Formation of a palladium thioketone complex from a thiophosphinoyl stabilized Li/Cl carbenoid

Article abstract of DOI:10.1021/om200584h

An in situ generated thiophosphinoyl stabilized Li/Cl carbenoid reacts with Pd(PPh₃)₄ with formation of a palladium thioketone complex. The presence of both the thiophosphinoyl and silyl groups as anion-stabilizing substituents is insufficient to stabilize the expected alkylidene complex and instead results in a rearrangement to the thioketone compound.

Full text of DOI:10.1021/om200584h

COMMUNICATION
pubs.acs.org/Organometallics
Formation of a Palladium Thioketone Complex from a
Thiophosphinoyl Stabilized Li/Cl Carbenoid
Viktoria H. Gessner*
Institut f€ur Anorganische Chemie, Julius-Maximilians-Universit€at W€urzburg, Am Hubland, 97074 W€urzburg, Germany
S Supporting Information
b
ABSTRACT: An in situ generated thiophosphinoyl stabilized
Li/Cl carbenoid reacts with Pd(PPh3)4 with formation of a
palladium thioketone complex. The presence of both the
thiophosphinoyl and silyl groups as anion-stabilizing substitu-
ents is insufficient to stabilize the expected alkylidene complex
and instead results in a rearrangement to the thioketone
compound.
ince the pioneering work by Fischer and Schrock, transition-
hinoyl)- and bis(iminophosphorano)methanediides, difficult to
synthesize and isolate in high yields.5,6 Thus, we aimed at geminal
dianions with sufficient stabilization of the negative charge to
allow for quantitative deprotonation. Several functional groups
are known as “anion-stabilizers”, in particular silyl groups, due to
the high polarizability of Si and the presence of low-lying σ*-SiR
orbitals, which allow for negative hyperconjugation.7,8 Herein,
we report our attempts to access a dianionic ligand from an R-
silyl-substituted phosphine sulfide. We show that, due to the
lower anion stabilization capability of the silyl group, the carbene
complex is no longer stable and rearranges to an unexpected
palladium thioketone complex.
The starting phosphine sulfide 1 was obtained in 76% yield by
straightforward lithiation of methyldiphenylphosphine sulfide
and subsequent treatment with trimethylchlorosilane (Scheme 1).
Addition of the lithium reagent to a solution of the electrophile is
of crucial importance to minimize the formation of the disilyl-
substituted compound by deprotonation of the more CH acidic
R-silyl phosphine sulfide 1 already formed in the reaction
mixture. To examine the possible dilithiation of the methylene
moiety to the geminal dianion 2, compound 1 was treated with 2
equiv of lithium base (MeLi, nBuLi, and tBuLi), which resulted in
a yellow ether solution. Monitoring of the reaction process by 31P
NMR spectroscopy showed the immediate consumption of 1
and the formation of a single species at δ 44.0 ppm. Trapping of
the reaction mixture with different electrophiles, however, only
afforded the products derived from the monometalated species.
Comparison with a monolithiation experiment identified the
formed lithium compound as the monolithiated compound 3.
An even longer reaction time and the addition of additives
(TMEDA, PMDETA) did not result in the clear formation of
2. This observation clearly demonstrates the considerable loss of
stabilization of the negative charge by replacing the phospho-
nium substituent with the SiMe3 group. In the case of the
bis(thiophosphinoyl)- and bis(iminophosphorano)methanes
S
metal complexes with a metalꢀcarbon double bond have
found wide-ranging applications.1 These complexes are generally
divided into two classes: the Fischer-type carbene and the
Schrock-type alkylidene complexes. The former usually bear π-
donor substituents and are viewed as singlet species, forming the
CdM bond via σ donation to the metal center and π back-
bonding from the metal into the empty pπ orbital. In contrast,
Schrock-type complexes usually bear alkyl substituents and the
metal center interacts with a triplet fragment with formation of a
covalent metalꢀcarbon bond. In this context, carbene complexes
formed from geminal dilithiated bis(thiophosphinoyl)methane
or bis(iminophosphorano)methane have gained special interest,
as they seem to contradict this general classification pattern
(Figure 1).2 Here, the metalꢀcarbon bond is formed by a four-
electron donation from the ligand to the metal, leaving a highly
nucleophilic carbon atom bound to anion stabilizing groups.3,4
Figure 1. Classifications of metal carbene complexes.
Fascinated by the unique properties of these carbene com-
plexes derived from geminal dianions, we were interested in
expanding this chemistry to further systems and functionalities.
We assumed that changing the substitution pattern at carbon
should significantly influence the electronic situation and thus
the reactivity of the complex. However, geminal dianions are
extremely sensitive species and, apart from the bis(thiophosp-
Received: July 4, 2011
Published: July 28, 2011
r
2011 American Chemical Society
4228
dx.doi.org/10.1021/om200584h Organometallics 2011, 30, 4228–4231
|

Organometallics
COMMUNICATION
double deprotonation usually occurs with simple organolithium
bases. Only in the case of sterically demanding systems is the use
of additional activating bases required.5
Scheme 1. Synthesis of Li/Cl Carbenoid 5a
This result prompted us to aim for an alternative approach to
the desired alkylidene complex. Le Floch and co-workers have
demonstrated the accessibility of the palladium complexes A via
the corresponding lithium carbenoid, which in their case could be
prepared by mild oxidation of the dilithium species.9 To follow
an approach via a carbenoid species, we aimed at the synthesis of
the chlorinated compound 4, which we expected to undergo a
preferred deprotonation reaction in comparison to a competitive
substitution of the chloride. At first, the chlorinated compound 4
was prepared in 95% yield by lithiation of 1 with n-butyllithium
followed by treatment with hexachloroethane (Scheme 1). For
the formation of the lithium carbenoid 5, 4 was treated in diethyl
ether with 1 equiv of methyllithium at ꢀ78 °C, which instantly
resulted in a yellow color. However, warming of the solution of 5
to room temperature resulted in decomposition at around ꢀ20 °C,
which was accompanied by a color change to black.10 This
thermolability of 5 also prevented its isolation. Nevertheless,
treatment with iodomethane at ꢀ78 °C gave the methylated
racemic product 6 and thus unequivocally confirmed the lithium
carbenoid as an intermediate species (Scheme 1). The methyla-
tion proceeds quantitatively, and 6 could be isolated in 94% yield
after workup. 1, 4, and 6 were characterized by elemental
analyses, mass spectrometry, and multinuclear NMR. Due to
the chiral centers in 4 and 6, the phenyl substituents at the
adjacent prochiral phosphorus atom are diastereotopic (see the
Supporting Information). 1 was additionally characterized by
X-ray crystallography (see the Supporting Information). Overall,
the formation of 6 confirmed the preferred deprotonation of 4
over a possible substitution reaction and thus the in situ acces-
sibility of the lithium carbenoid.11
a Reagents and conditions (yields): (a) 1 equiv of nBuLi, THF, ꢀ78 °C
to ꢀ30 °C, Me3SiCl, H2O; (b) 1 equiv of MeLi, Et2O, ꢀ78 °C to
ꢀ30 °C, quantitative by31P NMR; (c) C2Cl6, ꢀ78 °C (95%); (d) 1 equiv
of MeLi, Et2O, ꢀ78 °C to ꢀ50 °C; (e) CH3I (94%).
Scheme 2. Formation of the Thioketone Complex 7b and the
Proposed Mechanism
Compound 7b crystallizes in the triclinic space group P1. The
central motif consists of a PdꢀCꢀS moiety, in which the
palladium adopts a strongly distorted square planar (Figure 2c)
coordination (angles between 45.94(7) and 155.28(3)°) due to
the short bite of the CꢀS linkage. The same is true for the
coordination sphere around carbon, which considerably deviates
from the ideal tetrahedral arrangement. The PdꢀC (2.182(3) Å)
as well as the CꢀSi and CꢀP distances are in the typical range of
a single bond. In contrast, the CꢀS bond of 1.747(3) Å is shorter
than a typical CꢀS single bond, indicating some π character.
Both the short CꢀS bond and the flattened carbon coordination
sphere (torsion angle SiꢀCꢀPꢀS of 151.6(3)°) (Figure 3b) are
comparable to those for known thioketone transition-metal
complexes. Thus, 7b can be viewed as a side-on coordination
complex of the Pd(0) species to the CdS double bond.12 This is
also confirmed by computational studies of the model system
7b0, in which the PPh3 ligands are replaced by PH3 (for
computational details see the Supporting Information).13
The optimized geometry of 7b0 (e.g., PdꢀC = 2.158 Å, CꢀS =
1.745 Å) is in good agreement with the experimental data.
Natural bond orbital (NBO) analysis shows a typical CdS
double bond with a roughly sp2 hybridized carbon atom. As
evidenced by second-order perturbation theory, the π bond
interacts with the palladium fragment by strong donation into
a vacant metal orbital and back-bonding from the filled metal dxz
orbital into the π* (LUMO, Figure 3).
To test the applicability of lithium carbenoid 5 for the
synthesis of alkylidene complexes, freshly prepared 5 was reacted
with tetrakis(triphenylphosphine)palladium (Scheme 2). Addi-
tion of 5 to a THF solution of [Pd(PPh3)4] at ꢀ30 °C
immediately resulted in an orange color. 31P NMR studies of
the reaction mixture showed the formation of triphenylpho-
sphine and a second phosphorus species characterized by a set of
three doublets of doublets at δ 9.4, 21.3, and 25.8 ppm, the first
signal being shifted somewhat upfield to the expected alkylidene
complex 7a (see the Supporting Information). After workup
aimed at eliminating PPh3 and LiCl formed during the reaction,
the product was obtained as a yellow solid in 79% yield
(Scheme 2). However, when single crystals of 7 were obtained,
an X-ray diffraction study revealed that it was not the expected
alkylidene, but the thioketone complex 7b (Figure 2a). 7b
features a PdꢀCꢀS three-membered ring as the central struc-
tural motif and a noncoordinating phosphine unit, which is
consistent with the observed upfield-shifted signal in the 31P
NMR spectrum. Complex 7b is formed as a racemic mixture. The
stereogenic carbon atom results in diastereotopic phenyl sub-
stituents at the adjacent prochiral phosphorus atom and thus in
the corresponding splitting in the 1H and 13C NMR spectra (see
the Supporting Information).
4229
dx.doi.org/10.1021/om200584h |Organometallics 2011, 30, 4228–4231

Organometallics
COMMUNICATION
Figure 4. (a) Calculated model systems and (b) KS-HOMO and KS-
LUMO of the energy-optimized structure of hypothetical alkylidene
complex 7a0.
(Scheme 2). But why is 7b and not the alkylidene complex 7a the
stable product? It is noteworthy that the π interaction of the
MꢀC double bond is rather weak. This results from the unusual
electronic situation in complexes derived from geminal dianions
or carbenoid compounds. The energy-optimized model system
7a0 (PPh3 replaced by PH3) features a PdꢀC bond, which is
formed by a π-type interaction of the nπ orbital with the dxy
orbital of palladium (HOMO) and an interaction between the nσ
orbital and the dxz orbital (LUMO) (Figure 4). As the HOMO
is of antibonding nature, the π-type interaction is weaker and
thus the MꢀC bond longer than in “classical” metal carbene
complexes.9
This “weakness” of the alkylidene complex also becomes
evident from the energy optimization of the model systems 7a0
and 7b0. Although the alkylidene complex was also revealed to be
a minimum on the reaction coordinate, it is considerably less
stable (ΔH = 22.0 kcal molꢀ1) than the thioketone complex 7b0.
Remarkably, in the system A0 this difference only amounts to 2
kcal molꢀ1, which is also the result of the chelating nature of the
bis(thiophosphinoyl)methanide ligand. This is in line with the
higher negative charge at the carbenic atom in 7a0 (NPA charge
qC = ꢀ1.41) compared with that in A0 (qC = ꢀ1.38). This
suggests the negative charge to be the driving force for the
transformation of 7a to 7b (qC = ꢀ1.15), which is the result of
the lower anion-stabilizing capability of the silyl group compared
to that of the thiophosphinoyl substituent. The lower stabiliza-
tion is also in line with the so far failed dilithiation of 1 under
mild reaction conditions and the instability of the lithium
carbenoid 5.
Figure 2. Palladium complex 7b: (a) molecular structure; (b, c) views
of the Pd coordination sphere. Selected bond lengths (Å) and angles
(deg): C(4)ꢀS(1) = 1.747(3), C(4)ꢀP(1) = 1.833(3), C(4)ꢀSi(1) =
1.891(3), C(4)ꢀPd(1) = 2.182(3), P(2)ꢀPd(1) = 2.3142(9), P-
(3)ꢀPd(1) = 2.3465(8), Pd(1)ꢀS(1) = 2.2865(9); S(1)ꢀC(4)ꢀ
P(1) = 123.09(15), S(1)ꢀC(4)ꢀSi(1) = 113.47(14), P(1)ꢀ
C(4)ꢀSi(1) = 116.40(14), S(1)ꢀC(4)ꢀPd(1) = 70.18(9), P(1)ꢀC-
(4)ꢀPd(1) = 114.19(13), Si(1)ꢀC(4)ꢀPd(1) = 110.07(13), C-
(4)ꢀPd(1)ꢀS(1) = 45.94(7), P(2)ꢀPd(1)ꢀP(3) = 109.03(3),
C(4)ꢀS(1)ꢀPd(1) = 63.87(9). For crystallographic details see the
Supporting Information.
Figure 3. LUMO of model complex 7b0.
In conclusion, we have presented the unexpected formation of
a palladium thioketone complex 7b by reaction of Pd(PPh3)4
with a thiophosphinoyl-stabilized Li/Cl carbenoid. The presence
of both the thiophosphinoyl and a silyl group as anion-stabilizing
substituents was found insufficient to stabilize the analogous
alkylidene complex 7a, as known for the bis(thiophosphinoyl)-
methanide ligand. Theseresults clearly indicatethe crucial influence
of the substituents at the carbon atom on the electronic situation
and thus the reactivity of the corresponding metal complex. We
are currently studying the influence of the substitution pattern at
silicon and phosphorus on the formation of the thioketone
complex to obtain more detailed information about the mechan-
istic features of this reaction.
The selective formation of thioketone complexes has not been
reported before for analogous systems. Usually, thionation
reactions are preformed with Lawesson reagent or diphenylpho-
sphinodithioic acid on oxidized carbon compounds, particularly
carbonyls, amides, or esters.14 In the case of the lithium carbe-
noid 5 the thionation reaction only occurred by treatment with
[Pd(PPh3)4].15 The formation of an analogous thiocarbonyl
complex has been observed as a byproduct of the synthesis of a
uranium carbene complex.16 Accordingly, we propose that
carbenoid 5 also initially reacts with [Pd(PPh3)4] with formation
of the corresponding alkylidene complex 7a, which further
reacts to 7b by attack of the nucleophilic carbon atom at sulfur
4230
dx.doi.org/10.1021/om200584h |Organometallics 2011, 30, 4228–4231

Organometallics
COMMUNICATION
’ ASSOCIATED CONTENT
(7) Schleyer, P. v. R.; Clark, T.; Kos, A. J.; Spitznagel, G. W.; Rohde,
C.; Arad, D.; Houk, K. N.; Rondan, N. G. J. Am. Chem. Soc. 1984,
106, 6467–6475.
(8) M€uller, J. F. K.; Neuburger, M.; Springler, B. Angew. Chem., Int.
Ed. Engl. 1999, 38, 92–94.
(9) (a) Cantat, T.; Mꢀezailles, N.; Ricard, L.; Jean, Y.; Le Floch, P.
Angew. Chem., Int. Ed. 2004, 43, 6382–6385. (b) Cantat, T.; Jacques, X.;
Ricard, L.; Le Goff, X. F.; Mꢀezailles, N.; Le Floch, P. Angew. Chem., Int.
Ed. 2007, 46, 5947–5950.
(10) For examples of isolated Li/Hal carbenoids, see: (a) Konu, J.;
Chivers, T. Chem. Commun. 2008, 4995–4997. (b) Niecke, E.; Becker,
P.; Nieger, M.; Stalke, D.; Schoeller, W. W. Angew. Chem., Int. Ed. Engl.
1995, 34, 1849–1852. (c) M€uller, A.; Marsch, M.; Harms, K.; Lohrenz,
J. C. W.; Boche, G. Angew. Chem., Int. Ed. Engl. 1996, 35, 1518–1520.
(d) Boche, G.; Marsch, M.; M€uller, A.; Harms, K. Angew. Chem., Int. Ed.
Engl. 1993, 32, 1032–1033. (e) Boche, G.; Harms, K.; Marsch, M.;
M€uller, A. J. Chem. Soc., Chem. Commun. 1994, 1393–1394.
(11) Note that usage of excessive methyllithium resulted in the
formation of the dimethylated compound as a byproduct as result of the
substitution reaction.
S
Supporting Information. Text, figures, tables, and CIF
b
files giving experimental procedures, synthesis of all compounds,
spectroscopic and computational details, and crystallographic
data of the presented compounds. This material is available free
of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: vgessner@uni-wuerzburg.de.
’ ACKNOWLEDGMENT
V.H.G. thanks the Alexander von Humboldt Foundation for a
Feodor-Lynen return fellowship and Prof. Dr. Holger Braunschweig
for generous support.
(12) For examples of transition-metal thioketone complexes, see:
(a) Muraoka, T.; Nakamura, T.; Nakamura, A.; Ueno, K. Organometallics
2010, 29, 6624–6626. (b) Weisheit, T.; Petzold, H.; Go€rls, H.; Mloston,
G.; Weigand, W.; Wiedemann, R. Eur. J. Inorg. Chem. 2009, 3545–3551.
(c) Beck, R.; Sun, H.; Li, X.; Camadanli, S.; Klein, H.-F. Eur. J. Inorg.
Chem. 2008, 3253–3257. (d) Petzold, H.; Br€autigam, S.; G€orls, H.;
Weigand, W.; Celeda, M.; Mloston, G. Chem. Eur. J. 2006, 12,
8090–8095. (e) Bleeke, J. R.; Shokeen, M.; Wise, E. S.; Rath, N. P.
Organometallics 2006, 25, 2486–2500. (f) Schenk, W. A.; Vedder, B.;
Kl€uglein, M.; Moigno, D.; Kiefer, W. Dalton Trans. 2002, 3123–3128.
(g) Wolf, J.; Werner, H. Chem. Ber. 1996, 129, 29–31. (h) Yih, K.-H.;
Lee, G. H.; Wang, Y. Inorg. Chem. 2000, 39, 2445–2451. (i) Petzold, H.;
G€orls, H.; Weigand, W.; Romanski, J.; Mloston, G. Heteroat. Chem.
2007, 18, 584–590.
(13) All calculations were carried out using the Gaussian 03
(Revision E.01) set of programs and the B3PW91 functional. The
6-31+G* basis set was used for hydrogen, 6-311+G** for all other
nonmetal atoms (C, P, S, Si), and the LANL2TZ(f) basis set with the
corresponding ECP for palladium. The structures were optimized
without symmetry constraints: (a) Roy, L. E.; Hay, P. J.; Martin, R. L.
J. Chem. Theory Comput. 2008, 4, 1029. (b) Hay, P. J.; Wadt, W. R.
J. Chem. Phys. 1985, 82, 299. (c) Ehlers, A. W.; B€ohme, M.; Dapprich, S.;
Gobbi, A.; H€ollwarth, A.; Jonas, V.; K€ohler, K. F.; Stegman, R.;
Veldkamp, A.; Frenking, G . Chem. Phys. Lett. 1993, 208, 111. For more
detailed information and further basis sets tested, see the Supporting
Information.
’ REFERENCES
(1) (a) Klabunde, U.; Fischer, E. O. J. Am. Chem. Soc. 1967,
89, 7141–7142. (b) Schrock, R. R. J. Am. Chem. Soc. 1974, 96,
6796–6797.
(2) (a) Mindiola, D. J.; Scott, J. Nature Chem. 2011, 3, 15–17. (b)
Mills, D. P.; Soutar, L.; Lewis, W.; Blake, A. J.; Liddle, S. T. J. Am. Chem.
Soc. 2010, 132, 14379–14381. (c) Fustier, M.; Le Goff, X. F.; Le Floch,
P.; Mꢀezailles, N. J. Am. Chem. Soc. 2010, 132, 13108–13110.
(3) For reviews, see: (a) Jones, N. D.; Cavell, R. G. J. Organomet.
Chem. 2005, 690, 5485–5496. (b) Cantat, T.; Mꢀezailles, N.; Auffrant, A.;
Le Floch, P. Dalton Trans. 2008, 1957–1972. (c) Liddle, S. T.; Mills,
D. P.; Wooles, A. J. Organomet. Chem. 2010, 36, 29. (d) Harder, S.
Coord. Chem. Rev. 2011, 255, 1252–1267. (e) Liddle, S. T.; Mills, D. P.;
Wooles, A. J. Chem. Soc. Rev. 2011, 40, 2164–2176.
(4) For examples, see: (a) Babu, R. P. K.; McDonald, R.; Decker,
S. A.; Klobukowski, M.; Cavell, R. G. Organometallics 1999, 18,
4226–4229. (b) Cooper, O. J.; Mills, D. P.; McMaster, J.; Moro, F.;
Davies, E. S.; Lewis, W.; Blake, A. J.; Liddle, S. T. Angew. Chem., Int. Ed.
2011, 50, 2383–2386. (c) Cavell, R. G.; Babu, R. P. K.; Kasani, A.;
McDonald, R. J. Am. Chem. Soc. 1999, 121, 5805–5806. (d) Cantat, T.;
Ricard, L.; Mꢀezailles, N.; Le Floch, P. Organometallics 2006,
25, 6030–6038. (e) Mills, D. P.; Moro, F.; McMaster, J.; van Slageren,
J.; Lewis, W.; Blake, A. J.; Liddle, S. T. Nat. Chem. 2011, 3, 454–460.
(f) Tourneux, J.-C.; Berthet, J.-C.; Cantat, T.; Thuꢀery, P.; Mꢀezailles, N.;
Ephritikhine, M. J. Am. Chem. Soc. 2011, 133, 6162–6165. (g) Ma, G.;
Ferguson, M. J.; McDonald, R.; Cavell, R. G. Inorg. Chem. 2011, 50,
6500–6508.
(5) Examples of isolated bis(thiophosphinoyl)- and bis(imino-
phosphorano)methanediides: (a) Kasani, A; Babu, R. P. K.; McDonald,
R.; Cavell, R. G. Angew. Chem., Int. Ed. 1999, 38, 1483–1484. (b) Cantat,
T.; Ricard, L.; Le Floch, P.; Mꢀezailles, N. Organometallics 2006, 25,
4965–4976. (c) Cooper, O. J.; Wooles, A. J.; McMaster, J.; Lewis, W.;
Blake, A. J.; Liddle, S. T. Angew. Chem., Int. Ed. 2010, 49, 5570–5573.
(d) Orzechowski, L.; Jansen, G.; Harder, S. Angew. Chem., Int. Ed. 2009,
48, 3825–3829. (e) Demange, M.; Boubekeur, L.; Auffrant, A.;
Mꢀezailles, N.; Ricard, L.; Le Goff, X.; Le Floch, P. New J. Chem. 2006,
30, 1745–1754. (f) Orzechowski, L.; Jansen, G.; Harder, S. J. Am. Chem.
Soc. 2006, 128, 14676–14684. (g) Hull, K. L.; Carmichael, I.; Noll, B. C.;
Henderson, K. W. Chem. Eur. J. 2008, 14, 3939–3953.
(14) (a) Davis, T. P.; Barner, L. Synthesis 2003, 13, 1929–1958.
(b) Brillon, D. J. Sulfur Chem. 1992, 12, 297–338.
(15) Note that the formation of the thioketone compound has only
been observed in the presence of the Pd(0) species and not in trapping
reactions such as with iodomethane.
(16) Tourneux, J.-C.; Berthet, J.-C.; Cantat, T.; Thuꢀery, P.;
Mꢀezailles, N.; Le Floch, P.; Ephritikhine, M. Organometallics 2011,
30, 2957–2971.
(6) Examples for isolated geminal dianions: (a) Vollhardt, J.; Gais,
H.-J.; Lukas, K. L. Angew. Chem., Int. Ed. Engl. 1985, 24, 610–611.
(b) Vollhardt, J.; Gais, H.-J.; Lukas, K. L. Angew. Chem., Int. Ed. Engl.
1985, 24, 696–697. (c) Gais, H.-J.; Vollhardt, J. J. Am. Chem. Soc. 1988,
110, 978–980. (d) Linti, G.; Rodig, A.; Pritzkow, H. Angew. Chem., Int.
Ed. 2002, 41, 4503–4506. (e) Izod, K. Coord. Chem. Rev. 2002, 227,
153–173.
4231
dx.doi.org/10.1021/om200584h |Organometallics 2011, 30, 4228–4231