(Electro)catalytic C-C bond formation reaction with a redox-active cobalt complex

Article abstract of DOI:10.1039/c4cc03309d

Cooperativity between cobalt and non-innocent ligands in electron transfer processes has been utilized for (electro)catalytic C-C bond formation reactions.

Full text of DOI:10.1039/c4cc03309d

Showcasing work from the laboratory of Biprajit Sarkar,
Institut für Chemie und Biochemie, Freie Universität Berlin,
Germany.
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(Electro)catalytic C–C bond formation reaction with a
redox-active cobalt complex
(Electro)catalytic C–C bond formation reaction is reported for a
redox-active cobalt catalyst. Cooperativity between cobalt and
the redox-active ligands in electron transfer steps is responsible
for this (electro)catalytic process.
See Joris van Slageren,
Biprajit Sarkar et al.,
Chem. Commun., 2014, 50, 11104.
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(Electro)catalytic C–C bond formation reaction
with a redox-active cobalt complex†
Cite this: Chem. Commun., 2014,
50, 11104
Margarethe van der Meer,a Yvonne Rechkemmer,b Irina Peremykin,b
Stephan Hohloch,a Joris van Slageren*b and Biprajit Sarkar*a
Received 3rd May 2014,
Accepted 28th May 2014
DOI: 10.1039/c4cc03309d
www.rsc.org/chemcomm
Cooperativity between cobalt and non-innocent ligands in electron
transfer processes has been utilized for (electro)catalytic C–C bond
formation reactions.
Redox-active ligands have long fascinated coordination chemists
because of the unusual and curious electronic structures of their
metal complexes.1 More recently chemists have started developing
novel catalysts based on metal complexes of non-innocent
ligands.2 One feature of redox-active ligands which is often utilized
in such catalytic processes is their active participation in various
electron transfer steps. Quinones are an important class of redox-
active ligands, and historically metal complexes of the all oxygen
donating o-quinone ligand have been the most studied.1b More
recently, metal complexes of o-iminoquinones have been thor-
oughly investigated.1c,3 Such [O] for [NR] isoelectronic subs-
titutions often induce novel properties in the metal complexes;
redox-tuning, and steric protection through the R groups being
obvious gains of the approach.3d o-Diiminoquinones where both
the [O] groups have been replaced by [NR] have found rather
limited use.4 Some such [NR] containing ligands used for synthe-
sizing four-coordinate cobalt complexes have been listed in
Fig. 1.4,5 We have been pursuing the development of new redox-
active ligands where [O] donors are replaced by their isoelectronic
[NR] counterpart. Such a substitution often turns out to be
immensely helpful while using metal complexes of these ligands
for various catalytic transformations.6 Herein we present the cobalt
complex 1 with the ligand QꢀÀ. Geometric and electronic structure
of this complex has been probed by a battery of methods, and its
Fig. 1 Selected redox-active ligands with which four-coordinate cobalt
complexes have been reported together with the geometry and spin-
states of those cobalt complexes.
The complex 1 was synthesized by deprotonation of H2Q with
utility as an (electro)catalyst for C–C bond formation reactions has n-BuLi, its reaction with CoCl2, and the subsequent oxidation of
been investigated.
the reaction mixture with pure O2. Re-crystallization provided 1
in the pure form. The geometric structure of 1, which crystallizes
as a CH2Cl2 solvate, was determined by single crystal X-ray
diffraction studies. The cobalt center in 1 is in a (pseudo)tetra-
hedral environment; being coordinated by the nitrogen donor of
two different ligands (Fig. 2).
The t value for this structure is 0.46. The dihedral angle
between the two coordinating planes is 501. The Co–N bond
distances of about 1.92 Å points to a high spin (HS) Co(II)
a
¨
¨
Institut fur Chemie und Biochemie, Anorganische Chemie, Freie Universitat Berlin,
Fabeckstraße 34-36, D-14195, Berlin, Germany. E-mail: biprajit.sarkar@fu-berlin.de
b
¨
¨
Institut fur Physikalische Chemie, Universitat Stuttgart, Pfaffenwaldring 55, D-70569,
Stuttgart, Germany
† Electronic supplementary information (ESI) available: Details of synthesis,
crystallography, magnetic measurements, EPR spectroscopy, electrochemistry
and mechanistic studies. CCDC 902892. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/c4cc03309d
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Fig. 2 ORTEP plot of 1. Ellipsoids are drawn at 50% probability. Hydrogen
atoms and solvent molecules have been left out for clarity.
Fig. 3 Cyclic voltammogram of 1 with the addition of benzyl bromide.
THF/0.1 M Bu4NPF6, 298 K, 100 mV sÀ1
.
center. The intra-ring C–C bond display a quinoidal distortion
with two short and four long bonds (Table S2, ESI†). The C–N changes on the ligand backbone is thus shown to dramatically
bond lengths of 1.36 Å also points to the existence of these influence redox-induced chemical reactivity in these complexes. To
ligands as QꢀÀ, and hence neutral 1 can be formulated as prove the hypothesis of a catalytic reaction of complex 1 with
[(QꢀÀ)CoII,HS(QꢀÀ)]. However, the difference in intra-ring bond CH2Cl2 following the second reduction step, we recorded the cyclic
lengths for the various redox forms of ligands with all nitrogen voltammogram of this complex in THF. Gratifyingly, both
donors are known to be less compared to the various redox- reduction waves are electrochemically reversible in THF (Fig. S4,
forms of their [O,O] and [O,N] counterparts.7 Additionally, work ESI†). Reversibility was proven by comparing forward and reverse
by the groups of Wieghardt and Neese4c–e have shown the current heights (which is close to 1), and by plotting the square
existence of valence ambiguity in the corresponding cobalt root of the scan rate versus the current peak. The latter plot is
complex with [O,N] and [N,N] donor ligands (Fig. 1). With this linear proving that reduction proceeds reversibly under diffusion
background in mind, the temperature dependence of the control in THF (Fig. S5, ESI†). The oxidation waves become
magnetic susceptibility of 1 was investigated. The results irreversible in THF. This is a possible indication of oxidation-
resemble those of Sundermeyer et al.,4a indicating an S = 1/2 induced binding of THF to the cobalt center. Such five-coordinated
ground state due to strong antiferromagnetic exchange inter- cobalt complexes have been previously isolated as oxidation
actions between the central (HS) Co(II) ion and the surrounding products of the neutral four-coordinate cobalt complexes.4
radical ligands (Fig. S1, ESI†). The room-temperature effective
Inspired by the catalytic wave observed in CH2Cl2 after the
magnetic moment was determined to be 2.47 mB. Furthermore, second reduction of 1, we wanted to investigate the utility of 1
the strong g value anisotropy found in the 5 K EPR spectra of 1 as an (electro)catalyst for C–C bond formation reactions. To test
both in the solid state and in frozen solution confirmed a this, aliquots of benzyl bromide was added to a solution of 1 in
cobalt-centered S = 1/2 spin. Best fits were obtained with gJ = 4.0 THF. Each additional amount of benzyl bromide resulted in
Æ 0.08, g> = 1.78 Æ 0.01 for the solid and gJ = 3.75 Æ 0.08, g>
=
further increase in the catalytic current (Fig. 3), proving a
1.80 Æ 0.02 as well as a hyperfine coupling constant of AJ = 850 catalytic reaction of 1 with benzyl bromide after the second
Æ 30 MHz for the frozen solution. (Fig. S2, ESI†). This com- reduction wave.
bined approach delivered results that are best interpreted by
It should be noted, that the addition of benzyl bromide does
considering the resonance form [(QꢀÀ)CoII,HS(QꢀÀ)]. Thus, it is not lead to a change in the intensity or position of the first
seen that there are no simple correlations between ligand type, reduction wave. The catalytic current was determined for each
geometry at the metal center, and spin states for these classes wave after the addition of benzyl bromide. A plot of icat/ip versus
of cobalt complexes (ca. Fig. 1).
benzyl bromide concentration delivers a linear fit (Fig. S6,
Cyclic voltammogram of 1 in CH2Cl2/0.1 M Bu4NPF6 shows ESI†). Additionally, the plot of icat/ip versus the square root of
two reversible one-electron oxidation waves at À0.21 and 0.30 V the concentration of benzyl bromide is linear as well, proving
and a reversible one-electron reduction wave at À1.47 V vs. Fc/Fc+ that the reaction is first order in benzyl bromide concentration
(Fig. S3, ESI†). A second reduction wave is also observed in (Fig. S7, ESI†). Using the icat/ip ratio, we have calculated the
CH2Cl2, which shows the onset of a catalytic current. This observed rate constant (kobs) at various scan rates. kobs is seen to
observation suggests that complex 1 is capable of activating the be dependent on scan rate, with a stronger dependence
C–Cl bonds of CH2Cl2 catalytically. We note that most complexes observed at lower scan rates (Fig. S8, ESI†). This observation
listed in Fig. 1 show reversible reduction steps in dichloro- is consistent with the formation of the catalytically active
methane, indicating that they do not react with CH2Cl2. Only species after the second reduction process. At a scan rate of
the reduced form of complex [Co(L4)2] has been showed to 100 mV sÀ1, the value of kobs is approximately 10 sÀ1 displaying
perform C–Cl activation by Soper et al.5a Subtle substitution relatively fast reaction rates. We also tested benzyl chloride as a
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as [(QꢀÀ)CoII,HS(QꢀÀ)]. Despite the apparent similarity of this
complex with several reported cobalt complexes, unique cata-
lytic reactivities have been observed with it. Quantification of
the catalytic results and a possible mechanistic picture has
been presented. It is the subtle cooperative interplay between
cobalt and the redox-active ligands that make such catalytic
bond formation possible. The results presented here display
the catalytic utility of redox-active metal complexes in C–C bond
formation reactions, and provide impetus for carrying out
studies with a systematic variation of the ligand backbone.
Such investigations are likely to shed more light on the catalytic
activity of such metal complexes.
Deutsche Forschungsgemeinschaft (DFG, SA 1580/5-1, SL
104/2-1) is kindly acknowledged for financial support of this
project. We thank Prof. Dr M. Dressel for access to the SQUID
¨
facilities of the 1. Physikalisches Institut, Universitat Stuttgart.
Scheme 1 Proposed catalytic cycle for the (electro)catalytic C–C bond
formation reaction with 1.
Notes and references
1 (a) M. D. Ward and J. A. McCleverty, J. Chem. Soc., Dalton Trans., 2002,
275; (b) C. G. Pierpont, Coord. Chem. Rev., 2001, 216–217, 95;
(c) P. Chaudhuri, C. N. Verani, E. Bill, E. Bothe, T. Weyhermu¨ller
and K. Wieghardt, J. Am. Chem. Soc., 2001, 123, 2213.
substrate for this reaction. The trends observed are similar to
the reaction with benzyl bromide (Fig. S10–S12, ESI†). However,
reaction rates with benzyl chloride are slower. For example, at a
scan rate of 100 mV sÀ1, the value of kobs for the reaction with
benzyl chloride is about 2.8 sÀ1. This observation is consistent
with the breaking of the C–halide bond as the possible rate
determining step. Control reactions were also performed with
Bu4NBr to rule out the involvement of species derived from BrÀ
in catalysis. Such a reaction did not display any catalytic current
(Fig. S9 and S13, ESI†).
2 For selected examples see: (a) B. de Bruin, D. G. H. Hetterscheid,
A. J. J. Koekkoek and H.-J. Gru¨tzmacher, Prog. Inorg. Chem., 2007,
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We then turned our attention to the possible mechanism of
this (electro)catalytic reaction. A mixture of in situ generated
[Co(Q2À)2]2À and benzyl chloride were stirred together, and a
ESI mass spectrum of the mixture was recorded. The main
product observed from this mixture was the five-fold coordinated
cobalt complex [Co(QꢀÀ)2(CH2Ph)]À (Fig. S15, ESI†). Additionally,
a chromatographic work-up of the reaction mixture, and 1H NMR
characterization of the organic phase delivered dibenzyl as the
exclusive product (Fig. S16, ESI†). Control experiments showed
that CoCl2 without the ligand does not deliver any product
(Fig. S17, ESI†). Taking these observations into consideration, a
mechanism shown in Scheme 1 can be postulated. The complex
12À activates the C–X bonds of the substrates which lead to a
release of XÀ and the formation of the aforementioned five-fold
coordinated species. The formation of the C–C coupled dibenzyl
product leads to a release of 1À, the reduction of which regene-
rates the active catalyst 12À. This cycle ensures that the 1/1À redox
couple of complex 1 remains unchanged during the catalytic
cycle, as has been experimentally observed (Fig. 3).
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Summarizing, we have presented here a four-coordinate
cobalt complex with a redox-active ligand. The geometric and
electronic structure of this complex has been probed by a
variety of methods, and the complex 1 can be best described
11106 | Chem. Commun., 2014, 50, 11104--11106
This journal is ©The Royal Society of Chemistry 2014