Atom-transfer radical reactions under mild conditions with [{RuCl 2(1,3,5-C6H3iPr3)}2] and PCy3 as the catalyst precursors

Article abstract of DOI:10.1002/anie.200462037

Simply mixing the complex [{RuCl₂(1,3,5-C₆H ₃iPr₃)}₂] (1) with PCy₃ is sufficient to generate a catalyst suitable for highly efficient atom-transfer radical addition and polymerization reactions at exceptionally low temperatures (see scheme). The steric congestion at the metal caused by the bulky ligands is thought to be essential for the reactivity of the catalyst.

Full text of DOI:10.1002/anie.200462037

Communications
Living Polymerization
Atom-Transfer Radical Reactions under Mild
Conditions with [{RuCl2(1,3,5-C6H3iPr3)}2] and
PCy3 as the Catalyst Precursors**
Laurent Quebatte, Michel Haas, Euro Solari,
Rosario Scopelliti, Quoc T. Nguyen, and Kay Severin*
The complex [RuCl2(p-cymene)(PCy3)] (1) has emerged as a
versatile catalyst precursor for synthetically important trans-
formations such as ring-closing[1] and ring-opening olefin-
metathesis reactions[2] and atom-transfer radical polymeri-
zations (ATRP).[3] An attractive feature of this catalyst is the
fact that it can be prepared in situ from commercially
available [{RuCl2(p-cymene)}2] (2) and PCy3. Somewhat
surprising was the observation by Demonceau and co-work-
ers that complex 1—despite its good activity in ATRP
reactions—fails to catalyze atom-transfer radical additions
[*] L. Quebatte, Dr. M. Haas, Dr. E. Solari, Dr. R. Scopelliti,
Dr. Q. T. Nguyen, Prof. K. Severin
Institut des Sciences et Ingꢀnierie Chimiques
ꢁcole Polytechnique Fꢀdꢀrale de Lausanne (EPFL)
1015 Lausanne (Switzerland)
Fax: (+41)21-693-9305
E-mail: kay.severin@epfl.ch
Dr. Q. T. Nguyen
Institute of Materials, EPFL
1015 Lausanne (Switzerland)
[**] This work was supported by a postdoctoral fellowship from the
Ministꢂre de la Culture, de l’Enseignement Supꢀrieur et de la
Recherche, Luxembourg (M.H.) and by the Swiss National Science
Foundation. We thank Prof. Harm-Anton Klok (EPFL) for his
support with the GPC measurements.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Angewandte
Chemie
(ATRA) of CCl4 to olefins,[4] although ATRP and ATRA are
mechanistically very similar.[5] Herein, we show that by
replacing the (p-cymene)ruthenium complex 2 with the
trisisopropylbenzene complex [{RuCl2(1,3,5-C6H3iPr3)}2] (3)
it is possible to catalyze both ATRP and ATRA reactions
under exceptionally mild conditions with high efficacy.
Activation of 1 is thought to proceed by a thermally or
photochemically induced replacement of the arene
ligand.[1–3,6] We reasoned that a sterically more demanding
p ligand might facilitate this replacement due to steric
congestion with the PCy3 ligand. The commonly used
hexamethylbenzene complex [{RuCl2(C6Me6)}2] was not con-
sidered because of its low solubility and because it had been
reported that the reaction with PCy3 does not give the
monomeric complex [RuCl2(C6Me6)(PCy3)].[7] Instead, we
focused on the trisisopropylbenzene complex 3, which can be
obtained easily from 2 by arene exchange.[8]
had no influence from a light source. These results were
surprising in view of the higher intrinsic reactivity of CCl4 but
in accordance with the observation that complex 1 is not able
to promote the addition of CCl4 to styrene.[4]
The time course of reactions between styrene and CHCl3
catalyzed by complex 3 in the presence of one and two
equivalents of PCy3 is depicted in Figure 1 ([3] = 1 mol%).
First, we investigated the ATRP of methyl methacrylate
(MMA) by using ethyl 2-bromo-2-methylpropionate as the
initiator and a mixture of 3 and PCy3 as the catalyst precursors
(3/PCy3/initiator/MMA = 1:2:4:1600). The reaction was car-
ried out at a temperature of only 508C, which is significantly
below the 80–858C commonly employed for ruthenium-
catalyzed MMA polymerizations.[3,9,10] After 24 h, PMMA
could be isolated in 90% yield. The polymer was found to
¯
¯
Figure 1. Time course of reactions between styrene and CHCl3 cata-
have a very narrow molecular-weight distribution of Mw/Mn =
&
¯
¯
*
*
lyzed by 3 + 2PCy3 ( ), 3 + 1PCy3 ( ), and 2 + 2PCy3 ( ). Reaction
conditions: 2 or 3/styrene/CHCl3 =1:100:150; [2] or [3]=13.8 mm, tol-
uene, 408C, no light. The conversion is based on the consumption of
styrene as determined by GC.
1.19 (Mn and Mw are the number-average and weight-average
molecular mass, respectively; Mn = 35000, initiation effi-
¯
ciency f = 1.03).[11] Furthermore, a linear relationship of
ln([M]0/[M]) versus time was observed ([M] is the concen-
tration of the monomer; see the Supporting Information),
which suggests that polymerization had occurred in a
controlled fashion. A radical mechanism is supported by the
observed tacticity of rr:rm:mm = 63:30:7 and the fact that the
reaction was completely inhibited by galvinoxyl. A compar-
ison of the initial turnover frequencies (TOF)[12] revealed that
under these mild conditions, the new catalyst 3/PCy3 (TOF =
59 hÀ1) is one order of magnitude more active than the
previously reported system 2/PCy3 (TOF = 5 hÀ1), which was
considered to be one of the most active Ru-based catalyst
systems described so far.[13] Ethyl methacrylate can likewise
An induction period is clearly visible, which indicates that
catalyst activation must take place. A Ru/PCy3 ratio of 1:1 is
advantageous, although the reaction rates at a later stage of
this reaction (4–9 h) are similar to the reaction rates observed
when substoichiometric amounts of PCy3 are present. As in
the case of the polymerization reactions, the nature of the
p ligand was found to be crucial: reactions performed with the
p-cymene complex 2 instead of the trisisopropylbenzene
complex 3 gave zero conversion.[15]
To test the scope and the limitations of the new catalyst
system, 3/PCy3, we investigated ATRA reactions with differ-
ent olefins (Table 1). The CHCl3 adducts of the aromatic
olefins p-chlorostyrene, p-methoxystyrene, 1-vinylnaphtha-
lene, and styrene (entries 1–4) were obtained in yields of
between 69 and 95% with a ruthenium catalyst concentration
of 2–6 mol% at 408C (entries 1–3) or 608C (entry 4). It
should be noted that for a Ru-based catalyst, synthetically
useful yields above 80% have been described only for the
carbene complex [RuCl2(CHPh)(PCy3)2] (2.5–7.5 mol%, 65–
808C).[16] Using a substrate/3 ratio of 1000:1, we were able to
obtain the CHCl3 adduct of styrene in 57% yield after two
weeks. This corresponds to 285 turnovers per ruthenium
atom, which is, to the best of our knowledge, the highest value
ever reported.[17] MMA is a less suited substrate because
polymerization becomes a significant side reaction, in accord-
ance with the results described above (entry 5).
be polymerized by 3/PCy3 at 508C in a controlled fashion
(yield after 24 h: 89%, Mn = 40200, Mw/Mn = 1.10, f = 1.01).
Reactions with butyl acrylate as the monomer gave also a very
[11]
¯
¯
¯
high yield (99%) but revealed a bimodal molecular-weight
[14]
¯
¯
¯
distribution (Mn = 158500, Mw/Mn = 3.36, f = 0.32). Reac-
tions with styrene, on the other hand, gave only low amounts
of polymer (yield: 9%). It should be noted, however, that the
ATRP of styrene is generally carried out at temperatures
above 1008C.[9]
Encouraged by the success of the new catalyst system in
ATRP reactions, we investigated the ATRA of CCl4 and of
CHCl3 to styrene. Again, the catalyst was prepared in situ by
mixing 3 with two equivalents of PCy3 (3/styrene/CHCl3 or
CCl4 = 1:300:450). Two reactions were carried out in toluene
at 408C: one in the presence of a light source of moderate
intensity, the other in the dark. Reactions with CCl4 gave
nearly zero conversions, whereas a conversion of 65% (yield:
63%) was observed after 24 h for reactions with CHCl3 that
To obtain more information about the mode of activation
for reactions with the new catalyst system, we examined
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Table 1: ATRA reactions catalyzed by 3/PCy3 or by 5.
tetranuclear dinitrogen complex (6) of low
solubility had formed.
Entry
Cat.
Substr. A
Substr. B
T [8C]
t [h]
Conv./Yield [%]
Complex 5 represents the first structur-
ally characterized product of an arene-
displacement reaction with [{RuCl2(ar-
ene)}2] and PCy3 (see Figure 2). A plausible
mechanism of formation is the liberation of
trisisopropylbenzene from 4 to generate
[RuCl2(PCy3)]n, which subsequently reacts
with unconverted 3 and N2 to give the
mixed, chloro-bridged complex 5.[19] The
1
2
3
4
5
6
7
8
9
3/PCy3
3/PCy3
3/PCy3
3/PCy3
styrene
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CCl4
40
40
40
60
40
40
40
40
40
40
48
48
48
48
48
1
48
2
5
95/91[a]
93/84[a]
98/95[b]
73/69[b]
92/15[a]
25/19[c]
88/93[c]
99/98[d]
89/65[d]
67/66[d]
p-chlorostyrene
p-methoxystyrene
1-vinylnaphthalene
MMA
styrene
styrene
styrene
MMA
1-decene
3/PCy3
5
5
5
5
5
CCl4
CCl4
10
5
two
{(1,3,5-C6H3iPr3)Ru(m-Cl)3RuCl(P-
[a] 3/PCy3/olefin/CHCl3 =1:2:100:150, [3]=13.8 mm; [b] 3/PCy3/olefin/CHCl3 =3:6:100:150, [3]=
41.4 mm; [c] 5/olefin/CHCl3 =1:200:300; [5]=6.9 mm; [d] 5/olefin/CCl4 =1:600:900; [5]=2.3 mm. All
reactions were performed in toluene. The conversion (conv.) is based on the consumption of the olefin
and the yield is based on the formation of product as determined by GC or 1H NMR spectroscopy after
the time given.
Cy3)N} fragments are related by a crystallo-
graphic inversion center. The dinitrogen
ligand bridges the fragments in a nearly
linear fashion (Ru1-N1-N1A = 174.9(3)8)
À
and the N N bond length of 1.120(4) ꢀ is
similar to that reported for other complexes
1
[20]
solutions of 3 and PCy3 in [D8]toluene by H and 31P NMR
spectroscopy. At room temperature, an equilibrium between
3, PCy3, and the monomeric complex [RuCl2(1,3,5-
C6H3iPr3)(PCy3)] (4) was rapidly established, with 25% of
the ruthenium being present in the form of 3 and 75% in the
form of the monomer 4 (Scheme 1). This reaction was
with Ru-N N-Ru units.
The Ru Cl (Ru1/Ru2 Cl1/Cl2/
ꢀ
À
À
À
À
Cl3 = 2.41–2.53 ꢀ, Ru1 Cl4 = 2.3759(6) ꢀ) and the Ru P
(2.3121(7) ꢀ) bond lengths are within the expected range.
Figure 2. Molecular structure of complex 5 in the crystal. The hydrogen
atoms and the solvent molecules are omitted for clarity.
The tetranuclear complex 5 is a very active ATRA
catalyst. For the addition of CHCl3 to styrene, for example,
a yield of 19% was observed after only one hour at 408C
(Table 1, entry 6). The final yield after 48 h was similar to that
found for 3/PCy3 (entry 7). Interestingly, complex 5 can also
effect the addition of CCl4 to olefins (entries 8–10). The
observed TOFs are comparable to those of the best ATRA
catalysts described so far, despite the low reaction temper-
ature.[21] In this context it is interesting to note that two other
Scheme 1. In the presence of PCy3, the dimeric complex 3 is in equili-
brium with the monomeric complex 4. Partial loss of the arene ligand
leads to the formation of the tetranuclear complex 5.
followed by slow liberation of the arene ligand. At 408C,
this displacement proceeded with a half-life of t1/2 = 5 h. When
the reaction mixture was allowed to cool to room temper-
ature, an orange, crystalline complex precipitated. This
compound was identified as the tetranuclear complex 5
based on elemental and crystallographic analysis.[18] For
comparison, the reaction between the p-cymene complex 2
and PCy3 was investigated. In this case, the equilibrium was
found to be completely on the side of the monomeric complex
1, and arene displacement required significantly harsher
reaction conditions (t1/2 = 13 h, 608C). When the heating was
stopped after 12 h, an orange complex precipitated. Again,
the results of the elemental analysis suggested that a
ꢀ
À
À
complexes containing a {Ru N N Ru} structural motif were
reported to catalyze atom-transfer radical reactions.[21a,22] For
both complexes it was suggested that catalyst activation
proceeds by a CCl4-induced loss of the N2 ligand, and a similar
mode of activation appears likely for reactions with 5.
Since the mixture of 3 and PCy3 was inactive for CCl4
additions, CCl4 seemed to interfere with catalyst activation.
Control experiments showed that this is indeed the case. CCl4
was found to react immediately with PCy3 to give a
trichloromethylphosphonium salt.[23] The phosphine is thus
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Angewandte
Chemie
Chem. 2000, 606, 55 – 64; c) A. Hafner, A. Mꢁhlebach, P. A.
removed from the equilibrium between 3 and 4, thus
preventing the formation of the catalytically active ruthenium
complex.
van der Schaaf, Angew. Chem. 1997, 109, 2213 – 2216; Angew.
Chem. Int. Ed. Engl. 1997, 36, 2121 – 2124; d) A. Demonceau,
A. W. Stumpf, E. Saive, A. F. Noels, Macromolecules 1997, 30,
3127 – 3136; e) A. W. Stumpf, E. Saive, A. Demonceau, A. F.
Noels, J. Chem. Soc. Chem. Commun. 1995, 1127 – 1128; f) A.
Demonceau, A. F. Noels, E. Saive, A. J. Hubert, J. Mol. Catal.
1992, 76, 123 – 132.
In summary, we have demonstrated that a mixture of
complex 3 and PCy3 can be used to efficiently catalyze atom-
transfer radical reactions at exceptionally low temperatures.
Less than 0.07 mol% of complex 3 is required to quantita-
tively polymerize methacrylates in a controlled fashion at
508C. For the ATRA of the notoriously difficult substrate
CHCl3 to aromatic olefins, synthetically useful yields of
> 80% can be obtained with only 1–3 mol% of complex 3 at
408C. We have evidence that catalyst activation proceeds by a
PCy3-induced substitution of the arene ligand, and for the first
time a product of such a reaction has been structurally
characterized.
[3] a) F. Simal, D. Jan, L. Delaude, A. Demonceau, M.-R. Spirlet,
A. F. Noels, Can. J. Chem. 2001, 79, 529 – 535; b) F. Simal, S.
Sebille, L. Hallet, A. Demonceau, A. F. Noels, Macromol. Symp.
2000, 161, 73 – 85; c) F. Simal, D. Jan, A. Demonceau, A. F. Noels
in Controlled/Living Radical Polymerization (Ed.: K. Maty-
jaszwski), ACS Symposium Series 768, ACS, Washington, 2000,
chap. 16; d) F. Simal, A. Demonceau, A. F. Noels, Angew. Chem.
1999, 111, 559 – 562; Angew. Chem. Int. Ed. 1999, 38, 538 – 540.
[4] F. Simal, A. Demonceau, A. F. Noels, Tetrahedron Lett. 1999, 40,
5689 – 5693.
[5] a) M. Kamigaito, T. Ando, M. Sawamoto, Chem. Rev. 2001, 101,
3689 – 3745; b) K. Matyjaszewski, J. Xia, Chem. Rev. 2001, 101,
2921 – 2990.
[6] For an early report on arene-replacement reactions of [RuCl2(p-
cymene)(PR3)] complexes see: M. A. Bennett, A. K. Smith, J.
Chem. Soc. Dalton Trans. 1974, 233 – 241.
[7] M. A. Bennett, T.-N. Huang, J. L. Latten, J. Organomet. Chem.
1984, 272, 189 – 205.
[8] J. W. Hull Jr., W. L. Gladfelter, Organometallics 1984, 3, 605 –
613.
[9] For some recent examples of Ru-catalyzed ATRP reactions see:
a) T. Opstal, F. Verpoort, Angew. Chem. 2003, 115, 2982 – 2985;
Angew. Chem. Int. Ed. 2003, 42, 2876 – 2879; b) L. Delaude, S.
Delfosse, A. Richel, A. Demonceau, A. F. Noels, Chem.
Commun. 2003, 1526 – 1527; c) T. Opstal, K. Couchez, F.
Verpoort, Adv. Synth. Catal. 2003, 345, 393 – 401; d) F. Simal, S.
Delfosse, A. Demonceau, A. F. Noels, K. Denk, F. J. Kohl, T.
Weskamp, W. A. Herrmann, Chem. Eur. J. 2002, 8, 3047 – 3052;
e) M. Kamigaito, Y. Watanabe, T. Ando, M. Sawamoto, J. Am.
Chem. Soc. 2002, 124, 9994 – 9995; f) B. De Clercq, F. Verpoort,
Macromolecules 2002, 35, 8943 – 8947; g) B. De Clercq, F.
Verpoort, J. Mol. Catal. A 2002, 180, 67 – 76; S. Hamasaki, M.
Kamigaito, M. Sawamoto, Macromolecules 2002, 35, 2934 – 2940;
h) Y. Watanabe, T. Ando, M. Kamigaito, M. Sawamoto, Macro-
molecules 2001, 34, 4370 – 4374; i) T. Ando, M. Kamigaito, M.
Sawamoto, Macromolecules 2000, 33, 5825 – 5829.
[10] The ruthenium complex [RuH2(PPh3)4] was reported to catalyze
the controlled polymerization of MMA at 308C, but significantly
higher catalyst concentrations (MMA/Ru = 200:1) and reaction
times (300 h) were required: H. Takahashi, T. Ando, M.
Kamigaito, M. Sawamoto, Macromolecules 1999, 32, 6455 – 6461.
[11] Generally, values below 1.0 are observed. Currently, we have no
explanation for this discrepancy.
Experimental Section
Synthesis of complex 5: Complex 3 (100 mg, 133 mmol) was added to a
solution of PCy3 (37.3 mg, 133 mmol) in toluene (10 mL). The solution
was stirred for 4 h at 408C and then allowed to cool to room
temperature. After one week, orange crystals formed, which were
collected and washed with pentane (yield: 78.3 mg, 70%). Elemental
analysis calcd (%) for C66H114Cl8N2P2Ru4·2C6H5CH3: C 51.39, H 7.01,
N 1.50; found: C 51.53, H 6.95, N 1.15. The same complex was
obtained in reactions with a 3/PCy3 ratio of 1:2, but the yields were
lower. NMR spectra were not recorded owing to the low solubility of
5 in benzene or toluene. In chlorinated solvents the decomposition of
5 was observed.
General polymerization procedure: The monomer and a solution
of the initiator in toluene (ethyl 2-bromo-2-methylpropionate for
acrylates and 1-bromoethylbenzene for styrene; 0.1m) were added to
a Schlenk tube that contained the ruthenium complex 2 or 3
(6.25 mmol) and PCy3, such that the molar ratios [Ru]/[initiator]/
[monomer] were 1:2:800. n-Octane (50 mL) was added as the internal
standard for GC measurements. Immediately after the components
had been mixed, the tube was placed in a thermostatted oil bath
(508C), which was shielded from light. After a given period of time,
the mixture was cooled and diluted with THF (6 mL). The polymer
was then precipitated with hexane (acrylates) or with methanol
(styrene), isolated, and dried under vacuum. Remaining traces of
catalyst were removed by dissolving the polymer in toluene and by
adding silica gel. After the silica gel had been removed, the solvent
was removed by evaporation and the polymer was dried in vacuum.
All reactions were performed under an atmosphere of dry nitrogen
and with freshly distilled substrates and solvents. The molecular
weights and the molecular-weight distributions of the polymers were
determined by gel permeation chromatography (DMF, 608C) with
PMMA standards. The conversions were determined by GC and the
yields by mass.
[12] The initial TOF was calculated from the conversion of MMA per
ruthenium atom after 6 h as determined by gas chromatography.
[13] Some copper complexes are also able to catalyze the ATRP of
MMA at ambient temperatures, but the catalyst concentrations
are generally higher (MMA/Cu ꢁ 100:1). See for example:
a) D. P. Chatterjee, U. Chatterjee, B. M. Mandal, J. Polym. Sci.
Part A 2004, 42, 4132 – 4142; b) D. M. Haddleton, D. Kukulj, D. J.
Duncalf, A. M. Heming, A. J. Shooter, Macromolecules 1998, 31,
5201 – 5205.
Received: September 18, 2004
Published online: January 20, 2005
Keywords: homogeneous catalysis · living polymerization ·
.
polymers · ruthenium
[14] Bimodal GPC profiles have previously been observed in ATRP
reactions with ruthenium catalysts and butyl acrylate as the
monomer: O. Tutusaus, S. Delfosse, F. Simal, A. Demonceau,
A. F. Noels, R. Nfflæez, C. Viæas, F. Teixidor, Inorg. Chem.
Commun. 2002, 5, 941 – 945.
[1] a) A. Fꢁrstner, T. Mꢁller, J. Am. Chem. Soc. 1999, 121, 7814 –
7821; b) A. Fꢁrstner, L. Ackermann, Chem. Commun. 1999, 95 –
96.
[2] a) J. Baran, I. Bogdanska, D. Jan, L. Delaude, A. Demonceau,
A. F. Noels, J. Mol. Catal. A 2002, 190, 109 – 116; b) D. Jan, L.
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Angew. Chem. Int. Ed. 2005, 44, 1084 –1088
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Communications
[15] At higher temperatures or upon photochemical activation, the
ATRA of CHCl3 to styrene was also observed for reactions with
1 and PCy3.
[16] a) B. T. Lee, T. O. Schrader, B. Martín-Matute, C. R. Kauffman,
P. Zhang, M. L. Snapper, Tetrahedron 2004, 60, 7391 – 7396;
b) J. A. Tallarico, L. M. Malnick, M. L. Snapper, J. Org. Chem.
1999, 64, 344 – 345.
[17] To the best of our knowledge, the highest turnover number
(TON) for a Ru-catalyzed addition of CHCl3 to styrene was
reported for the complex [Cp*RuCl(PPh3)2] (TON = 207, 858C,
24 h; Cp* = pentamethylcyclopentadienyl): F. Simal, L. Wlo-
darczak, A. Demonceau, A. F. Noels, Eur. J. Org. Chem. 2001,
2689 – 2695.
[18] Crystal data for 5·2C6H5CH3: C80H130Cl8N2P2Ru2, Mr = 1869.68,
monoclinic, space group P2(1)/c, a = 13.1038(8), b = 19.3979(10),
c = 17.8631(8) ꢀ, a = 90, b = 109.074(5), g = 908, V=
4291.2(4) ꢀ3, Z = 2, 1calcd = 1.447 gcmÀ3
,
m = 1.018 mmÀ1
,
F(000) = 1932, crystal dimensions 0.19 ” 0.18 ” 0.13 mm3. Data
collection: KM4/Sapphire CCD, T= 140(2) K, MoKa radiation,
l = 0.71073 ꢀ, q = 3.20–25.028, À15 ꢂ h ꢂ 15, À23 ꢂ k ꢂ 23,
À20 ꢂ l ꢂ 21, 24640 reflections collected, 7567 independent
reflections, Rint = 0.0329, semi-empirical absorption correction
(MULABS), max./min. transmission: 0.9485/0.7504. Refine-
ment: Nref = 7567, Npar = 473, R1 [I > 2s(I)] = 0.0252, wR2 (all
data) = 0.0601, largest difference peak 0.475 eꢀÀ3, largest differ-
ence minimum À0.673 eꢀÀ3. Structure solution and refinement
by SHELX97 (G. M. Sheldrick, SHELX97, Programs for Crystal
Structure Analysis, University of Göttingen, Göttingen (Ger-
many), 1998). H atoms were placed in calculated positions by
using the riding model. CCDC-247810 contains the supplemen-
tary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
[19] For some recent publications about mixed, halogeno-bridged
complexes see: a) S. Gauthier, R. Scopelliti, K. Severin Organo-
metallics 2004, 23, 3769 – 3771; b) S. Gauthier, L. Quebatte, R.
Scopelliti, K. Severin, Chem. Eur. J. 2004, 10, 2811 – 2821; c) S.
Gauthier, L. Quebatte, R. Scopelliti, K. Severin, Inorg. Chem.
Commun. 2004, 7, 708 – 712; d) K. Severin, Chem. Eur. J. 2002, 8,
1514 – 1518, and references therein.
[20] a) L. Bonomo, C. Stern, E. Solari, R. Scopelliti, C. Floriani,
Angew. Chem. 2001, 113, 1497 – 1500; Angew. Chem. Int. Ed.
2001, 40, 1449 – 1452; b) V. Rodriguez, I. Atheaux, B. Donna-
dieu, S. Sabo-Etienne, B. Chaudret, Organometallics 2000, 19,
2916 – 2926; c) D. G. Gusev, F. M. Dolgushin; M. Y. Antipin,
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