Group VIII carbamoyl complexes as catalysts for alkyne hydrocarboxylation and electrochemical proton reduction

Article abstract of DOI:10.1016/j.jorganchem.2019.03.019

A series of group VIII carbamoyl complexes, [M(2-NHC(O)C₅H₄N)(CO)₂(2-SC₅H₄N)] [where M = Fe, Ru and Os], was found to be efficient and regioselective catalysts for the intramolecular hydroxycarboxylat

Full text of DOI:10.1016/j.jorganchem.2019.03.019

Journal of Organometallic Chemistry 889 (2019) 40e44
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Group VIII carbamoyl complexes as catalysts for alkyne
hydrocarboxylation and electrochemical proton reduction
*
Chandan Kr Barik, Malcolm E. Tessensohn, Richard D. Webster, Weng Kee Leong
Division of Chemistry & Biological Chemistry, Nanyang Technological University, 21 Nanyang Link, 637371, Singapore
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 20 February 2019
Received in revised form
A series of group VIII carbamoyl complexes, [M(2-NHC(O)C H N)(CO) (2-SC H N)] [where M ¼ Fe, Ru
5
4
2
5 4
and Os], was found to be efficient and regioselective catalysts for the intramolecular hydroxycarbox-
ylation of -alkynoic acids, yielding exocyclic enol lactones for ring sizes up to 7 atoms, and endocyclic
enol lactones for ring sizes up to 12 atoms. They also catalysed the regioselective intermolecular
hydroxycarboxylation reaction between propargylic alcohol and carboxylic acids to form -oxo-esters.
a,u
21 March 2019
Accepted 22 March 2019
Available online 27 March 2019
b
These complexes could also function as electrocatalysts in proton reduction, and evaluation of their
redox potentials revealed that the iron complex was much more efficient than the ruthenium or osmium
analogues.
Keywords:
Group 8 metals
Carbamoyl
©
2019 Elsevier B.V. All rights reserved.
Catalysis
Alkyne hydrocarboxylation
Proton reduction
1
. Introduction
esters (Scheme 1). Both these classes of products are important and
useful. The enol lactones are present in many natural products with
biological activity [15e17], and may occur as an endocyclic ring
[18], or as an exocyclic substructure [19], with the latter serving as
useful intermediates in the syntheses of more complex compounds
such as parthenolide, helenalin and arglabin [20e22]. Similarly, the
The hydrogenases are naturally-occurring metalloenzymes which
are involved in the utilization of hydrogen [1e3]. There are three
classes of hydrogenases of which the [Fe]-hydrogenase has a simple
metal cofactor which comprises a single iron centre (Fig. 1a) [4e7].
There has been a lot of research focused on functional as well as
structural mimics of this metal cofactor [8e11]. We recently reported
the synthesis and physical properties of metallacyclic carbamoyl
complexes 1n as structural analogues of this cofactor for the group
VIII triad, viz., iron, ruthenium and osmium (Fig. 1b) [12e14]. An
interesting aspect of these complexes is that the 2-mercaptopyridine
ligand is hemilabile; it imparts stability to the complexes and yet is
capable of de-coordination at the pyridine-N [12]. This property
makes the complexes potential catalysts and indeed, as we would like
to report here, they are efficient catalysts for alkyne hydrox-
ycarboxylation and electrochemical proton reduction.
acetonyl ester motif of
natural compounds and biologically active steroids [23e25].bib25
The cyclization of -alkynoic acids represents an effective
a,u-oxo-esters can be found in several
a,u
synthetic approach to the enol lactones; the exocyclic and endo-
cyclic products being obtained via Markovnikov and anti-
Markovnikov addition, respectively [15,26]. A large number of
metal-based catalysts have been reported for the intra- and inter-
molecular hydrocarboxylation of terminal alkynes - molybdenum
[27], ruthenium [28e31], rhodium [32], palladium [33], platinum
[34], and gold [35,36]. Among these, only two ruthenium com-
i
plexes, viz., [TpRu{PhCNC(Ph)CCPh)(PMe Pr
2
}] and [TpRuH(PPh
3
)
2
],
have been reported for the intramolecular reaction [37]; the former
gives the endocyclic enol lactone and the latter a mixture of both
the exocyclic and endocyclic enol lactones, but neither is selective
for the exocyclic enol lactone.
2
. Results and discussion
2.1. Alkyne hydrocarboxylation
The complexes
1
catalyze the intramolecular hydro-
carboxylation of a wide range of terminal alkynes (Table 1). The
hydrocarboxylation of 5-hexynoic acid in the presence of 1b, for
example, gives the corresponding exocyclic enol lactone 2b,
regioselectively and exclusively via intramolecular nucleophilic
attack of the acid. The cyclization takes place smoothly in refluxing
The hydrocarboxylation of terminal alkynes can occur intra-
molecularly to afford enol lactones, or intermolecularly to b-oxo-
*
Corresponding author.
E-mail address: chmlwk@ntu.edu.sg (W.K. Leong).
https://doi.org/10.1016/j.jorganchem.2019.03.019
022-328X/© 2019 Elsevier B.V. All rights reserved.
0

C.K. Barik et al. / Journal of Organometallic Chemistry 889 (2019) 40e44
41
regioselectively; complex 1c shows similar activity. The molecular
structure of one of the products, 4d, has been confirmed crystal-
lographically (Table S4). In the absence of the alcohol functionality,
regioselectivity is lost and all three possible products, with the E
product as the major species, is obtained; this is illustrated for the
reaction with p-ethynyltoluene (Table 2).
The catalytic pathway is presumably similar to those previously
suggested; the proposed pathway for the intramoleular alkyne
hydrocarboxylation is shown in Scheme 2 [28,32]. We have already
established that the carbamoyl ligand exerts a very strong trans
effect and hence it will labilize decoordination of the nitrogen atom
of the 2-mercaptopyridine ligand in 1n [22]. The resulting complex
A allows for coordination of the alkyne moiety to B, and intra-
Fig. 1. Structure of the (a) active site of [Fe]-hydrogenase, and (b) metallacyclic car-
bamoyl complexes 1n.
molecular nucleophilic attack by the carboxylate group at the
a- or
b-position (to C and C', respectively), followed by protonolysis,
would afford either the Markovnikov (exocyclic) or anti- Markov-
nikov (endocyclic) product, respectively. An attempt to identify a
species corresponding to intermediate B was carried out via a re-
action between 1b and propargyl alcohol but no such species was
observable. The change in selectivity with the chain length may
have to do with steric factors; with a shorter chain, the a-position is
more accessible, and formation of a terminal alkenyl complex is
also favoured, while this is decreased for a longer chain length. The
intermolecular hydrocarboxylation probably follows a similar
Scheme 1. Hydrocarboxylation reaction of terminal alkynes.
pathway, favouring attack by the carboxylate group at the
tion of the metal-bound alkyne.
a-posi-
toluene within 12 h and with 1.4% catalyst loading. Optimization of
the catalyst loading for 1b suggests an optimal loading of 1.4 mol-%
2
.2. Electrocatalytic proton reduction
(
Table S1). The product obtained depends on the chain length of the
alkynoic acid, HOOCCH (CH CCH; the exocyclic enol lactone 2 is
obtained for n ¼ 1e3 (entries 2e4) whereas endocyclic enol lactone
results for n ¼ 7 (entry 5), under the same reaction conditions; a
2
2 n
)
A recent report on two complexes which are structurally similar
to 1n, viz., [Fe(2-CH C(O)C NCH OCOR)(CO) (2-SC N)] where
R ¼ Me (6a) or Ph (6b), as potential electrocatalysts for proton
reduction prompted us to assess the complexes 1n for a similar
function [38]. The cyclic voltammograms (CV) of 1a-c display
chemically irreversible oxidation and reduction waves (measured
in acetonitrile, scan rate of 0.1 V s ) which are similar to those
reported for 6a and 6b (Fig. S1). As may be expected, the first
oxidation potential increased in the order 1a(Fe) < 1b(Ru) < 1c(Os)
2
H
5 3
2
2
5 3
H
3
control reaction without 1b affords no product (entry 1). All the
cyclization products are allowed under Baldwin's rules. Cyclization
for the larger cyclic enol lactone is also not stereoselective, giving a
mixture of both the Z- and E-enol lactones. The yield also appears to
decrease with an increase in the ring size, and may be ascribed to
the higher thermodynamic stability of the smaller rings. An
attempt at the cyclization of 5-hexynoic acid with 1a fails due to
catalyst decomposition at higher temperature, and 1c shows
similar catalytic efficiency and selectivity to that of 1b (entries 6
and 7).
ꢀ1
þ
(
þ0.75, þ0.77 and þ 0.83 V versus Fc/Fc , respectively), and the
potential for 1a is slightly more positive than those reported for 6a
þ
and 6b (0.60 and 0.68 V versus Fc/Fc , respectively). The small
Complex 1b is also a good catalyst for the intermolecular
hydrocarboxylation of propargyl alcohol, giving the
b-oxo-ester
Table 2
Intermolecular hydrocarboxylation of terminal alkynes.
Table 1
Intramolecular hydrocarboxylation of terminal alkynes.
Entry
Catalyst
n
Product ratio 2n:3n
Yield (%)a
Turnover number
0
Product, % (E:Z:G)a
1
2
3
4
5
6
7
8
1a
1b
1b
1b
1b
1c
1c
1c
1
1
2
3
7
1
2
7
0
0
0
Entry
R
R
Turnover number
100:0
100:0
100:0
0:100
100:0
100:0
93
98
57
80
62
77
68
71
75
44
61
47
59
52
1
2
3
4
5
6
7
H
CH
CH
CH
CH
CH
2
OH
2
OH
2
OH
2
OH
2
OH
4a, 90
4b, 88
4c, 90
4d, 84
4e, 80
5a, 92 (55:28:17)
5b, 90 (5:3:2)
69
68
69
64
61
70
69
o-Me
o-OMe
p-CH
p-Br
H
b
2
Br
b
p-tolyl
p-tolyl
0:100
o-Me
a
NMR yields with anisole as internal standard. bA 55:45 mixture of Z/E isomers,
a
1
NMR yields with anisole as internal standard. Ratio of the products obtained by
integration of the 1H NMR spectrum.
determined by integration of the H NMR spectrum.

42
C.K. Barik et al. / Journal of Organometallic Chemistry 889 (2019) 40e44
Scheme 2. Proposed catalytic cycle for the alkyne hydrocarboxylation.
differences in the potentials for 1a-c suggest that this first oxidation
is ligand- rather than metal-centred, based on comparisons against
the CVs of the free ligands 2-aminopyridine and 2-
mercaptopyridine (Fig. S3). Consistent with this, a DFT study car-
ried out on 1b shows that the HOMO resides mostly on the sulphur
atom (see supplementary information).
potentials) were noted on the reverse scan when the initial direc-
tion was swept towards negative potentials. A CPE experiment on
1b suggests that the same number of electrons are transferred for
both the oxidation and reduction waves [39].
The cathodic current increased with the amount of trifluoro-
acetic acid (TFA) added; that for 1a is shown in Fig. 2, and this
behavior is consistent with catalytic proton reduction [40,41]. This
was verified with a combination of CPE and GC experiments. A TOF
A similar trend is observed for the first reduction potential,
which increased (became more negative) in the order
þ
ꢀ1
1
a(Fe) < 1b(Ru) < 1c(Os) (ꢀ1.90, ꢀ2.22 and ꢀ2.27 V versus Fc/Fc ,
of 2.4 h was measured for 1a at a constant potential of ꢀ2.11 V
þ
respectively); the larger differences suggest metal-centred reduc-
tion, although the computed LUMO is essentially ligand-centred.
The oxidation and reduction processes were found to be indepen-
dent of each other, although additional oxidation processes (that
were not observed when the initial direction was towards positive
(versus Fc/Fc couple) over a period of 30 min. The estimated cur-
þ
rent produced at a fixed over-potential (ꢀ0.21 V versus Fc/Fc
couple) was much higher than that for the other two complexes
(Fig. S2), thus indicating that 1a is much more efficient, and can be
attributed to its lower first reduction potential [42,43].
Crossover of the forward and reverse scans may indicate some
4 6 3
Fig. 2. Cyclic voltammogram of 1a (1.0 mM)/n-Bu NPF (0.1 M) in CH CN and TFA at
ꢀ
1
0
(red),
2
(blue),
5
(green), 10 (pink) and 20 (black) mM. Scan rate ¼ 0.1 V s
,
ꢁ
þ
T ¼ 25 ± 2 C with the starting potential at ꢀ0.6 V vs. Fc/Fc . Solid and dashed lines
represent initial scans towards the positive and negative directions, respectively. (For
interpretation of the references to colour in this figure legend, the reader is referred to
the Web version of this article.)
Scheme 3. Proposed mechanism for the electrocatalytic proton reduction.

C.K. Barik et al. / Journal of Organometallic Chemistry 889 (2019) 40e44
43
deposition on the surface but this was observed only at higher
concentrations of TFA. Although the mechanism for the electro-
catalytic proton reduction has not been studied in detail, it is
probably similar to that reported for the other hydrogenase model
complexes; a simplified pathway is depicted in Scheme 3. It is likely
that initial protonation and reduction occurs at the S atom and the
hydrogen is subsequently transferred to the metal centre
solutions at the end of each measurement. Bulk controlled potential
electrolysis (CPE) experiments were performed in two-
a
compartment CPE cell divided by a porosity 5 sintered glass frit
[39]. Identically sized glassy carbon cylindrical rods (length 6 cm,
outer diameter 1.8 cm and internal diameter 1.2 cm) were
employed as the working and auxiliary electrodes and symmetri-
cally arranged with respect to each other.
[
42e44].bib44
Acknowledgements
3
. Conclusion
This work was supported by Nanyang Technological University
and the Ministry of Education (Research Grant No. M4011793).
C.K.B is grateful to the university for a Research Scholarship. We
acknowledge the help of Drs Yongxin Li and Rakesh Ganguly with
the crystallographic work.
We have shown in this work that the group VIII metal complexes
[
M(2-NHC(O)C
5
H
3
NR)(CO)
2
(2-SC
5
4
H N)] (where M ¼ Fe, Ru or Os)
are good and selective catalysts for the intra- and intermolecular
hydroxycarboxylation of terminal akynes, as well as electro-
catalysts for proton reduction. While the iron complex was found to
be the most efficient in proton reduction, it was unstable under the
conditions for alkyne hydrocarboxylation and, in that case, the
ruthenium complex was much better.
Appendix A. Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.jorganchem.2019.03.019.
4
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