CHEMCATCHEM
COMMUNICATIONS
DOI: 10.1002/cctc.201300294
Synthesis of Air-Stable and Recyclable CpCoI-Complexes
Indre Thiel, Anke Spannenberg, and Marko Hapke*[a]
The advances in transition metal-catalysed [2+2+2] cycloaddi-
tion reactions of alkynes and heterocumulenes have demon-
strated that a vast number of transition metals are able to cat-
alyse this atom-economic cyclotrimerisation reaction.[1] Never-
theless, group 9 transition metals, and especially cobalt, are of
particular importance and interest owing to their long-stand-
ing history and versatility in this area.[1u] The most commonly
used cobalt-based complexes such as [CpCo(CO)2] (1) or
[CpCo(cod)] (cod=1,5-cyclooctadiene) require high tempera-
tures, irradiation with light, or
the second one for dimethyl fumarate as an electron-deficient
olefin at elevated temperatures proceeded only in the case of
triphenylphosphite and dimethyl fumarate in excellent yield
(98%). As we intend to establish convenient and efficient syn-
thetic routes for the new complexes, we set forth to pursue
a different approach and developed a synthetic strategy start-
ing from the commercially available [CpCo(CO)2] (1) and two
successive ligand replacements (Scheme 1). The first CO ligand
is easily exchanged for a phosphite ligand by simply stirring
both to be activated. Only
a few examples such as the
Jonas reagent, [CpCo(H2C=
CH2)2] or our recently developed
catalyst [CpCo(H2C=CHSiMe3)2][2]
are already active at or below
room temperature and have
found application. However,
one disadvantage of all these
CpCoI systems is their sensitivity
towards air, requiring inert han-
dling and reaction conditions.
Up to now, only Gandon et al.
Scheme 1. Synthesis of [CpCo(CO)(phosphite)] and [CpCo(dimethyl fumarate)(phosphite)] complexes.
reported on the preparation and application of air-stable com-
plexes of the type [CpCo(CO)(dialkyl fumarate)].[3]
the two liquids at room temperature for 24 h.[7] Removal of the
second CO ligand also takes place at room temperature but
only under radiation with light.[8] The synthetic approach start-
ing from 1 was exemplarily performed for P(OPh)3 as well as
for P(OEt)3 as a more electron-rich phosphite and both steps
can be achieved in excellent yields (Scheme 1). While com-
plexes 2a-b are air-sensitive as most CpCoI-complexes, 3a-
b are air-stable solids and can be stored on the bench for
months.
We recently developed CpCoI systems with two different li-
gands, namely an olefin and a phosphite ligand, representing
a novel class of catalysts that are both reactive and stable
complexes.[4] These systematic studies corroborated the advan-
tages heteroleptic ligand combinations can provide for the
properties of transition metal precatalysts.[5] The phosphite–
olefin combination proved especially advantageous because it
principally allowed the variation of the electronic s donor/p ac-
ceptor abilities and steric demands of each type of ligand. To
further expand the library of these [CpCo(olefin)(phosphite)]
compounds and to unearth novel interesting properties, we
set out to synthesise complexes with different olefin–phos-
phite combinations. Especially electron-poor olefins, displaying
improved p acceptor abilities, should provide a higher stability
for the precatalyst owing to tighter bonding to the metal
Interestingly, the sequential substitution has to be in the
order 1) phosphite, then 2) olefin to yield the clean com-
pounds 3a–b in excellent yields. If dimethyl fumarate is react-
ed with 1 the mixed [CpCo(CO)(dimethyl fumarate)] complex
(4) is formed in very good yields.[3a] Addition of one equivalent
of triethylphosphite and irradiation of the reaction mixture
with light, only yields 40% of 3a. The reaction mixture con-
tains additional 2a (32%), [CpCo{P(OEt)3}2] (20%) and free di-
centre.[6] Initial experiments starting from [CpCo(H2C= methyl fumarate (8%). This suggests that the phosphite can
CHSiMe3)2] and replacement of the first trimethylvinylsilane
ligand for a phosphite P(OR)3 followed by the substitution of
substitute both the CO as well as the olefin ligand in 4, leading
to both monosubstituted phosphite complexes as well as the
bisphosphite complex through a second substitution reaction.
It can therefore be assumed that under these conditions the
phosphite ligand coordinates more strongly to the cobalt than
the dimethyl fumarate. We were able to obtain crystals of 3a
and b suitable for X-ray analysis from pentane solutions at 48C
(see the Supporting Information).[9] The characteristic data for
3a and b as the bond length for the coordinated double bond
[a] I. Thiel, Dr. A. Spannenberg, Dr. M. Hapke
Leibniz-Institut fꢀr Katalyse e.V. an der Universitꢁt Rostock
Albert-Einstein-Str. 29a, Rostock (Germany)
Fax: (+49)3811281-51213
Supporting information for this article is available on the WWW under
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 0000, 00, 2 – 4
&
2
&
ÞÞ
These are not the final page numbers!