Dinuclear Pd(i) complexes - Solely precatalysts? Demonstration of direct reactivity of a Pd(i) dimer with an aryl iodide

Article abstract of DOI:10.1039/c3sc52054d

This report provides experimental, computational and spectroscopic data in support of the direct reactivity of a Pd(i) dimer with an aryl iodide, resulting in Br/I halogen exchange between the complex and the aryl iodide. The reactivity could not be achieved through analogous Pd(0) conditions, demonstrating the distinct reactivities at such multiple Pd-sites. Computational studies support that the direct oxidative addition to ArI by the dinuclear metal complex is energetically feasible.

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ARTICLE TYPE
Dinuclear Pd(I) Complexes – Solely Precatalysts? Demonstration of
Direct Reactivity of a Pd(I) Dimer with an Aryl Iodide**
Karl J. Bonney,a Fabien Proutiere,a,b and Franziska Schoenebeck*a
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
This report provides experimental, computational and spectroscopic data in support of the direct reactivity
of a Pd(I) dimer with an aryl iodide, resulting in Br/I halogen exchange between the complex and the aryl
iodide. The reactivity could not be achieved through analogous Pd(0) conditions, demonstrating the
distinct reactivities at such multiple Pdꢀsites. Computational studies support that the direct oxidative
10 addition to ArI by the dinuclear metal complex is energetically feasible.
Palladium is a versatile element that exists in complexes of
oxidation states (0) to (IV). While rich chemistry has emerged
When we exposed one equivalent of Pd(I)ꢀBrꢀdimer 1 to 10
equivalents of 9ꢀiodoanthracene in THF, we detected that 1.8
over the past decades involving mononuclear Pd(0) species, 45 equivalents of ArBr had formed relative to Pd(I)ꢀBrꢀdimer 1 after
particularly in relation to cross coupling reactions involving
15 Pd(0)/Pd(II) catalysis,1 much less is known about the reactivities
at Pd centers of odd oxidation states. A number of dinuclear Pd(I)
and Pd(III) species featuring distinct PdꢀPd bonds have been
3 h at room temperature (see Scheme 2), indicating that Brꢀatoms
of 1 had been incorporated into the arene. A 31PꢀNMR
spectroscopic analysis of the mixture showed that Pd(I)ꢀBrꢀdimer
1 had converted to Pd(I)ꢀIꢀdimer 2 in the same process. Thus, a
synthesized,2,3 and in some cases have been isolated from or 50 net halide exchange between dimer 1 and ArI had taken place.
detected in catalytic transformations.4 In this context, we recently
20 demonstrated5 that conditions that are typically encountered in
Suzuki, Sonogashira or Stille reactions, i.e. the combination of
CuBrn salt and Pd(0)L2 (L = PtBu3) catalyst, lead to in situ
formation of the dinuclear Pd(I) complex {[PtBu3]PdBr}2 16,7
(see Scheme 1). Similarly, complex 1 has also been shown to
25 form in syntheses of Pd(II) intermediates that are relevant in
crossꢀcoupling reactions.8 There is only little understanding of the
precise role of such multiple Pdꢀsites in catalysis and their
,
Scheme 2. ArI→ArBr halide exchange with Pd(I) dimer 1.15 16
reactivities,9,10 and the generally posed questions in this context
55
are whether such species are just sideꢀproducts that are formed in
There are very few successful aromatic (pseudo)halide
30 the catalytic cycles, are offꢀcycle precursors to the actual active
exchange reactions with Pdꢀcomplexes.17,18 Examples are
species11 or whether they could in fact also directly react with
Buchwald’s Pd(0)/Pd(II) catalyzed aromatic fluorination,
aryl halides.12 Our previous work13 has shown that Pd(I) dimer 1 ꢀ
bromination and chlorination of aryl triflates at >100°C.19 The
in accord with the general belief11,7 and studies of related Pd(I)
60 observed reactivity with Pd(I)ꢀBrꢀdimer 1 seemed therefore rather
distinct, and we wondered whether a Pd(0) complex, that might
potentially form from the Pd(I)ꢀBrꢀdimer 1 in situ, could give rise
to this halide exchange. Our previous studies on the reactivity of
Pd(I)ꢀBrꢀdimer 1 with more challenging ArX substrates (with
65 X=Cl or OTf) had shown that the reactivity under Suzuki crossꢀ
coupling conditions is consistent with monoligated Pd(0)PtBu3 in
THF. However, the generation of the active Pd(0)PtBu3 species
required the combination of ArB(OH)2 and KF,13 and those are
absent in our present study. Moreover, the direct release of Pd(0)
70 species from the dinuclear Pd(I) complex via disproportionation
is also unlikely: our previous computational studies had indicated
that disproportionation of the Pd(I)ꢀBrꢀdimer 1 into Pd(0)PtBu3
and Pd(II)Br2PtBu3 species would have a prohibitively high
reaction free energy of 38.1 kcal/mol.13 The alternative
dimers14 ꢀ acts as a highly efficient preꢀcatalyst for Pd(0) in
35 Suzuki coupling reactions of ArCl or ArOTf.
Herein we present data in support of the direct reactivity of the
Pd(I) complex 1 with an aryl iodide and give an example of
distinct catalysis at these multiple Pdꢀsites.
tBu3P
Cu
Br Cu
Br
Br
PtBu3
CuBr2
tBu3P Pd0 PtBu3
tBu3P (I)Pd
Pd(I) PtBu3
+
Br
Cu
PtBu3
Br
THF, r.t.,
<15 min
Cu
Br
tBu3P
1
40 Scheme 1. Oxidation of Pd(0)(PtBu3)2 to dinuclear Pd(I) complex
1.5
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dissociation to Pd(0)(PtBu3)2 and Pd(II)Br2 was even more
disfavored (ꢁGrxn=65.4 kcal/mol).13
dinuclear Pd(I) complexes.21 The halide exchange reaction of
iodoanthracene to ArBr therefore proceeds without a detectable
intermediate: the dinuclear Pd(I) complexes are the only
phosphineꢀcontaining species.
To search for additional experimental evidence to
unambiguously exclude the involvement of Pd(0) in the present
halide exchange reaction, we prepared Pd(II) complexes derived
from 9ꢀiodoꢀ and 9ꢀbromoanthracene that would be expected to
be the key intermediates, if a Pd(0)PtBu3 were to be reactive via
the generally accepted Pd(0)/Pd(II) cycle (see Scheme 3). We
subsequently attempted the reductive elimination of ArBr at r.t.
5
50
10 for 3 h in THF, in (i) the presence of one equivalent of
iodoanthracene,32 (ii) the presence of PdBr2 with one equivalent
of PtBu3 and (iii) the presence of Pd(I)ꢀBrꢀdimer 1 (see Scheme
3). We did not observe more than trace amounts of ArBr in any of
these reactions and Pd(II) complex was still present in all cases.
15 Although reductive eliminations of ArBr from analogous Pd(II)
complexes have previously been reported, those were achieved at
higher temperature (70°C) over longer reaction times (20 h) and
in the presence of a large excess of ligand (10ꢀ20 equivalents).20
These data rule out the possibility of a Pd(0)/Pd(II) sequence in
20 the I/Br exchange reaction under Pd(I) dimer conditions.
Figure 1. 31PꢀNMR study of the halide exchange reaction
between 9ꢀiodoanthracene (10 equiv.) and Pd(I)ꢀBrꢀdimer 1 (1
equiv.) in THF at room temperature versus internal standard
55 PO(OMe)3.
Given that a Pd(0)/Pd(II) sequence has already been excluded as
a likely mechanism, the collected data could otherwise be
60 consistent with a halide exchange that (i) occurs directly at the
dinuclear Pd(I) complex or (ii) proceeds via the intermediacy of
Pd(I) radicals, formed after homolytic dissociation of the Pd(I)
complex. A mononuclear Pd(I)/Pd(III) halide exchange could
then take place via a mononuclear pathway and after regeneration
65 of the openꢀshell Pd(I) monomer, a dinuclear Pd(I) complex
could be reformed.22 In order to gain detailed molecular insights
into the favored reaction pathway, we applied computational
studies.23 The dissociation energy of the Pd(I) dimer to the
corresponding Pd(I) radicals was calculated to be 22.6
70 kcal/mol.24,25 This is in line with previously reported
experimental bond energy data of a related Pd(I) dimer, which
was determined to be between 21 and 29 kcal/mol.26 For the
alternative dinuclear pathway we were able to locate three
distinctly different transition states for halide exchange at
75 iodobenzene (see Figure 2), featuring (i) direct oxidative addition
by PdꢀPd (TSA), in which the coordination number at Pd is
increased and the CꢀI and CꢀPd bond formations take place
concertedly; (ii) a TS in which the Pd(I) dimerꢀcation (resulting
from initial Br‾ displacement by PhI) acts as a Lewis acid on CꢀI
80 and is simultaneously substituted by Br‾ (TSB); and (iii) a
concerted coordination of halide ions and exchange at aryl (TSC)
was also calculated. The oxidative addition, i.e. TSA was
calculated to be significantly favored (by ∆∆G‡ ≈ 30 kcal/mol)
over the alternative TSs B and C. Moreover, the overall activation
85 barrier to adopt TSA was calculated to be ∆G‡ = 14.8 kcal/mol,
which is substantially lower than the dissociation energy of the
Pd(I) dimer to the corresponding Pd(I) radicals (22.6 kcal/mol).27
Scheme 3. Attempted reductive elimination from Pd(II).
25
To gain further insight, we monitored the halide exchange
reaction of 9ꢀiodoanthracene in the presence of Pd(I)ꢀBrꢀdimer 1
in THF by 31PꢀNMR spectroscopy. Figure 1 gives the spectra
recorded at (i) 0 min, (ii) 30 min, (iii) 120 min and (iv) 180 min
30 reaction time. Three distinct signals were observed after 30 min,
of which two can be unambiguously assigned as Pd(I)ꢀBrꢀdimer 1
(87 ppm) and the diiodo analogue 2 (103 ppm). Notably, the
Pd(II) complex that is expected to form in “standard”
Pd(0)/Pd(II) reactions was not detected (its characteristic signal is
35 at 62 ppm, see page S65 in the SI), in line with our conclusions
above. A third signal at 97 ppm was observed, however. We
presume that the latter corresponds to the mixed Pd(I)ꢀBrꢀIꢀdimer
3, featuring one bromine and one iodine bridge. To test this
hypothesis, Pd(I)ꢀBrꢀdimer
1
and Pd(I)ꢀIꢀdimer
2
were
40 independently mixed in various ratios (see Figure S13 in the SI,
page S15), resulting in essentially identical 31PꢀNMR spectra to
those seen in the halide exchange reaction of 9ꢀiodoanthracene.
This shows that Br/I exchanges occur readily between these
Pd(I)ꢀdimer frameworks, in line with results reported previously
45 by Balch who described the scrambling of halogens for related
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Figure 2. Calculated24,24 TSs for dinuclear oxidative addition of Pd(I) dimer 1 (TSA) to PhI and disfavored alternatives (TSB, TSC) (top). Color code of
structures: Pd (turquoise), I (purple), Br (red), P (orange), C (grey), H (white). Free energy pathway in kcal/mol (bottom).
5
This suggests that ArI→ArBr halide exchange may proceed
directly via oxidative addition at the dinuclear Pd(I) complex,
leaving the PdꢀPd bond intact. The PdꢀPd bond length in TSA is
calculated to be 2.79 Å and the HOMO shows a bonding
were reactive enough to add ArI, catalytic halide exchange would
be possible. We found that 9ꢀiodoanthracene 4 in the presence of
10 equivalents of Bu4NBr and 20 mol% of Pd(I)ꢀiodoꢀdimer 3 is
converted to 9ꢀbromoanthracene 5 in 82% yield (see Figure 3)
interaction along the PdꢀPd axis (see Figure 2). The 40 after 4 days at ambient temperature (25°C).29 Slight heating of the
10 corresponding free energy pathway for halide exchange was
mixture (35°C) accelerated the reaction, and 62 % of 5 was
obtained after 20 h. This confirms that catalytic halide exchange
via dinuclear catalysis is indeed possible.
subsequently calculated (Figure 2). After dinuclear oxidative
addition, the intermediate readily exchanges Br and I. Reductive
elimination to give mixed Pd(I)ꢀBrꢀIꢀdimer 3 and ArBr then
follows. The overall process is calculated to be exergonic. The
15 mixed Pd(I)ꢀBrꢀIꢀdimer 3 could then undergo oxidative addition
to another ArI molecule. This process is of slightly higher
activation barrier (∆∆G‡ = 3.3 kcal/mol). Overall, the halide
exchange process is thermodynamically driven (∆Grxn= ꢀ12
kcal/mol). The reductive elimination of ArBr is calculated to be
20 the rate determining step. In line with this, our preliminary kinetic
studies indicated zeroꢀorder kinetics in both Pd(I)ꢀBrꢀdimer 1 and
iodoanthracene for the first I/Br exchange from 1 to 3 (see SI,
page S80 for further information).
25
Catalytic halide exchange reactions reported to date frequently
involve Cuꢀ or Niꢀcatalysts.17,28 We tested the possibility of
catalytic halide exchange with 1. Given that we saw facile I/Br
exchanges at the Pd(I) complex in our 31PꢀNMR spectroscopic
investigations, we hypothesized that we might be able to
30 regenerate Pd(I)ꢀBrꢀdimer 1 or the mixed Pd(I)ꢀBrꢀIꢀdimer 3 from
the diiodo analogue 2. Indeed, upon addition of an excess of
Bu4NBr to Pd(I)ꢀIꢀdimer 2, our 31PꢀNMR studies indicated that
the equilibrium shifted slightly towards the mixed Pd(I)ꢀBrꢀIꢀ
dimer 3, but the diiodo Pd(I)ꢀdimer 2 was still predominant (see
35 SI, Figure S23, page S27). Nevertheless, if the mixed species
45 Figure 3. Catalytic aromatic halide exchange.
When we employed standard Pd(0) catalytic conditions with 9ꢀ
iodoanthracene as control experiments, involving Pd(PtBu3)2 and
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Bu4NBr (10 equiv.), at best trace amounts of ArBr could be
achieved through analogous Pd(0) conditions. We anticipate that
detected (see Table S3 on page S29 in the SI). Moreover, addition 50 the data reported herein will increase our understanding of the
of isopropyl iodide – a reagent that has previously been shown to
generate Pdꢀbased radicals22,30 – to Pd(0) conditions [using
Pd(0)Ln with L = PPh3, PtBu3] along with Bu4NBr (10 equiv.),
also did not give rise to significant bromination, suggesting that
the halide exchange does not proceed via Pd(I)ꢀbased radicals. In
line with this, the addition of the radical traps cyclohexadiene or
dihydroanthracene to the Pd(I) dimerꢀcatalyzed transformation
10 shown in Figure 3 did not have any marked effect. Separate tests
involving AIBN (10 mol%) / Bu3SnH (10 mol%), i.e. standard
conditions to generate aryl radicals, with 10 equivalents of
Bu4NBr did not give rise to formation of ArBr. Halide exchange
via a radicalꢀnucleophilic aromatic substitution (SRN1) sequence
15 can therefore also be ruled out as a mechanism (see SI, page S20
onwards for details on these reactions, Schemes S6ꢀS10). These
results suggest that the halide exchange with Pd(I)ꢀBrꢀdimer 1
does not occur via a mechanism involving radicals nor a
Pd(0)/Pd(II) cycle and instead supports the direct reaction of the
20 Pd(I) dimer with the electrophile.
Finally, for a preliminary investigation of scope we also
exposed Pd(I) dimer 1 to alternative aryl iodides, and these also
gave brominated arenes (see Figure 4). Dimerꢀderived sideꢀ
products were also observed under these unoptimized reaction
25 conditions.31 While the activation free energy barrier for
oxidative addition by Pd(I)ꢀBrꢀdimer 1 to iodoanthracene is
calculated to be ∆G‡=10.7 kcal/mol, the barriers for the substrates
in Figure 4 are ≥ 14.0 kcal/mol. Thus, it is likely that only a
single bromination event takes place under these conditions,
30 formally stopping at the less reactive mixed Pd(I)ꢀBrꢀIꢀdimer 3
that fragments over time in solution.31 Our future work is directed
at optimizing these transformations and exploring the full scope
of halogen exchange at dinuclear Pd complexes.
reactivities of Pd(I) dimers and their potential role in catalysis.
5
Acknowledgements
We thank the Swiss National Science Foundation, ETH Zürich,
the RWTH Aachen University and the NRW Returning Scholars
55 Program for financial support. The major portion of this work
was conducted at ETH Zürich. Calculations were performed on
the ETH cluster “Brutus”.
Notes and references
a
Present address: Institute of Organic Chemistry, RWTH Aachen
60 University, Landoltweg 1, 52056 Aachen, Germany
E-mail: franziska.schoenebeck@rwth-aachen.de
Homepage: http://www.schoenebeck.oc.rwth-aachen.de/
b
Laboratory for Organic Chemistry, ETH Zürich, Wolfgang-Pauli-
Strasse 10, 8093 Zürich, Switzerland
65 † Electronic Supplementary Information (ESI) available: Contains:
additional experiments, details on experimental procedures, spectroscopic
data, computational information, Cartesian coordinates, full reference 23.
See DOI: 10.1039/b000000x/
70
1
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75
2
First preparation of a Pd(I) dimer: A. D. Gelman, E. Meilakh, Dokl.
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Figure 4. Alternative ArI in halogen exchange with Pd(I) dimer 1.15,16
3
4
First synthesis of a Pd(III)ꢀPd(III) complex: F. A. Cotton, L. Gu, C.
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100
105
110
In conclusion, with
a
combination of experimental,
40 computational and spectroscopic studies, we have obtained strong
support for the direct reactivity of the dinuclear Pd(I) complex
{[PtBu3]PdBr}2 (1) with aryl iodides, resulting in Br/I halide
exchange between the complex and the arene. The computational
data suggest that oxidative addition by the dinuclear metal
45 complex to ArI has a relatively low activation barrier; and this
mechanistic pathway was significantly favored over the
calculated alternatives. An example of a catalytic aromatic halide
exchange reaction was also presented, which could not be
P. D. W. Boyd, A. J. Edwards, M. G. Gardiner, C. C. Ho, M.ꢀH.
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C. C. C. Johansson Seechurn, H. Li, T. J. Colacot, J. Chou, C. J.
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First synthesis of 1: R. Vilar, D. M. P. Mingos, C. J. Cardin, J. Chem.
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600, 198.
27 See SI for alternative computational methods applied to the Pd(I)
radical vs dinuclear oxidative addition pathways (Figure S24, Table
S4, page S31).
5
6
28 A. Klapars, S. L. Buchwald, J. Am. Chem. Soc., 2002, 124, 14844; A.
A. Cant, R. Bhalla, S. L. Pimlott, A. Sutherland, Chem. Commun.,
2012, 48, 3993; X. Feng, Y. Qu, Y. Han, X. Yu, M. Bao, Y.
Yamamoto, Chem. Commun., 2012, 48, 9468.
5
10
15
20
25
30
75
29 The barrier for Pd(I)ꢀIꢀBrꢀdimer 3 to add to 9ꢀiodoanthracene is ∆∆G‡
= 3.0 kcal/mol lower than for the addition to PhI.
7
First application as a precatalyst in crossꢀcoupling reactions: J. P.
Stambuli, R. Kuwano, J. F. Hartwig, Angew. Chem. Int. Ed., 2002, 80 30 G. Manolikakes, P. Knochel, Angew. Chem. Int. Ed., 2009, 48, 205.
41, 4746; T. Hama, X. Liu, D. A. Culkin, J. F. Hartwig, J. Am. Chem.
Soc., 2003, 125, 11176; M. Prashad, X. Y. Mak, Y. Liu, O. Repic, J.
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Soc., 2004, 126, 1184.
31 For example, PtBu3•HBr is formed; 31PꢀNMR spectra given in SI in
Figure S1, S4, S7, S10. The mechanism of fragmentation is currently
under study. The sideꢀproducts do not trigger halide exchange
however. See SI, page S78 for tests.
85 32 In this experiment, the temperature was 25°C (ambient temperature).
For comparison, a 1:1 mixture of iodoanthracene and Pd(I)ꢀBrꢀdimer
1 gave 93% bromoanthracene under the analogous conditions [the
yield is relative to ArI starting material used, not 1].
31 For example, PtBu3•HBr is formed; 31PꢀNMR spectra given in SI in
Figure S1ꢀS5, page S5 onwards. The mechanism of fragmentation is
currently under study. The sideꢀproducts do not trigger halide
exchange however. See SI, page S22 for tests.
8
9
Catalysis by Di- and Polynuclear Metal Cluster Complexes, ed. R. D.
Adams and F. A. Cotton, WileyꢀVCH, New York, 1998.
10 For reactivities of related Pd(I) dimers with CO2, see: D. P.
Hruszkewycz, J. Wu, J. C. Green, N. Hazari, T. J. Schmeier,
Organometallics, 2012, 31, 470; D. P. Hruszkewycz, J. Wu, N.
Hazari, C. D. Incarvito, J. Am. Chem. Soc., 2011, 133, 3280; for
reactivity with arenes, see: T. Murahashi, K. Takase, M. Oka, S.
Ogoshi, J. Am. Chem. Soc., 2011, 133, 14908; for other studies and
applications using Pd(I) dimers, see: T. Hama, D. A. Culkin, J. F.
Hartwig, J. Am. Chem. Soc., 2006, 128, 4976; S. E. Denmark, J. D.
Baird, Org. Lett., 2006, 8, 793; T. E. Barder, J. Am. Chem. Soc.,
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Khan, L. J. Gooßen, Org. Lett., 2012, 14, 3716.
11 T. J. Colacot, Platinum Metals Rev., 2009, 53, 183; U. Christmann,
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12 For a recent commentary, see: R. S. Paton, J. M. Brown, Angew.
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13 F. Proutiere, M. Aufiero, F. Schoenebeck, J. Am. Chem. Soc., 2012,
134, 606.
35 14 U. Christmann, D. A. Pantazis, J. BenetꢀBuchholz, J. E. McGrady, F.
Maseras, R. Vilar, J. Am. Chem. Soc., 2006, 128, 6376; C. Jimeno, U.
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15 Conditions: Pd(I)ꢀBrꢀdimer 1 (0.04 M), ArI (10 equiv.). Reactions
40
carried out in the dark, in the glovebox. The yield was determined
with calibrated SFC or 1HꢀNMR.
16 The number of equiv ArBr formed are relative to Pd(I)ꢀBrꢀdimer 1.
The maximum amount of ArBr formed is 2.0 equiv (=100% yield), as
1.0 equiv. of 1 can give 2.0 equiv. of ArBr.
45 17 T. D. Sheppard, Org. Biomol. Chem., 2009, 7, 1043.
18 Halogenation via Pd (III/IV) complexes is a promising alternative: A.
J. Hickman, M. S. Sanford, Nature, 2012, 484, 177; K. M. Engle, T.ꢀ
S. Mei, X. Wang, J.ꢀQ. Yu, Angew. Chem. Int. Ed., 2011, 50, 1478;
A. J. Canty, Dalton Trans., 2009, 47, 10409.
50 19 D. A. Watson, M. Su, G. Teverovskiy, Y. Zhang, J. GarcíaꢀFortanet,
T. Kinzel, S. L. Buchwald, Science, 2009, 325, 1661; X. Shen, A. M.
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20 A. H. Roy, J. F. Hartwig, J. Am. Chem. Soc., 2003, 125, 13944.
21 C. T. Hunt, A. L. Balch, Inorg. Chem., 1982, 21, 1641; for analogous
55
60
65
70
scrambling with Pd(III)ꢀdimers: M. C. Nielsen, E. Lyngvi, F.
Schoenebeck, J. Am. Chem. Soc., 2013, 135, 1978.
22 For a recent review of catalysis derived from transition metal radicals,
see: U. Jahn, Top. Curr. Chem., 2012, 320, 323.
23 Gaussian09, Revision A.01, M. J. Frisch et al. (see SI for full
reference).
24 Calculated at CPCM (THF) M06L/6ꢀ311++G(d,p)//B3LYP/6ꢀ
31G(d), with SDD for Pd, I.
25 For appropriateness of M06L, see: N. Sieffert, M. Bühl, Inorg.
Chem., 2009, 48, 4622; S. O. N. Lill, P. Ryberg, T. Rein, E.
Bennström, P.ꢀO. Norrby, Chem. Eur. J., 2012, 18, 1640; for B3LYP,
see: C. L. McMullin, J. Jover, J. N. Harvey, N. Fey, Dalton Trans.,
2010, 39, 10833; C. J. Cramer, D. G. Truhlar, Phys. Chem. Chem.
Phys., 2009, 11, 10757.
26 C. L. Lee, B. R. James, D. A. Nelson, R. T. Hallen, Organometallics,
1984, 3, 1360.
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