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

Hohloch, Stephan

Meer, Margarethe van der

Peremykin, Irina

Rechkemmer, Yvonne

Sarkar, Biprajit

Slageren, Joris van

Institutional Repository of the Freie Universität Berlin

As featured in:See Joris van Slageren, Biprajit Sarkar et al., Chem. Commun., 2014, 50, 11104.Showcasing work from the laboratory of Biprajit Sarkar, Institut für Chemie und Biochemie, Freie Universität Berlin, Germany.(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.Registered charity number: 207890www.rsc.org/chemcomm11104 | Chem. Commun., 2014, 50, 11104--11106 This journal is©The Royal Society of Chemistry 2014Cite this:Chem. Commun., 2014,50, 11104(Electro)catalytic C–C bond formation reactionwith a redox-active cobalt complex†Margarethe van der Meer,a Yvonne Rechkemmer,b Irina Peremykin,bStephan Hohloch,a Joris van Slageren*b and Biprajit Sarkar*aCooperativity between cobalt and non-innocent ligands in electrontransfer processes has been utilized for (electro)catalytic C–C bondformation reactions.Redox-active ligands have long fascinated coordination chemistsbecause of the unusual and curious electronic structures of theirmetal complexes.1 More recently chemists have started developingnovel catalysts based on metal complexes of non-innocentligands.2 One feature of redox-active ligands which is often utilizedin such catalytic processes is their active participation in variouselectron transfer steps. Quinones are an important class of redox-active ligands, and historically metal complexes of the all oxygendonating o-quinone ligand have been the most studied.1b Morerecently, 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protection through the R groups beingobvious gains of the approach.3d o-Diiminoquinones where boththe [O] groups have been replaced by [NR] have found ratherlimited use.4 Some such [NR] containing ligands used for synthe-sizing four-coordinate cobalt complexes have been listed inFig. 1.4,5 We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 beimmensely helpful while using metal complexes of these ligandsfor various catalytic transformations.6 Herein we present the cobaltcomplex 1 with the ligand Q. Geometric and electronic structureof this complex has been probed by a battery of methods, and itsutility as an (electro)catalyst for C–C bond formation reactions hasbeen investigated.The complex 1 was synthesized by deprotonation of H2Q withn-BuLi, its reaction with CoCl2, and the subsequent oxidation ofthe reaction mixture with pure O2. Re-crystallization provided 1in the pure form. The geometric structure of 1, which crystallizesas a CH2Cl2 solvate, was determined by single crystal X-raydiﬀraction studies. The cobalt center in 1 is in a (pseudo)tetra-hedral environment; being coordinated by the nitrogen donor oftwo diﬀerent ligands (Fig. 2).The t value for this structure is 0.46. The dihedral anglebetween the two coordinating planes is 501. The Co–N bonddistances of about 1.92 Å points to a high spin (HS) Co(II)Fig. 1 Selected redox-active ligands with which four-coordinate cobaltcomplexes have been reported together with the geometry and spin-states of those cobalt complexes.a Institut fu¨r Chemie und Biochemie, Anorganische Chemie, Freie Universita¨t Berlin,Fabeckstraße 34-36, D-14195, Berlin, Germany. E-mail: biprajit.sarkar@fu-berlin.deb Institut fu¨r Physikalische Chemie, Universita¨t Stuttgart, Pfaﬀenwaldring 55, D-70569,Stuttgart, Germany† Electronic supplementary information (ESI) available: Details of synthesis,crystallography, magnetic measurements, EPR spectroscopy, electrochemistryand mechanistic studies. CCDC 902892. For ESI and crystallographic data inCIF or other electronic format see DOI: 10.1039/c4cc03309dReceived 3rd May 2014,Accepted 28th May 2014DOI: 10.1039/c4cc03309dwww.rsc.org/chemcommChemCommCOMMUNICATIONPublished on 29 May 2014. Downloaded by Freie Universitaet Berlin on 16/10/2014 09:50:34. View Article OnlineView Journal  | View IssueThis journal is©The Royal Society of Chemistry 2014 Chem. Commun., 2014, 50, 11104--11106 | 11105center. The intra-ring C–C bond display a quinoidal distortionwith two short and four long bonds (Table S2, ESI†). The C–Nbond lengths of 1.36 Å also points to the existence of theseligands as Q, and hence neutral 1 can be formulated as[(Q)CoII,HS(Q)]. However, the difference in intra-ring bondlengths for the various redox forms of ligands with all nitrogendonors are known to be less compared to the various redox-forms of their [O,O] and [O,N] counterparts.7 Additionally, workby the groups of Wieghardt and Neese4c–e have shown theexistence of valence ambiguity in the corresponding cobaltcomplex with [O,N] and [N,N] donor ligands (Fig. 1). With thisbackground in mind, the temperature dependence of themagnetic susceptibility of 1 was investigated. The resultsresemble those of Sundermeyer et al.,4a indicating an S = 1/2ground state due to strong antiferromagnetic exchange inter-actions between the central (HS) Co(II) ion and the surroundingradical ligands (Fig. S1, ESI†). The room-temperature effectivemagnetic moment was determined to be 2.47 mB. Furthermore,the strong g value anisotropy found in the 5 K EPR spectra of 1both in the solid state and in frozen solution confirmed acobalt-centered S = 1/2 spin. Best fits were obtained with gJ = 4.0 0.08, g> = 1.78  0.01 for the solid and gJ = 3.75  0.08, g> =1.80  0.02 as well as a hyperfine coupling constant of AJ = 850 30 MHz for the frozen solution. (Fig. S2, ESI†). This com-bined approach delivered results that are best interpreted byconsidering the resonance form [(Q)CoII,HS(Q)]. Thus, it isseen that there are no simple correlations between ligand type,geometry at the metal center, and spin states for these classesof cobalt complexes (ca. Fig. 1).Cyclic voltammogram of 1 in CH2Cl2/0.1 M Bu4NPF6 showstwo reversible one-electron oxidation waves at 0.21 and 0.30 Vand a reversible one-electron reduction wave at1.47 V vs. Fc/Fc+(Fig. S3, ESI†). A second reduction wave is also observed inCH2Cl2, which shows the onset of a catalytic current. Thisobservation suggests that complex 1 is capable of activating theC–Cl bonds of CH2Cl2 catalytically. We note that most complexeslisted in Fig. 1 show reversible reduction steps in dichloro-methane, indicating that they do not react with CH2Cl2. Onlythe reduced form of complex [Co(L4)2] has been showed toperform C–Cl activation by Soper et al.5a Subtle substitutionchanges on the ligand backbone is thus shown to dramaticallyinfluence redox-induced chemical reactivity in these complexes. Toprove the hypothesis of a catalytic reaction of complex 1 withCH2Cl2 following the second reduction step, we recorded the cyclicvoltammogram of this complex in THF. Gratifyingly, bothreduction waves are electrochemically reversible in THF (Fig. S4,ESI†). Reversibility was proven by comparing forward and reversecurrent heights (which is close to 1), and by plotting the squareroot of the scan rate versus the current peak. The latter plot islinear proving that reduction proceeds reversibly under diffusioncontrol in THF (Fig. S5, ESI†). The oxidation waves becomeirreversible in THF. This is a possible indication of oxidation-induced binding of THF to the cobalt center. Such five-coordinatedcobalt complexes have been previously isolated as oxidationproducts of the neutral four-coordinate cobalt complexes.4Inspired by the catalytic wave observed in CH2Cl2 after thesecond reduction of 1, we wanted to investigate the utility of 1as an (electro)catalyst for C–C bond formation reactions. To testthis, aliquots of benzyl bromide was added to a solution of 1 inTHF. Each additional amount of benzyl bromide resulted infurther increase in the catalytic current (Fig. 3), proving acatalytic reaction of 1 with benzyl bromide after the secondreduction wave.It should be noted, that the addition of benzyl bromide doesnot lead to a change in the intensity or position of the firstreduction wave. The catalytic current was determined for eachwave after the addition of benzyl bromide. A plot of icat/ip versusbenzyl bromide concentration delivers a linear fit (Fig. S6,ESI†). Additionally, the plot of icat/ip versus the square root ofthe concentration of benzyl bromide is linear as well, provingthat the reaction is first order in benzyl bromide concentration(Fig. S7, ESI†). Using the icat/ip ratio, we have calculated theobserved rate constant (kobs) at various scan rates. kobs is seen tobe dependent on scan rate, with a stronger dependenceobserved at lower scan rates (Fig. S8, ESI†). This observationis consistent with the formation of the catalytically activespecies after the second reduction process. At a scan rate of100 mV s1, the value of kobs is approximately 10 s1 displayingrelatively fast reaction rates. We also tested benzyl chloride as aFig. 2 ORTEP plot of 1. Ellipsoids are drawn at 50% probability. Hydrogenatoms 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 s1.Communication ChemCommPublished on 29 May 2014. Downloaded by Freie Universitaet Berlin on 16/10/2014 09:50:34. View Article Online11106 | Chem. Commun., 2014, 50, 11104--11106 This journal is©The Royal Society of Chemistry 2014substrate for this reaction. The trends observed are similar tothe reaction with benzyl bromide (Fig. S10–S12, ESI†). However,reaction rates with benzyl chloride are slower. For example, at ascan rate of 100 mV s1, the value of kobs for the reaction withbenzyl chloride is about 2.8 s1. This observation is consistentwith the breaking of the C–halide bond as the possible ratedetermining step. Control reactions were also performed withBu4NBr to rule out the involvement of species derived from Brin catalysis. Such a reaction did not display any catalytic current(Fig. S9 and S13, ESI†).We then turned our attention to the possible mechanism ofthis (electro)catalytic reaction. A mixture of in situ generated[Co(Q2)2]2 and benzyl chloride were stirred together, and aESI mass spectrum of the mixture was recorded. The mainproduct observed from this mixture was the five-fold coordinatedcobalt complex [Co(Q)2(CH2Ph)] (Fig. S15, ESI†). Additionally,a chromatographic work-up of the reaction mixture, and 1H NMRcharacterization of the organic phase delivered dibenzyl as theexclusive product (Fig. S16, ESI†). Control experiments showedthat CoCl2 without the ligand does not deliver any product(Fig. S17, ESI†). Taking these observations into consideration, amechanism shown in Scheme 1 can be postulated. The complex12 activates the C–X bonds of the substrates which lead to arelease of X and the formation of the aforementioned five-foldcoordinated species. The formation of the C–C coupled dibenzylproduct leads to a release of 1, the reduction of which regene-rates the active catalyst 12. This cycle ensures that the 1/1 redoxcouple of complex 1 remains unchanged during the catalyticcycle, as has been experimentally observed (Fig. 3).Summarizing, we have presented here a four-coordinatecobalt complex with a redox-active ligand. The geometric andelectronic structure of this complex has been probed by avariety of methods, and the complex 1 can be best describedas [(Q)CoII,HS(Q)]. Despite the apparent similarity of thiscomplex with several reported cobalt complexes, unique cata-lytic reactivities have been observed with it. Quantification ofthe catalytic results and a possible mechanistic picture hasbeen presented. It is the subtle cooperative interplay betweencobalt and the redox-active ligands that make such catalyticbond formation possible. The results presented here displaythe catalytic utility of redox-active metal complexes in C–C bondformation reactions, and provide impetus for carrying outstudies with a systematic variation of the ligand backbone.Such investigations are likely to shed more light on the catalyticactivity of such metal complexes.Deutsche Forschungsgemeinschaft (DFG, SA 1580/5-1, SL104/2-1) is kindly acknowledged for financial support of thisproject. We thank Prof. Dr M. Dressel for access to the SQUIDfacilities of the 1. Physikalisches Institut, Universita¨t Stuttgart.Notes and references1 (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¨llerand K. Wieghardt, J. Am. Chem. Soc., 2001, 123, 2213.2 For selected examples see: (a) B. de Bruin, D. G. H. Hetterscheid,A. J. J. Koekkoek and H.-J. Gru¨tzmacher, Prog. Inorg. Chem., 2007,55, 247; (b) P. J. Chirik and K. Wieghardt, Science, 2010, 327, 794;(c) V. K. K. Praneeth, M. R. Ringenberg and T. R. Ward, Angew. Chem.,2012, 124, 10374; (d) M. K. Tsai, J. Rochford, D. E. Polyansky,T. Wada, K. Tanaka and J. T. Muckerman, Inorg. Chem., 2009,48, 4372; (e) Forum Issue on Redox-Active Ligands, Inorg. Chem.,2011, 50, 9737–9914; ( f ) Cluster Issue, Cooperative and Redox Non-Innocent Ligands in Directing Organometallic Reactivity, Eur.J. Inorg. Chem., 2012, 340–580; (g) V. Lyaskovskyy and B. de Bruin,ACS Catal., 2012, 2, 270.3 For selected examples see: (a) A. I. Poddel’sky, V. K. Cherkasov andG. A. Abakumov, Coord. Chem. Rev., 2009, 253, 291; (b) Z. Sun,H. Chun, K. Hildebrandt, E. Boethe, T. Weyhermu¨ller, F. Neese andK. Wieghardt, Inorg. Chem., 2002, 41, 4295; (c) P. Ghosh, A. Begum,D. Herebian, E. Boethe, K. Hildebrand, T. Weyhermu¨ller andK. Wieghardt, Angew. Chem., Int. Ed., 2003, 42, 563;(d) M. R. Ringenberg, S. L. Kokatam, Z. M. Heiden andT. B. Rauchfuss, J. Am. Chem. Soc., 2008, 130, 788; (e) D. Das,H. Agarwala, A. Dutta Chowdhury, T. Patra, S. M. Mobin, B. Sarkar,W. Kaim and G. K. Lahiri, Chem. – Eur. J., 2013, 19, 7384.4 (a) M. M. Khusniyarov, K. Harms, O. Burghaus, J. Sundermeyer,B. Sarkar, W. Kaim, J. van Slageren, C. Duboc and J. Fiedler, DaltonTrans., 2008, 1355; (b) A. L. Balch and R. H. Holm, J. Am. Chem. Soc.,1966, 88, 5201; (c) D. Herebian, K. Wieghardt and F. Neese, J. Am.Chem. Soc., 2003, 125, 10997; (d) E. Bill, E. Bothe, P. Chaudhuri,K. Chlopek, D. Herebian, S. Kokatam, K. Ray, T. Weyhermu¨ller,F. Neese and K. Wieghardt, Chem. – Eur. J., 2005, 11, 204;(e) K. Chlopek, E. Bothe, F. Nesse, T. Weyhermu¨ller andK. Wieghardt, Inorg. Chem., 2006, 45, 6298.5 (a) A. L. Smith, K. I. Hardcastle and J. D. Soper, J. Am. Chem. Soc.,2010, 132, 14358; (b) W. I. Dzik, J. I. van der Vlugt, J. N. H. Reek andB. de Bruin, Angew. Chem., Int. Ed., 2011, 50, 3356.6 (a) N. Deibel, D. Schweinfurth, S. Hohloch, J. Fiedler and B. Sarkar,Chem. Commun., 2012, 48, 2388; (b) N. Deibel, D. Schweinfurth,S. Hohloch, M. Delor, I. V. Sazanovich, M. Towrie, J. Weinstein andB. Sarkar, Inorg. Chem., 2014, 53, 1021; (c) S. Hohloch, P. Braunsteinand B. Sarkar, Eur. J. Inorg. Chem., 2012, 546.7 S. Bhattacharya, P. Gupta, F. Basuli and C. G. Pierpont, Inorg. Chem.,2002, 41, 5810.Scheme 1 Proposed catalytic cycle for the (electro)catalytic C–C bondformation reaction with 1.ChemComm CommunicationPublished on 29 May 2014. Downloaded by Freie Universitaet Berlin on 16/10/2014 09:50:34. View Article Online

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

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

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