Access to phosphorus-rich zirconium complexes

Article abstract of DOI:10.1002/anie.201103634

The missing member [Cp′₂Zr(η²-P ₃CtBu)] (Cp′=η⁵-C₅H ₃tBu₂) of the phosphorus-containing bicyclobutane zirconium complex family was synthesized by reaction between tetraphosphazirconocene derivative [Cp′₂Zr(η²- P₄)] and a phosphaalkyne with the release of a formal P₂ unit. Synthetic evidence of another unknown isomer [Cp′ ₂Zr(η²-P₂(CtBu)₂)] was found; according to DFT calculations, it is the stable form if a Cp′ ₂Zr moiety is used.

Full text of DOI:10.1002/anie.201103634

Communications
DOI: 10.1002/anie.201103634
Phosphorus Ligands
Access to Phosphorus-Rich Zirconium Complexes**
Ulf Vogel, Miriam Eberl, Maria Eckhardt, Andreas Seitz, Eva-Maria Rummel,
Alexey Y. Timoshkin, Eugenia V. Peresypkina, and Manfred Scheer*
Dedicated to Professor John F. Nixon on the occasion of his 75th birthday
Activation and transfer of small molecules is a current
research area in chemistry. Among the numerous target
molecules, the interest in those containing phosphorus is
substantial.[1] Recent developments of transfer reagents make
use of Nb and to some extent W complexes to transfer P2, P3,
and Pn(CR)m moieties to other transition metals or to
generate novel phosphorus-rich cage compounds.[2]
form square-pyramidal cationic [EP2(CtBu)2]+ (E = P, As)
moieties,[8–10] and unexpected neutral P,C[11] and P,C hetero-
element cages,[12] to find unusual pathways to triphospholyl
anions.[13,14] In contrast, the all-phosphorus analogue of A, the
tetraphosphacyclobutane complex C, which was synthesized
in 1988 by Scherer et al.,[15] has to date not been used for
subsequent reactions. One reason might be its limited
stability; it is only stable if Cp’’ (Cp’’ = h5-C5H3tBu2-1,3) is
present as auxiliary ligand. Even for Cp* ligands, this
compound decomposes at room temperature. In contrast,
no evidence has yet been reported on the existence of the
corresponding triphospha derivative B. However, by using C
ꢀ
as starting material in the reaction with tBuC P, we found a
straightforward synthesis to this triphosphabicyclobutane
derivative B and other phosphorus-rich organic molecules,
which will in future be new synthetic tools for Zr-mediated
transfer reactions.
When 1 and tert-butylphosphaalkyne are allowed to react
in boiling toluene for four days, the 31P NMR spectrum of the
reaction mixture shows an absence of the two triplets of 1,
whereas the triplet and the doublet of the major product
[Cp’’2Zr(h2-P3CtBu)] (2) emerge among the signals of the cage
compounds 3 and 4 [Equation (1)].
Looking closer in this field, the roots of phosphorus-
containing transfer reagents go back to early developments of
Binger and Regitz, who obtained the diphosphabicyclobutane
zirconium complex A in 1987 by reaction of “Cp2Zr” with
[3]
ꢀ
tBuC P. They demonstrated the use of A as a starting
material for the synthesis of unprecedented molecules such as
tetraphosphacubane[4] and numerous P,C cage compounds[5,6]
under halogenation conditions. Later, Francis et al. contrib-
uted by showing that this starting material is useful for
creating nido-type cage compounds of main-group ele-
ments.[7] Recently, Russell et al. used A as a starting material
by reacting with element halides of Group 15 elements to
[*] Dr. U. Vogel, Dr. M. Eberl, M. Eckhardt, A. Seitz, E.-M Rummel,
Prof. Dr. M. Scheer
Institut fꢀr Anorganische Chemie der Universitꢁt Regensburg
93040 Regensburg (Germany)
E-mail: manfred.scheer@chemie.uni-regensburg.de
Homepage: http://www.chemie.uni-regensburg.de/Anorganische_-
Chemie/Scheer/index.html
The brown compound 3 crystallizes from the reaction
mixture in moderate yields. Compound 2 can be separated
from 3 by column chromatography, which however leads to a
decrease of its yield. Compound 4 crystallizes as single
crystals from this fraction and the residue contains almost
pure 2. Unfortunately, numerous attempts to obtain single
crystals of 2 that were suitable for X-ray structural analysis
failed as yet. However, the red, crystalline bis(di-tert-butyl-
cyclopentadienyl)-1,2,4-triphospha-bicyclo[1.1.0]butane-2,4-
diylzirconium complex 2 could be characterized without
doubt by 31P NMR spectroscopy and mass spectrometry.[16] In
the 31P NMR spectrum of 2, an A2X spin system is evident
with one doublet at 179.7 ppm and a triplet at À236.5 ppm in
an integration ratio of 2:1; both signals show a 1J(P,P)
Dr. A. Y. Timoshkin
Department of Chemistry, St. Petersburg State University
University pr. 26, 198504 Old Peterhoff, St. Petersburg (Russia)
Dr. E. V. Peresypkina
Nikolaev Institute of Inorganic Chemistry Siberian Division of RAS
Acad. Lavrentyev prosp. 3
630090 Novosibirsk (Russia)
[**] This work was supported by the Deutsche Forschungsgemeinschaft
and the Fonds der Chemischen Industrie. The COSTaction CM0802
PhoSciNet is gratefully acknowledged.
Supporting information for this article, including complete syn-
thesis and spectroscopic details for 2–4 and 6, crystallographic
details and complete details and references for the DFT calcula-
tions, is available on the WWW under http://dx.doi.org/10.1002/
anie.201103634.
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coupling of 185 Hz. As the 31P NMR spectroscopic data of 2
are very similar to those of 1 (A2X2 spin system with two
triplets at 173.7 and À205.1 ppm, with a coupling constant of
1J(P,P) = 203 Hz)[15] as well as [Cp2Zr{P2(CtBu)2}] (singlet at
À247 ppm),[3] it can be concluded that 2 possesses the
structure of isomer B. Furthermore, FD mass spectroscopy
shows the molecular ion peak of 2 at m/z 606 with the correct
isotope pattern. Transition metal complexes containing a
triphosphabicyclo[1.1.0]butane ligand such as in 2 have not
been
reported;
however,
a
few
uncoordinated
triphosphabicyclo[1.1.0]butanes with organic substituents
RP3CR’2 (R = tBu3C6H2, (Me3Si)2CH, (Me3Si)2NN(SiMe3);
R’ = Me3Si)[17] are known.
The formation pathway for 2 is not clear, but it is possible
that 1 eliminates a P2 unit and the formed unsaturated
Figure 1. Molecular structure of 3 in the solid state (hydrogen atoms
omitted for clarity). Selected bond lengths [ꢂ] and angles [8]: Zr1–P1
2.6911(19), Zr1–P3 2.699(2), P1–P4 2.193(2), P1–P2 2.210(3), P2–P6
2.211(3), P2–P3 2.219(2), P3–P5 2.181(2), P4–C2 1.876(7), P4–P5
2.225(3), P5–C1 1.872(6), P6–C2 1.890(6), P6–C1 1.871(5), C1–C2
1.567(9); P1-Zr1-P3 76.85(6), Zr1-P1-P2 81.79(7), Zr1-P1-P4 97.66(8).
ꢀ
zirconium complex reacts with tBuC P to give 2. The
ꢀ
eliminated P2 units react with tBuC P to give carbon–
phosphorus cage compounds such as 4, which represents a
ꢀ
tetramer of tBuC P extended by one P2 moiety. Interestingly,
all of the isolated products are formed in an almost equimolar
ratio as found in the 31P NMR of the crude reaction mixture.
Moreover, 3 can be viewed as an addition product of two
molecules of phosphaalkyne to the starting material 1
(Scheme 1). To shed light into the reaction pathway, the
structure of a hypothetical P8 molecule.[20] In 3, the cuneane
core is made up by six phosphorus atoms and two carbon
À
atoms with the ZrCp’’2 fragment inserted into one P P bond
of the cuneane core; this core has bond lengths in the usual
À
À
À
À
range of P P, C P, and C C single bonds. The P P bond
lengths vary between 2.181(2) and 2.225(3) ꢁ and are in the
À
same range as the P P bonds in the P8 core of (1,2-
[19d]
C6H4)2P8
or in the tricyclic compound [(1,2-
C6H4)2H2P6{W(CO)5}4].[21] The Cp’’ ligands around the zirco-
nium atom are in an antiparallel arrangement and face in
different directions. That might explain the extreme differ-
ences in the chemical shifts for the CH protons attached to the
Cp’’ rings. The CH group of the top ring faces towards the
P6C2 cage and is strongly influenced by the lone pair of the
phosphorus atom P2 (Figure 1), whereas the HCCH unit of
the Cp ring faces in a different direction. For the bottom ring,
the situation is the other way round, but it does not interact
with the lone pair of the phosphorus atoms. If the rings cannot
rotate freely, which is more likely due to the steric repulsion
of the bulky tBu substituents, the protons are fixed in very
different magnetic environments, and thus show large differ-
ences in their chemical shifts.
Scheme 1. Proposed formation of 3 by addition of two phosphaalkyne
units to 1.
2
ꢀ
arsenic complex [Cp’’2Zr(h -As4)] was reacted with tBuC P
to give a mixture of products. To date, crystals of an arsenic
analogue of 3 could be isolated, and its X-ray structure shows
clearly the occupancy of phosphorus in striking distance to the
carbon atoms. This result shows that insertion of the
À
À
phosphaalkyne occurs in a P P and As As bond, respec-
tively, of the bridge-head atoms not bound to Zr.
The brown crystals of [Cp“2Zr{h2-P6(CtBu)2}] (3) are
readily soluble in common organic solvents, such as n-hexane
and toluene. The 1H NMR spectrum of 3 at room temperature
shows four signals for the CH protons of the Cp’’ ring and
three signals for the tBu groups. This inequivalence of the
signals, combined with large differences in the chemical shifts
of the Cp’’ ring protons (5.50 and 7.29 or 5.32 and 7.95 ppm),
lead to the assumption that Cp’’ ring rotation is hindered. A
similar behavior with an inequivalence of the protons of the
CpR ring is observed in [{h5-P2(CtBu)3}CpZrCl2] as well as in
[{h5-P3(CtBu)2}CpZrCl2].[18] The 31P{1H} NMR spectrum of 3
shows a higher-order spectrum that could be simulated as an
AA’MM’XY spin system.[16]
Like 2, the yellow plates of 4 show high solubility in all
common organic solvents. The 31P NMR of 4 exhibits a
higher-order spectrum that could be simulated as an
ADEFPQ spin system.[16]
The main feature of the molecular structure of 3 is a
ZrP6C2 core, which displays a cuneane-like structural motif
(Figure 1).[16] This motif is sometimes found in polyphospho-
rus compounds, for example in Hittorfꢀs phosphorus allotrope
and some polyphosphorus complexes and compounds.[19] The
cuneane structure is also calculated to be the minimum
The molecular structure of 4 (Figure 2)[16] shows a frame-
work that is similar to the carbon–phosphorus cage 3 with one
tBuCP unit replacing the Cp’’2Zr fragment. However, the
molecule contains four CtBu groups instead of only two in 3,
and there are no vicinal CtBu groups. Overall seven isomers
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Whereas the formation of B and B’ is almost energetically
equivalent for the case of the Cp ligand (the energy difference
between isomers is only a minuscule 0.03 kJmolÀ1), for the
Cp’’ ligand isomer B is more stable by 45 kJmolÀ1 (in
agreement with experimental isolation of B and not B’).
Isomer A is slightly (by 7 kJmolÀ1) favored by the Cp ligand;
however, if the Cp’’ ligand is present at the Zr atom, the
formation of the unknown isomer A’ is strongly favored over
isomer A (by 72 kJmolÀ1). To investigate this result, we
ꢀ
treated [Cp’’2ZrCl2] with tBuC P after reducing the Zr
complex. Indeed it was possible to detect the expected
compound A’ by NMR spectroscopy. The 31P NMR chemical
shift is found at 185.5 ppm, which corresponds well to those of
1 (+173.7 ppm) and 2 (+179.1 ppm) for the Zr bound P
atoms. Although A’ was one of the major products out of
numerous formed products in this reaction, its isolation
failed.[27] An interesting hint for the possibility of its
formation was found when [Cp’’2Zr(CO)2] was reacted with
Figure 2. Molecular structure 4 in the solid state (hydrogen atoms are
omitted for clarity). Selected bond lengths [ꢂ]: C2’–P4 1.713(5), P4–P5
2.235(3), P5–C1’ 1.849(5), C1’–P2 2.188(7), P2–C2 1.842(5), C2–P1’
1.842(5), P2–P3 2.192(4), P3–C2 1.857(5), P1–P1’ 2.192(2), C1–P5
1.713(6), C1–P1 1.866(5).
ꢀ
tBuC P [Equation (2)]. The blue-green product 6 has a
diphosphacyclopentadienone ligand, which is bound by P and
O at the Zr moiety as a consequence of head-to-head coupling
of the phosphaalkyne in which a CO molecule is inserted into
of the formula P6(CtBu)4 have been described;[22] one of them
(5) is closely related to the isomer 4, which has now been
discovered; both compounds differ in the position of the
À
a P C bond of the phosphaalkyne. Also in this case, the
atoms P4 and C2’ in the C P bond.[23] In 4, the position of the
zirconium-bound P atom shows a 31P NMR chemical shift at
153.8 ppm, with a large P,P coupling constant of 430 Hz, which
=
=
tBuC group and phosphorus atom in the tBuC P unit are
=
interchanged with respect to 5. As a result, 4 contains three
is in accordance with the P P bond found in the X-ray
À
À
À À
P P units, whereas 5 features only one P P unit, one P P P
chain, and an isolated P atom. Thus, the chemical shifts in the
structure analysis.
31P NMR spectrum differ slightly in 4 and 5.[23] The P P and
À
À
P C bonds in 4 are all within the range of normal single
À
bonds, except for the P4 C2’ bond, which corresponds to a
=
P C bond (1.713(5) ꢁ). In the solid state structure of 4, an
interesting disordering of the molecules is observed.[16] The
molecule possesses a crystallographic twofold axis which goes
À
through the P1 P1’ bond. Therefore, the phosphorus atoms
P2, P3, P4, P5, the carbon atom C2, and two tBu groups are
disordered over two positions with occupation factors of
50%. Cages containing only phosphorus and CtBu groups,
such as the tetraphosphacubane (PCtBu)4 ,[4] can also be
obtained by oligomerization of neat tBuCP at higher temper-
atures[24] or by cyclooligomerization mediated by transition-
metal complexes[25] as well as by CuI.[26]
In the molecular structure of 6 (Figure 3),[16] the five-
membered ring of the novel ligand shows bonds in the usual
=
=
=
range of P P, C P, and C C double bonds, which indicates a
delocalized p system over the five-membered ring. Both the
Moreover, it is very astonishing that so far only isomer A
and not A’ has been synthesized, and that we succeeded in
synthesizing B and not B’. To shed light onto this problem,
DFT calculations were carried out (Scheme 2).[16]
Figure 3. Molecular structure of 6 in the solid state (hydrogen atoms
omitted for clarity). Selected bond lengths [ꢂ]: Zr1–P1 2.6525(13),
Zr1–O1 2.066(3), Zr1–C1 2.568(4), P1–P2 2.1139(16), P1–C1 1.785(4),
P2–C3 1.769(4), O1–C1 1.364(5), C1–C2 1.418(5), C2–C3 1.436(6).
Scheme 2. Relative energies of the isomers in kJmolÀ1, depending
on the CpR ligand at the Zr atom (B3LYP/pVDZ level of theory; ECP
on Zr).
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[8] C. Fish, M. Green, R. J. Kilby, J. M. Lynam, J. E. McGrady, D. A.
Pantazis, C. A. Russell, A. C. Whitwood, C. E. Willans, Angew.
Chem. 2006, 118, 3710 – 3713; Angew. Chem. Int. Ed. 2006, 45,
3628 – 3631.
[9] C. Fish, M. Green, J. C. Jeffery, R. J. Kilby, J. M. Lynam, J. E.
McGrady, D. A. Pantazis, C. A. Russell, C. E. Willans, Chem.
Commun. 2006, 1375 – 1377.
[10] D. A. Pantazis, J. E. McGrady, J. M. Lynam, C. A. Russell, M.
Green, Dalton Trans. 2004, 2080 – 2086.
[11] C. Fish, M. Green, R. J. Kilby, J. E. McGrady, D. A. Pantazis,
C. A. Russell, Dalton Trans. 2008, 3753 – 3758.
[12] C. Fish, M. Green, J. C. Jeffery, R. J. Kilby, J. M. Lynam, J. E.
McGrady, D. A. Pantazis, C. A. Russell, C. E. Willans, Angew.
Chem. 2006, 118, 6837 – 6841; Angew. Chem. Int. Ed. 2006, 45,
6685 – 6689.
[13] J. L. Lynam, M. C. Copsey, M. Green, J. C. Jeffery, J. E.
McGrady, C. A. Russell, J. M. Slattery, A. C. Swain, Angew.
Chem. 2003, 115, 2884 – 2888; Angew. Chem. Int. Ed. 2003, 42,
2778 – 2782.
[14] C. Fish, M. Green, J. C. Jeffery, R. J. Kilby, J. M. Lynam, C. A.
Russell, C. E. Willans, Organometallics 2005, 24, 5789 – 5791.
[15] O. J. Scherer, M. Swarowsky, H. Swarowsky, G. Wolmershꢄuser,
Angew. Chem. 1988, 100, 738 – 739; Angew. Chem. Int. Ed. 1988,
27, 694 – 695.
oxygen outside of the ring as well as the P1 atom within the
ring are coordinated by the Zr atom, with bond lengths in the
expected range.[28] Owing to the coordination of the O atom,
=
the C O double bond is slightly elongated.
In summary, we have shown that the tetraphosphazirco-
2
ꢀ
nocene derivative [Cp’’2Zr(h -P4)] (1) and tBuC P are useful
starting materials for the synthesis of novel phosphorus–
carbon cage compounds. In contrast to known concepts,
phosphorus-rich derivatives of these classes of compounds are
obtained by this method. Based on this reaction, the novel
complex [Cp’’2Zr(h2-P3CtBu)] (2) as well as the cuneane-like
complex [Cp’’2Zr{h2-P6(CtBu)2}] (3) were synthesized; the
ꢀ
latter can be regarded as an addition product of two tBuC P
units at 1. Compound 2 is the missing member B of the
bicyclobutane zirconium complex family A–C. Moreover,
synthetic evidence of another as yet unknown isomer A’ by
using Cp’’2Zr moieties hints at the possibility of tuning the
steric influence of adjacent complex fragments to form
previously unknown isomers. In light of the rich chemistry
of A in the past, the novel phosphorus-rich zirconium
derivatives 2 (type B) and 3 will no doubt play a decisive
role in the future for zirconium-mediated transfer reactions to
obtain novel P-rich complexes and main-group-element
cages.
[16] See the Supporting Information for details.
[17] a) R. Appel, B. Niemann, M. Nieger, Angew. Chem. 1988, 100,
957 – 958; Angew. Chem. Int. Ed. 1988, 27, 957 – 958; b) V.
Thelen, D. Schmidt, M. Nieger, E. Niecke, W. W. Schoeller,
Angew. Chem. 1996, 108, 354 – 356; Angew. Chem. Int. Ed. 1996,
35, 313 – 315; c) E. Niecke, O. Altmeyer, M. Nieger, J. Chem.
Soc. Chem. Commun. 1988, 945 – 946.
Received: May 27, 2011
Published online: August 24, 2011
[18] J. Panhans, F. W. Heinemann, U. Zenneck, J. Organomet. Chem.
2009, 694, 1223 – 1234.
Keywords: density functional calculations · phosphaalkynes ·
phosphorus · phosphorus–carbon cages · zirconium
.
[19] a) M. E. Barr, B. R. Adams, R. R. Weller, L. F. Dahl, J. Am.
Chem. Soc. 1991, 113, 3052 – 3060; b) M. Scheer, U. Becker, E.
Matern, Chem. Ber. 1996, 129, 721 – 724; c) S. N. Konchenko,
N. A. Pushkarevsky, M. T. Gamer, R. Kçppe, H. Schnçckel, P. W.
Roesky, J. Am. Chem. Soc. 2009, 131, 5740 – 5741; d) C. P. Butts,
M. Green, T. N. Hooper, R. J. Kilby, J. E. McGrady, D. A.
Pantazis, C. A. Russell, Chem. Commun. 2008, 856 – 858.
[20] a) L. Guo, H. Wu, Z. Jin, J. Mol. Struct. 2004, 677, 59 – 66;
b) B. M. Gimarc, D. S. Warren, Inorg. Chem. 1993, 32, 1850 –
1856; c) R. Janoschek, Chem. Ber. 1992, 125, 2687 – 2689; d) see
also reference [1a].
[21] M. Stubenhofer, C. Kuntz, G. Balꢂzs, M. Zabel, M. Scheer,
Chem. Commun. 2009, 1745 – 1747.
[22] M. M. Al-Ktaifani, W. Bauer, U. Bergstrꢄßer, B. Breit, M. D.
Francis, F. W. Heinemann, P. B. Hitchcock, A. Mack, J. F. Nixon,
H. Pritzkow, M. Regitz, M. Zeller, U. Zenneck, Chem. Eur. J.
2002, 8, 2622 – 2633.
[23] M. M. Al-Ktaifani, P. B. Hitchcock, J. F. Nixon, Inorg. Chim.
Acta 2003, 356, 103 – 108.
[24] A. Mack, M. Regitz, Chem. Ber./Recueil 1997, 130, 823 – 834.
[25] J. F. Nixon, Chem. Soc. Rev. 1995, 24, 319 – 328.
[26] U. Vogel, J. F. Nixon, M. Scheer, Chem. Commun. 2007, 5055 –
5057.
[27] By applying a large variety of reducing agents under various
reaction conditions, a clean dechlorination of the starting
material and thus the isolation of A’ was not achieved. To
date, the best results were obtained with nBuLi (55% of A’ in
the 31P NMR spectrum of the crude reaction mixture).
[28] R. J. Long, D. J. Jones, V. C. Gibson, A. P. White, Organo-
metallics 2008, 27, 5960 – 5967.
[1] Recent reviews on P4 activation: a) M. Scheer, G. Balꢂzs, A.
Seitz, Chem. Rev. 2010, 110, 4236 – 4256; b) B. M. Cossairt, N. A.
Piro, C. C. Cummins, Chem. Rev. 2010, 110, 4161 – 4177; c) M.
Caporali, L. Gonsalvi, A. Rossin, M. Peruzzini, Chem. Rev. 2010,
110, 4178 – 4235.
[2] a) B. M. Cossairt, C. C. Cummins, Angew. Chem. 2010, 122,
1639 – 1642; Angew. Chem. Int. Ed. 2010, 49, 1595 – 1598;
b) N. A. Piro, C. C. Cummins, Angew. Chem. 2009, 121, 952 –
956; Angew. Chem. Int. Ed. 2009, 48, 934 – 938; c) B. M. Cossairt,
C. C. Cummins, Angew. Chem. 2008, 120, 8995 – 8998; Angew.
Chem. Int. Ed. 2008, 47, 8863 – 8866; d) N. A. Piro, C. C.
Cummins, Inorg. Chem. 2007, 46, 7387 – 7393.
[3] P. Binger, B. Biedenbach, C. Krꢃger, M. Regitz, Angew. Chem.
1987, 99, 798 – 799; Angew. Chem. Int. Ed. 1987, 26, 764 – 765.
[4] T. Wettling, B. Geissler, R. Schneider, S. Barth, P. Binger, M.
Regitz, Angew. Chem. 1992, 104, 761 – 762; Angew. Chem. Int.
Ed. 1992, 31, 758 – 759.
[5] P. Binger, T. Wettling, R. Schneider, F. Zurmꢃhlen, U.
Bergstrꢄßer, J. Hoffmann, G. Maas, M. Regitz, Angew. Chem.
1991, 103, 208 – 211; Angew. Chem. Int. Ed. Engl. 1991, 30, 207 –
210.
[6] B. Geißler, S. Barth, U. Bergstrꢄßer, M. Slany, J. Durkin, P. B.
Hitchcock, M. Hofmann, P. Binger, J. F. Nixon, P. von R.
Schleyer, M. Regitz, Angew. Chem. 1995, 107, 485 – 488;
Angew. Chem. Int. Ed. 1995, 34, 484 – 487.
[7] a) M. D. Francis, P. B. Hitchcock, Chem. Commun. 2002, 86 – 87;
b) M. D. Francis, P. B. Hitchcock, Organometallics 2003, 22,
2891 – 2897.
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ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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