observed with BINAP, SEGPHOS, BICP, and so
forth, the ligand exchange with 1 in MeOH leads to the cation-
ic complex [Rh(DIPAMP)(cod)]+ as the main species (88%).
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perature and by adding the
ligand very slowly. Indeed, when
the synthesis was carried out at
À788C,
[{Rh(Me-DuPHOS)(m2-
Cl)}2] was obtained in quantita-
tive yield.
A similar behavior was ob-
served when using the nbd and
coe precursors 2 and 3 at room
temperature. They afford the de-
sired product in 56 and 82%
yield, respectively, together with
the known cationic complex
[Rh(Me-DuPHOS)2]+, in 41 and
12% yield, respectively.
Figure 11. 31P NMR spectrum of a solution of [{Rh(cod)(m2-Cl)}2] (1) and two equivalents of DIOP in THF at room
temperature.
parison of the spectroscopic data with those reported in sever-
al publications.[46]
If 4 is used as the rhodium source, the in situ procedure is
less efficient. The yield of [{Rh(Me-DuPHOS)(m2-OH)}2] is at best
40% and less selective. In fact, a number of unidentified spe-
cies are formed.
With the nbd complex 2 and two equivalents of DIOP at
room temperature in THF, only 1% of the target compound
[{Rh(DIOP)(m2-Cl)}2] is formed. The highest yield in THF at room
temperature (74%) was obtained using the coe precursor 3.
DPPB
Only traces of the desired complex [{Rh(DPPB)(m2-Cl)}2] are
formed by treating the cod complex 1 with two equivalents of
achiral DPPB, whether at room temperature in THF or at ele-
vated temperature (1258C) in toluene.[38] Several unidentified
complexes were detected as shown in Figure 13a.
Me-DuPHOS
Addition of Me-DuPHOS to [{Rh(cod)(m2-Cl)}2] (1) in a ratio of
2:1 in THF at room temperature similarly resulted in the forma-
tion of more than one species. These, however, could be iden-
tified as the target compound [{Rh(Me-DuPHOS)(m2-Cl)}2]
(62%),[47] the intermediate [(cod)Rh(m2-Cl)2Rh(Me-DuPHOS)],
and the cationic complex [Rh(Me-DuPHOS)2]+. Regardless of
the use of precursor 1, 2, or 3, the reaction with two equiva-
lents of Me-DuPHOS always affords the cationic complex as
suggested by 31P NMR spectroscopy. The molecular structure
of [{Rh(Me-DuPHOS)(m2-Cl)}2] and the corresponding 31P NMR
spectrum are shown in Figure 12.
At room temperature in THF, precursor 2 quantitatively leads
to the formation of just one species, which, however, was iden-
tified as the monomeric pentacoordinated neutral complex
[Rh(DPPB)(nbd)(Cl)][26], Figure 13b.
The best precursor for the in situ synthesis of [{Rh(DPPB)(m2-
Cl)}2] is the coe complex 3, which, under the same reaction
conditions, provided quantitative yield (99%) of the desired
complex, Figure 13c.
Equally unselective are the in situ procedures that rely on 4.
In THF at room temperature, the target compound accounts
for only 13% of the rhodium content. In toluene, under other-
wise identical conditions, the yield did not exceed 45%.
The formation of the cationic rhodium bis(diphosphine)
complex [Rh(diphosphine)2]+ is an undesired side reaction in
the in situ synthesis of [{Rh(diphosphine)(m2-Cl)}2]. It can be
suppressed, at least in this case, by lowering the reaction tem-
DPPP
The reaction of
1 with two
equivalents of achiral DPPP in
THF at room temperature, a pro-
cedure that is applied for the in
situ formation of the catalyst
precursor in, for example, the
Pauson–Khand-type reaction of
enynes,[11b] affords the target
compound
[{Rh(DPPP)(m2-Cl)}2]
(44%) and the monomeric pen-
tacoordinated neutral complex
[Rh(DPPP)(cod)(Cl)] (50%). Both
complexes have been fully char-
acterized by X-ray analysis and
NMR spectroscopy.[26]
Figure 12. 31P NMR spectrum of the solution obtained by dissolving crystals of [{Rh(Me-DuPHOS)(m2-Cl)}2] in THF at
room temperature and the X-ray structure of [Rh{(S,S)-Me-DuPHOS}(m2-Cl)}2] (ORTEP, 30% probability ellipsoids).
Hydrogen atoms are omitted for clarity. See below for a short discussion of the structural data. Selected bond
lengths and angles are listed in Table S1 of the Supporting Information.
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logue neutral complexes that
have been found in the Cam-
bridge Structural Database.
The rhodium complexes dis-
play a distorted square-planar
coordination geometry. The RhÀ
P bond lengths fall in the ex-
pected range between 2.176(1)
and 2.241(1) ꢁ. In the case of the
C1-symmetric ligand PPF-PtBu2,
in which the two phosphorus
donors possess different elec-
tronic properties, the two RhÀP
distances are clearly different in
the complex.
The bite angles (P-Rh-P) fall in
the range between 75.23(10)8
for the bidentate ligands in [{Rh-
(DtBPM)(m2-Cl)}2]
(YUWFEB;
DtBPM=bis(di-tert-butylphosphi-
no)methane)[48] and 100.29(4)8 in
[{Rh(DiPPF)(m2-Cl)}2]. The bite
angles in the seven-membered
chelated complexes that contain
BINAP, SEGPHOS, and BICP are
around 908. The RhÀRh distances
fall between 3.025(1) (DPPE) and
3.718(2) ꢁ (DiPPF). The longest
distance reported so far, 3.511 ꢁ,
was found in the analogous
complex that contains the MeO-
F12-BIPHEP ligand.[49] The authors
Figure 13. 31P NMR spectra of the reaction solution of the in situ ligand exchange of a) [{Rh(cod)(m2-Cl)}2] (1), b)
[{Rh(nbd)(m2-Cl)}2] (2), and c) [{Rh(coe)2(m2-Cl)}2] (3) with DPPB (1:2) in THF at room temperature.
Like the case of DPPB, the reaction of the nbd complex 2
with DPPP in THF quantitatively leads to a single complex, the
corresponding monomeric pentacoordinated neutral complex
[Rh(DPPP)(nbd)(Cl)].
explain the weakening of the interaction between Rh···Rh as
a consequence of the less s-donating ability of the phospho-
rus donors, which is unfavorable for the dz2 orbital of Rh.
Characteristic bond lengths and angles of the intermediates
with the general formula [(diolefin)Rh(m2-Cl)2Rh(diphosphine)]
described in this study are collected in Table S2 of the Support-
ing Information. The RhÀP distances as well as the bite angles
are comparable to those found in the complexes reported in
Tables S1 and S2 of the Supporting Information. Quite impres-
sive is the increase in the RhÀRh distance from 3.287(1) ꢁ in
[{Rh(BINAP)(m2-Cl)}2] to 3.581(1) ꢁ in [(cod)Rh(m2-Cl)2Rh(BINAP)].
This is clearly caused by an increase of the angle between the
planes Cl-Rh1-Cl/Cl-Rh2-Cl from 126.18 to 168.40(4)8. The
reason for this structural modification is not clear; in the case
of the complexes [{Rh(DPPE)(m2-Cl)}2]/[(cod)Rh(m2-Cl)2Rh(DPPE)],
the RhÀRh distances are, on the contrary, very similar.
Even in this case, however, the target compound [{Rh(DPPP)-
(m2-Cl)}2] can be successfully synthesized in 98% yield starting
from the coe complex 3 under otherwise identical conditions.
When the DPPP ligand is used in substoichiometric amounts
(3/DPPP 1:1) in THF at room temperature, the conversion ap-
parently is not complete, as shown by the 31P NMR spectrum.
The formation of the target compound (3%), the presence of
the precursor (3%), and the absence of free ligand in solution
suggest that an equilibrium like the one already described for
BINAP and SEGPHOS (Scheme 2) is actually established.
As already shown in the case of DPPB, the use of the m2-OH-
bridged complex 4 affords [{Rh(DPPP)(m2-OH)}2] in just 17%
yield.
Conclusion
The results of the systematic investigations reported above
show that the commonly applied in situ method for the prepa-
ration of neutral Rh complexes of the type [{Rh(PP)(m2-X)}2] by
addition of two equivalents of disphosphine ligand to precur-
sors 1–5 might not be considered as straightforward and selec-
tive as generally thought. The outcome of the in situ proce-
Structural details
Characteristic bond lengths and angles of six neutral com-
plexes with the general formula [{Rh(diphosphine)(m2-Cl)}2] de-
scribed in this study are collected in Table S1 in the Supporting
Information. Also included are the values that refer to six ana-
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dure, which is applied in a plethora of catalytic processes, is
strongly affected by the nature of the diolefin present in the
catalyst precursor used as the rhodium source, the ancillary (di-
phosphine) ligand, the solvent, and the temperature.
in situ procedure carried out in MeOH always led to cationic
complexes, which were frequently formed in quantitative yield.
The result of the in situ formation of the active species
mainly determines the outcome of the subsequent catalysis. If
several metal species are formed, the activity as well as the se-
lectivity of the catalytic process will be affected, since it cannot
be assumed that different rhodium complexes all promote the
same reaction mechanism.
The results show that the expected neutral complexes are
obtained quantitatively just with a couple of ligands under se-
lected conditions. For example, in THF at room temperature
the yield in the desired complex is practically quantitative with
the ligands BINAP, SEGPHOS, DM-SEGPHOS, BICP, PPF-PtBu2,
and DiPPF if the cod precursor 1 is used. Under otherwise
identical conditions the best precursor is coe complex 3 for
DPPB and DPPP as ligands.
Although it is not possible to define a common behavior for
the formation of the complexes described above, it is still pos-
sible to recognize some trends:
However, our results show that, depending on the reaction
conditions, either different rhodium species exist simultaneous-
ly (e.g., DIPAMP) or in some cases unexpected complexes are
formed in quantitative yield (e.g., DPPB or DPPP). The general
structures of such unexpected complexes, which were charac-
terized by NMR spectroscopy and, where possible, also by X-
1) The exchange of the diolefins in the catalyst precursor pro-
ceeds stepwise and, as the results presented so far prove,
the first diolefin is replaced considerably faster than the
second one.
2) In MeOH such exchange unexpectedly affords complexes
of the type [Rh(diphosphine)(diolefin)]+.
ray
analysis,
are
the
following:
[(diolefin)Rh(m2-
3) An equilibrium exists, not yet described, between the inter-
mediate product of such exchange on one side and the
catalyst precursor and desired product on the other.
Cl)2Rh(diphosphine)], [Rh(diphosphine)(diolefin)]+, [Rh(diphos-
phine)2]+, and [Rh(diphosphine)(diolefin)(Cl)].[50] They are de-
picted in Scheme 4.
Another important issue for catalysis is the temperature at
which the reaction is performed, since the nature of the spe-
cies that arises from the in situ synthesis of the catalyst precur-
sor also depends on the temperature. This influence of tem-
perature on the course of such in situ synthesis is strikingly evi-
dent in the case of ligands DPPE and Me-DuPHOS. The forma-
tion of [{Rh(Me-DuPHOS)(m2-Cl)}2] from 1 in THF is quantitative
only if the reaction is carried out at À788C, whereas that of
[{Rh(DPPE)(m2-Cl)}2] requires 1258C.
On the basis of the above results, it might be concluded
that special care should be taken in the evaluation of catalyst
performance in those processes that rely on the in situ tech-
nique for “catalyst” preparation. This is because the actual cat-
alyst might either be only one of several species present in so-
lution or, in the worst case, not the expected one.
Acknowledgements
Last but not least to affect the course of the in situ forma-
tion of the rhodium species is the solvent, as shown, for exam-
ple, for BINAP (Figure 3). A large number of catalytic reactions
are carried out in CH2Cl2. Based on our experience, mixtures of
unknown species often resulted if CH2Cl2 was used. Instead the
We thank Dr. Elisabetta Alberico (Istituto di Chimica Biomoleco-
lare, CNR, Italy) for helpful discussions and the DFG for financial
support. We thank Takasago International Corporation for gener-
ously supplying us with the SEGPHOS, DM-SEGPHOS, and DTBM-
SEGPHOS ligands.
Keywords: cations
·
ligand
effects · P ligands · rhodium ·
solvent effects
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[40] Owing to the small amounts handled in the experiment, it was very dif-
ficult to exactly determine the amount of precipitated solid.
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(m2-OH)}2]: 31P NMR (202.4 MHz, C6D6): d=74.4 ppm (d, J=190 Hz).
[42] The ethylene molecules coordinated to rhodium in the anion were only
detectable in the 1H NMR spectrum at low temperature. See the Sup-
porting Information.
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[17] All spectra, further comments, and other information that support our
conclusions are to be found in the Supporting Information.
[18] The molecular structure of [{Rh(BINAP)(m2-Cl)}2]—single crystals were
isolated from CH2Cl2—has already been reported in reference [16].
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Cl)}2].
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[45] By simple recrystallization—that is, from iPrOH—[{Rh(DIOP)(m2-Cl)}2] can
be isolated as a pure compound.
[23] C. Fischer, T. Beweries, A. Preetz, H.-J. Drexler, W. Baumann, S. Peitz, U.
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[47] [{Rh(Me-DuPHOS)(m2-Cl)}2], like other dinuclear complexes, is surprising-
ly air-sensitive. After just 10 min exposure of the solid to air, 24% of the
ligand is oxidized. See also: A. Cꢆtꢇ, A. A. Boezio, A. B. Charette, Angew.
Chem. 2004, 116, 6687–6690; Angew. Chem. Int. Ed. 2004, 43, 6525–
6528. (Literature: d=72.3 ppm in MeOH; this study, d=69.6 ppm in
THF.)
[24] As a result, the color change often observed during the in situ proce-
dure (e.g., from orange to red for 1 as starting complex and BINAP as
ligand) does not necessarily account for the formation of the desired
target complex, but rather only for the formation of the intermediate
[(diolefin)Rh(m2-Cl)2Rh(diphosphine)].
[25] a) C. A. Busacca, R. Raju, N. Grinberg, N. Haddad, P. James-Jones, H. Lee,
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2001, 66, 8854–8858; For Rh complexes with oxidized BINAP see: c) A.
[48] P. Hofmann, C. Meier, W. Hiller, M. Heckel, J. Riede, M. U. Schmidt, J. Or-
ganomet. Chem. 1995, 490, 51–70.
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[50] Similar observations were found for the ligand-exchange process with
[Rh(cod)(acac)] as starting material and diphosphine ligands, which
leads to five-membered chelate ring complexes (e.g., DPPE, DIPAMP,
Me-DuPHOS). As described here, the formation of the cationic complex
[Rh(PP)2]+ can be avoided by slow addition of the ligand at low tem-
perature (À788C). See: K. Angermund, W. Baumann, E. Dinjus, R. Forni-
Received: July 8, 2014
Revised: August 18, 2014
Published online on && &&, 0000
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FULL PAPERS
Rh revisited: The commonly applied in
situ method for the preparation of
neutral Rh complexes of the type
A. Meißner, A. Preetz, H.-J. Drexler,
W. Baumann, A. Spannenberg, A. Kçnig,
D. Heller*
[{Rh(PP)(m2-X)}2] by addition of diphos-
phine ligand (2 equiv) to different rhodi-
um precursors might not be as straight-
forward and selective as generally
thought. The outcome is strongly affect-
ed by the nature of the diolefin present
in the ancillary (diphosphine) ligand, the
catalyst precursor, the solvent, and the
temperature (see figure).
&& – &&
In Situ Synthesis of Neutral Dinuclear
Rhodium Diphosphine Complexes
[{Rh(diphosphine)(m2-X)}2]: Systematic
Investigations
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemPlusChem 0000, 00, 1 – 13
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These are not the final page numbers!