Decomposition mechanism of a cobalt-coordinated phosphite-olefin ligand under irradiation

Article abstract of DOI:10.1016/j.jorganchem.2014.04.013

We herein report on the different coordination behavior of phosphite-olefin ligands containing either a terminal olefin moiety or a related compound containing an internal olefin moiety to a CpCo(I)-center. While the former coordinates to the metal center via P atom and olefin moiety upon irradiation, photolysis of the latter led to the subsequent formation of the phosphorus-free [CpCo(η⁴-1,3-butadiene)] complex (6). A mechanistic rational of the decomposition reaction based on an Arbuzov-like rearrangement is suggested.

Full text of DOI:10.1016/j.jorganchem.2014.04.013

Journal of Organometallic Chemistry 763-764 (2014) 60e64
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Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Decomposition mechanism of a cobalt-coordinated phosphiteeolefin
ligand under irradiation
*
Indre Thiel, Haijun Jiao, Anke Spannenberg, Marko Hapke
Leibniz-Institut für Katalyse e.V. an der Universität Rostock, Albert-Einstein-Str. 29a, 18059 Rostock, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
We herein report on the different coordination behavior of phosphiteeolefin ligands containing either a
terminal olefin moiety or a related compound containing an internal olefin moiety to a CpCo(I)-center.
While the former coordinates to the metal center via P atom and olefin moiety upon irradiation,
Received 28 February 2014
Received in revised form
10 April 2014
photolysis of the latter led to the subsequent formation of the phosphorus-free [CpCo(h
4-1,3-butadiene)]
Accepted 14 April 2014
complex (6). A mechanistic rational of the decomposition reaction based on an Arbuzov-like rear-
rangement is suggested.
Keywords:
Cobalt
Ó 2014 Elsevier B.V. All rights reserved.
Phosphite
Olefin
Arbuzov rearrangement
Photolysis
Introduction
described by our group [2e]. There are only two further reports on
this kind of degradation, resulting in tetrameric CpCo-clusters [5].
Over the past decades intensive studies on CpCo(I)-complexes
Reactions of CpCo(I)-sources with phosphine and phosphite li-
gands leading to oligonuclear Co-complexes have been described
occasionally. One example is the report of the reactivity of
[CpCo(CO)2] (1) with free phosphines yielding phosphorus-bridged
clusters as described by Lal De and Maiti [6]. An interesting
example for the decomposition of a phosphorus compound is
(Cp:
h
5-cyclopentadienyl) as catalysts for [2 þ 2 þ 2] cycloaddi-
tion reactions have been reported [1]. While their catalytic per-
formances under thermal as well as photochemical conditions
have been studied extensively, the degradation or reactivity to-
wards cluster formation of only several olefin-ligated CpCo(I)-
complexes has been investigated and various respective cluster
complexes have been characterized [2]. The reaction of with al-
delivered by the reaction of tert-butylphosphaethyne with [
u-(di-
tert-butyl)phosphanyl (ethylcyclopentadienyl)cobalt]-complexes,
kynes can lead to mononuclear [CpCo(
h
4-diene)]-complexes,
where the C^P bond was split completely and a trinuclear m3
carbyne-m3-phosphidotricobalt cluster formed [7].
-
affording highly stable cyclobutadiene complexes, which can
however be subject to oxidative degradation with sterically hin-
dered cyclobutadienes [3]. A number of oligonuclear CpCo-cluster
complexes has been investigated by Wadepohl et al. [4]. They
Therefore, the decomposition of phosphorus ligands poses a
serious problem in catalysis [8]. Depending on the ligands of the
transition metal complexes various routes of decomposition can
be envisioned [9]. Since our group recently focussed on the
synthesis of CpCo(I)-bisphosphite as well as CpCo(I)-phosphitee
olefin complexes, the decomposition of coordinated phosphite
ligands was especially interesting. While phosphites are more
stable towards oxidation compared to phosphines, a number of
different other reaction pathways is possible including hydroly-
sis, alcoholysis, trans-esterification, OeC and PeO bond cleavage
as well as the Arbuzov rearrangement. There are few reports on
CpCo(I)- [10] as well as CpRh(I)-complexes [9] with phosphite
ligands undergoing an Arbuzov rearrangement leading to
phosphite decomposition and an alkylated metal center
(Scheme 1).
demonstrated that the reaction of [CpCo(h
2-C2H4)2] or in situ
generated low-valent complexes starting from cobaltocene/K with
various cycloalkenes resulted in the formation of trinuclear CpCo-
hydrido cluster complexes. As a result of the dehydrogenation
reaction of the cyclic olefin, the corresponding cycloalkyne is co-
ordinated by three CpCo-fragments. The formation of a tetrameric
cluster from [CpCo(H2C]CHSiMe3)2], affording a butterfly-type
coordination of the derived trimethylsilylacetylene has been
* Corresponding author.
E-mail address: marko.hapke@catalysis.de (M. Hapke).
http://dx.doi.org/10.1016/j.jorganchem.2014.04.013
0022-328X/Ó 2014 Elsevier B.V. All rights reserved.

I. Thiel et al. / Journal of Organometallic Chemistry 763-764 (2014) 60e64
61
Scheme 1. Documented Arbuzov rearrangements in the coordination sphere of a
transition metal.
In general aliphatic phosphites are found to be more prone to
the Arbuzov rearrangement than arylphosphites. To the best of our
knowledge these rearrangement reactions have exclusively been
reported for conventional trialkylphosphites and there has not
been any account of decomposition of a coordinated phosphorus-
olefin ligand. We herein report on the degradation of a cobalt-
bound phosphiteeolefin ligand under irradiation with light,
resulting in the phosphorus-free and purely olefin-ligated
[CpCo(h
4-1,3-butadiene)] (6) complex.
Results and discussion
We recently described the synthesis of the first CpCo(I)-
complex with a chelating phosphiteeolefin ligand [11]. Interest-
ingly in this specific case no stabilizing effect due to the chelati-
sation effect was observed. To investigate the influence of the
ligands into more detail the application of a different chelating
phosphiteeolefin ligand seemed to be promising. We chose to
compare the cyclic phosphite L1 and the linear phosphite L2
(Scheme 2). DFT calculations suggested e after carrying out BP86
density functional theory computations to examine the thermal
stability e that the coordination of L1 and L2 via the phosphite
group leading to the formation of both 4 as well as 5 were
endothermic but possible under reaction conditions. Starting from
[CpCo(CO)2] (1) as well as the cyclic phosphite L1 and linear
phosphite L2, the formation of the corresponding complexes
[CpCo(CO)(L2)] (2) and [CpCo(CO)(L1)] (3) have been computed to
be endergonic by 13.42 and 11.50 kcal/mol, respectively, indicating
that the formation of complexes 2 and 3 is thermodynamically not
favorable.
Since the formed CO gas molecule can be removed from the
reaction mixture to the gas-phase, the chemical potential of the
possible equilibrium of the system is changed and the reaction
shifted to favor the formation of complexes 2 and 3. This has indeed
been observed since the synthesis of 2 and 3 respectively pro-
ceeded smoothly at room temperature. Thus, starting from
[CpCo(CO)2] (1) the first carbonyl ligand could easily be replaced by
a phosphite at room temperature and the respective complexes
with one CO ligand were prepared in very good yields (Scheme 3).
Crystals of 3 suitable for X-ray analysis were obtained from a n-
pentane solution at 4 ꢀC. As shown in Fig. 1 the phosphiteeolefin
ligand is only coordinated via the phosphorus atom and one CO
ligand is still attached to the cobalt center.
Scheme 2. Calculations on the formation of the new complexes 2e5 from 1e3 at
BP86/TZVP.
reaction to yield the chelating complexes 4 and 5 anyway. However,
the irradiation with light and the release of the second CO ligand
only led to a successful chelation in the case of ligand L2 and the
generation of 4. Irradiation of 3 led to no chelating compound 5
despite the fact that the formation of complex 5 is less endergonic
than that of complex 4.
If 3 was irradiated with light at 25 ꢀC or even at temperatures
as low as ꢁ20 ꢀC a black solid precipitated and no 31P NMR signal
could be detected in the red solution. Filtration of the solution
over neutral aluminum oxide and removal of the solvent afforded
a red, highly volatile solid in 41% yield. NMR and MS analysis
provided evidence for the formation of [CpCo(h
4-1,3-butadiene)]
(6). Pruett and Myers already described 6 as a volatile red solid
prepared from 1 and excess 1,3-butadiene at elevated tempera-
tures [12], but only Bergman et al. published a full characterization
of 6, the NMR data being in accordance with our findings [13].
Starting from complexes 2 and 3 we further computed the
possible intramolecular substitution reaction of the CO ligand with
the olefin unit within L2 and L1. As expected, these substitution
reactions with the formation of complexes 4 and 5 are also ther-
modynamically not favorable and endergonic by 16.93 and
9.76 kcal/mol, respectively. Since the same equilibrium shift due to
the release of the second CO ligand applies, we were hopeful for the
Scheme 3. Synthesis of chelating CpCo(I)-phosphiteeolefin complexes.

62
I. Thiel et al. / Journal of Organometallic Chemistry 763-764 (2014) 60e64
Scheme 5. Postulated possible degradation pathway.
Fig. 1. ORTEP drawing of the molecular structure of 3. Ellipsoids are set at 30% prob-
ability. Hydrogen atoms are omitted for clarity.
rearrangement to take place it seems to be essential to have a
vacant coordination site at the cobalt center.
Our computations show that the degradation of complex 3,
following an intramolecular isomerization reaction with the for-
mation of an alkyl complex, which e in this case e subsequently
dissociates into the corresponding butadiene complex 6 and
phosphenate “(PhO)PO2” as a side product, is endergonic by
8.68 kcal/mol, and therefore, thermodynamically not favorable.
Since phosphenates like (PhO)PO2 have been reported to be highly
instable and generally short lived species [22] and favor the for-
mation of larger molecules by oligomerization [23], we have
computed the trimerization by the formation of a six-membered
ring structure. This trimerization reaction is highly exergonic by
51.87 kcal/mol (17.29 kcal/mol per (PhO)PO2). Owing to the for-
mation of the phosphenate clusters to be the driving force of the
reaction, this leaves the overall reaction to be exergonic by
8.61 kcal/mol. These energetic data reasonably explain the
observed formation of complex 6 instead of 5 and support our
proposed degradation mechanism involving an Arbuzov rear-
rangement as the essential step.
Vollhardt et al. reported on 6 in their investigation of the isom-
erization reaction of late transition metal-1,3-butadiene-derived
complexes [14,15].
The degradation mechanism of 3 is not yet fully understood
(Scheme 4). It seems that the cleavage of the CeO bond is preferred
over the PeO bond although both bond energies are very similar
(CeO: 85.5 kcal/mol; PeO: 80 kcal/mol) [16]. A similar observation
has been made by McDonald et al. [17], who reported on the same
effect of a CO2-laser on phosphite compounds describing the
decomposition of various alkyl phosphites when subjected to this
CO2 laser for 20 s. Alcohols, which are the decomposition products
from normal, thermal pyrolysis, were never found therefore hinting
at a CeO bond cleavage leaving the PeO bond intact.
A number of degradation pathways for L1 in 3 is conceivable,
involving for example homolytic CeO bond cleavage or a formal
cycloreversion (Scheme 5).
However, we believe that an Arbuzov-like rearrangement with a
subsequent second rearrangement causes the formation of 1,3-
butadiene and complex 6 respectively (Scheme 5). Arbuzov rear-
rangements of coordinated phosphite ligands have been reported
for a number of different metal complex fragments such as CpRh
[9,18], CpNi [19], CpRu [20], CpFe [10], but also CpCo [10]. In our
case we postulate the formation of the phosphenate “(PhO)PO2” as
a side product. In contrast to the reported photo-Arbuzov rear-
rangement of uncoordinated cyclic phosphites [21], we never
observed any decomposition or rearrangement of L1 if L1 was
irradiated with light on its own or as a ligand of the respective
bisphosphite complex [CpCo(L1)2]. For this Arbuzov-like
Conclusions
We reported on the light-induced photolysis of a CpCo(I)-
coordinated cyclic phosphiteeolefin ligand leading to the com-
plex [CpCo(h
4-butadiene)] (6) in moderate yield but high purity
and proposed an Arbuzov-like rearrangement leading to a CeO
bond cleavage and the removal of all traces of phosphorus from the
solution by the formation of easily polymerized phosphenates. This
observation contrasts the coordination behavior on irradiation of a
structurally related phosphiteeolefin ligand containing an open-
chain, terminal olefin moiety, which leads to the coordination of
the olefin by replacing another neutral ligand.
Experimental section
Compound 2 and 4 [11] as well as the phosphiteeolefin ligand
L2 [24] were synthesized according to published procedures. For
the photochemistry two halogen lamps (460 W each, lmax ¼ 363,
406, 419, 436, 548, 588, 591 nm) have been used for irradiation of
the thermostated Schlenk-type reaction vessel.
Scheme 4. Photolysis of 3 and the formation of 6.

I. Thiel et al. / Journal of Organometallic Chemistry 763-764 (2014) 60e64
63
Synthesis of L1
Acknowledgments
L1 was synthesized after a modified procedure from Ingenuin
et al. [25] Sodium (138 mg, 5 mmol) was washed with n-hexane
(3ꢂ) before 3-buten-1-ol (3.3 mL, 38 mmol) was added and gas
evolution was observed. Then triphenylphosphite (10 mL,
58 mmol) was added to the solution which turned redebrown and
a solid precipitated. This suspension was heated at 100 ꢀC for 2 h
before the mixture was allowed to cool to room temperature. The
redebrown oil was removed from the white precipitate via syringe
and the last residues of phenol were removed by bulb-to-bulb
distillation (100 ꢀC, 0.5 mbar). After 6 h at this temperature no
more phenol was recovered and the desired phosphiteeolefin
We thank Prof. Dr. Uwe Rosenthal for his enduring support and
helpful discussions and comments on the cluster complexes. We
thank the DFG (HA3511/3-1) for financial support and the Fonds der
Chemischen Industrie for a Kekulé scholarship for I. Thiel.
Appendix A. Supplementary material
CCDC 988726 contains the supplementary 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.
could be distilled at 180 ꢀC and 0.5 mbar. 1H NMR (C6D6)
d
¼ 2.37
(m, 2H), 3.93 (q, J ¼ 6.8 Hz, 1H), 4.13 (q, J ¼ 6.8 Hz, 1H), 4.98e5.10
Appendix A. Supplementary data
(m, 2H), 5.67e5.83 (m, 1H), 6.96e7.11 (m, 6H), 7.17e7.29 (m, 4H)
ppm; 31P NMR (C6D6)
(s), 61.6 (s), 115.3 (s), 117.3 (s), 120.2 (s), 120.4 (s), 120.8 (s), 123.8 (s),
124.3 (s), 129.6 (s), 129.7 (s), 134.3 (s) ppm.
d
¼ 129.2 (s) ppm; 13C NMR (C6D6)
d
¼ 35.2
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.jorganchem.2014.04.013.
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