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Chemical Communications

www.rsc.org/chemcomm Volume 49 | Number 99 | 25 December 2013 | Pages 11577–11708

COMMUNICATION Hyunwoo Kim et al.Controlling helical chirality of cobalt complexes by chirality transfer from vicinal diamines

This journal is c The Royal Society of Chemistry 2013 Chem. Commun., 2013, 49, 11623--11625 11623

Cite this: Chem. Commun.,2013,49, 11623

Controlling helical chirality of cobalt complexes bychirality transfer from vicinal diamines†

Min-Seob Seo,ab Kiseong Kimab and Hyunwoo Kim*ab

Stereoselective cis–trans isomerism of a CoIII–N2O2 complex has been

achieved by coordinating chiral vicinal diamines. The induced metal-

centered helical chirality can be exploited for chirality amplification

and asymmetric coordination chemistry.

There has been considerable fascination with asymmetric coordina-tion chemistry since the discovery of metal-centered chirality byAlfred Werner in 1911.1 In particular, asymmetric octahedral metalcomplexes have received increasing attention in catalysis, materialscience, and pharmaceutics.2 Unlike well-established asymmetriccontrol of carbon-centered chirality, controlling metal-centered chir-ality2a–c has proved to be challenging. In most examples, metal-centered chirality has been generated by stereoselective assembly ofmetals and chiral ligands.2c–e However, octahedral metal complexescan have a propeller-like helical arrangement with achiral ligandssuch as ethylenediamine (en) and 2,20-bipyridine (bpy). The resultingoctahedral M(en)3 or M(bpy)3 complexes exhibit helical chiralitywith D and L configurations.1 Indeed, stereoselective generation ofhelical octahedral metal complexes with achiral ligands is a greaterchallenge than with chiral ligands, and only recently it has beenshown that stereodynamic metal-centered chirality of M(bpy)3 can becontrolled by chiral auxiliaries,3 chiral anions,4 and chiral catalysts,5

as well as long-range chiral induction.6 Given that the asymmetriccoordination chemistry is still under development, it is highlydesirable to expand such concepts to a wide range of stereodynamicmetal complexes. Here we report stereoselective generation of metal-centered helical chirality of an unprecedented CoIII N2O2 complex bychirality transfer from chiral vicinal diamines.

The tetradentate N2O2 salen ligand is one of the well-knownprivileged ligands for developing stereoselective catalysts pioneeredby Jacobsen and Katsuki.7 Most metal salen complexes show

a trans configuration, but some cis-b metal salen complexes ofTiIV, AlIII, and RuII have been reported and used for stereo-selective catalysis.8 Such geometrical isomerization of metalsalen complexes appears to originate from the nature of metals andligands. In addition, we propose that the geometry conversionfrom trans to cis would be possible in metal salen complexes bycoordinating a bidentate ligand due to the availability of the cis-binding site only in the cis configuration (Scheme 1). Interestingly,metal centers for the cis configuration become stereogenic whilethose for the trans configuration are achiral. Thus, stereoselectivegeneration of metal-centered helical chirality can be studied duringisomerization by chiral bidentate ligands. We have also studied thepossibility of asymmetric coordination chemistry by substitutionwith an achiral ligand (Scheme 1). To the best of our knowledge, thestereoselective transition from an achiral trans to a chiral cisconfiguration in metal salen complexes has not yet been reported.

We prepared a salen-type ligand 3 by replacing an aldimine witha ketimine of 2,20-dihydroxybenzophenone (1)9 because the ketiminestructure is known to favor the formation of cis-b-CoIII complexes.10

A new N2O2 salen-type ligand (3) was prepared as illustrated inScheme 2. When ethylenediamine was mixed with 2,20-dihydroxy-benzophenone (1) at 60 1C, 1 : 1 imine compound 2 was formed andprecipitated out of a MeOH solution (85% yield). Unsymmetricalbisimine 3 was then synthesized by the reaction of 2 with 3,5-di-tert-butyl-2-hydroxybenzaldehyde in 84% yield. A [Co-3]OTs complex was

Scheme 1 Asymmetric cis–trans isomerization in M(N2O2) induced by a chiralbidentate ligand (L*) and asymmetric coordination chemistry.

a Department of Chemistry, KAIST, Daejeon 305-701, Korea.

E-mail: hwkim@kaist.edub Center for Nanomaterials and Chemical Reactions, Institute for Basic Science

(IBS), Daejeon 305-701, Korea

† Electronic supplementary information (ESI) available: Synthetic procedures,and spectroscopic, spectrometric, calculation and crystallographic details. CCDC959826. For ESI and crystallographic data in CIF or other electronic format seeDOI: 10.1039/c3cc46887a

Received 9th September 2013,Accepted 3rd October 2013

DOI: 10.1039/c3cc46887a

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11624 Chem. Commun., 2013, 49, 11623--11625 This journal is c The Royal Society of Chemistry 2013

prepared by following the published procedure of [Co-salen]OTs.11

When 1 equiv. of rac-dpen (rac-1,2-diphenylethylenediamine) wasadded to a methanolic solution of [Co-3]OTs at ambient tempera-ture, several tert-butyl peaks were initially observed (see ESI†). To oursurprise, when the solution was heated to 70 1C for 6 h, a remarkablyclean 1H NMR spectrum resulted with only two sets of tert-butylpeaks in a 7 : 1 ratio (Scheme 2b and Fig. 1a). The major product wasisolated in an 18 : 1 ratio by recrystallization in EtOH/pentane(Fig. 1b). When the major product was heated to 70 1C in methanol,the aforementioned 7 : 1 product ratio was obtained. Although theproduct ratio was expected to be 13 : 1 at 25 1C, no minor productwas observed when the pure major product was dissolved in CD3ODor DMSO-d6 for 7 days at 25 1C. The experimental data indicate thatthe thermodynamic products obtained at 70 1C do not interconvertat ambient temperature due to the kinetic barrier.

The crystal structure of [Co-3-(rac)-dpen]OTs revealed the majorcompound as a cis-b octahedral complex (Fig. 2). The cis-b-[Co-3]OTsis chiral with D and L configurations (Scheme 2b). In principle, rac-dpen can chelate to cis-b-[Co-3]+ to form diastereomeric mixtures ofD-[Co-3-(R,R)-dpen]+ and L-[Co-3-(R,R)-dpen]+ as well as D-[Co-3-(S,S)-dpen]+ and L-[Co-3-(S,S)-dpen]+. Interestingly, the unit cell consistsof only two octahedral CoIII complexes where (R,R)-dpen chelatedto the D-form of [Co-3]+, and (S,S)-dpen chelated to the L-form of[Co-3]+ (ESI†). The crystals of [Co-3-(rac)-dpen]OTs were obtained indiastereomeric purity close to 100% as a racemate. Indeed, the1H NMR spectrum of crystals dissolved in DMSO-d6 is identical to thatof the major complex prepared from the solution (Fig. 1b). BecauseCoIII complexes are known to be kinetically inert to substitution, thesolid-state structure (X-ray) and the major diastereomer in solution(1H NMR) can be considered to be the same.

Computation can be used to help explain the selective chelationof dpen by cis-b-[Co-3]OTs. DFT computation shows that D-[Co-3-(R,R)-dpen]+ is more stable than L-[Co-3-(R,R)-dpen]+ by about1.2 kcal mol�1 (Scheme 2b).12 This energy difference translates toan equilibrium ratio of about 5.6 for D-[Co-3-(R,R)-dpen]+ to L-[Co-3-(R,R)-dpen]+ at 70 1C. It appears that steric hindrance between thephenyl group of dpen and the tert-butyl groups of 3 is responsiblefor the energy difference. Considering additional effects such assolvents and counter anions, the calculated value is in excellentagreement with the experimental results. Previously, cis-a ZnII com-plexes were used for recognition of chiral vicinal diamines.13

Although rigid C2 symmetric chiral ligands were used, the obtainedstereoselectivity is moderate ranging from 2 : 1 to 5 : 1. It appears thatthe cis-b complex exhibits higher selectivity in diamine recognitionthan the cis-a complex.

Since the chirality of (R,R)-dpen is selectively imprinted ontocis-b-[Co-3]+ with a D-configuration, the helical contents of the CoIII

complex can be verified by circular dichroism (CD) spectroscopy.14

Fig. 3 shows CD and UV-vis spectra of [Co-3-(R,R)-dpen]OTs and[Co-3-(S,S)-dpen]OTs. There are strong Cotton effects of the sameamplitude but of opposite sign in [Co-3-(R,R)-dpen]OTs and [Co-3-(S,S)-dpen]OTs. Since the CD signal of (R,R)-dpen appears only at250 nm, the CD signal at 300–700 nm is due to the metal-to-ligand-charge-transfer (MLCT) bands of [Co-3]+. Moreover, electronic CDcalculation yielded simulated CD spectra that closely matched withthe experimental CD spectra (ESI†).12 As observed in Fig. 3, theobserved CD signals are in a good linear relationship with the

Scheme 2 (a) Synthesis of N2O2 salen-type ligand 3 and (b) stereoselectivechirality transfer from (R,R)-dpen to [Co-3]OTs.

Fig. 1 Partial 1H NMR spectra of a [Co-3-(R,R)-dpen]OTs showing signals of tert-butyl groups. (a) Mixtures at 70 1C and (b) recrystallized product.

Fig. 2 ORTEP representation (50% probability) of the crystal structure ofD-[Co-3-(R,R)-dpen]+. Its enantiomer L-[Co-3-(S,S)-dpen]+, tosylate, and solventethanol are not shown. All hydrogens except for those in dpen and phenol areomitted for clarity.

Fig. 3 UV-vis and CD spectra of [Co-3-(R,R)-dpen]OTs and [Co-3-(S,S)-dpen]OTsand CD spectra of (R,R)-dpen (100 mM in acetonitrile, 1 cm cell, at 20 1C).

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enantiomeric excess (ee) of dpen (ESI†). In principle, [Co-3]OTs canbe used for sensing the enantiopurity of dpen.

In order to increase the observed stereoselectivity (7 : 1 with dpen),we used a more sterically bulky chiral diamine that may increase theenergy difference between two helical isomers, based on the crystaland calculated structures. Indeed, when 1,2-bis(2,4,6-trimethyl-phenyl)ethylenediamine was used, only a single isomer was formedto any observable extent in 1H NMR spectra (Scheme 2b). Lastly, westudied asymmetric coordination chemistry of an octahedral cis-b-CoIII complex by replacing the chiral diamine with an achiral ligand.Heating D-[Co-3-(R,R)-dpen]OTs with an excess amount of ethylene-diamine (en) gave a clean product of [Co-3-en]OTs, but the complexwas CD inactive (Fig. 4). In contrast, when excess bipyridine (bpy)was used in the presence of trifluoroacetic acid (TFA), the dpenwas cleanly replaced by bpy, and the resulting complexes wereCD active (Fig. 4). Although we were not able to determine thestructure and stereoselectivity of [Co-3-bpy]+, the active CDspectra indicated that the helical chirality could be retained. Weare currently exploring the asymmetric coordination chemistry ofCoIII complexes.

In summary, we have demonstrated stereoselective cis–transisomerization in a CoIII–N2O2 complex induced by a chiralvicinal diamine. (R,R)-Dpen chelates [Co-3]OTs to form D-[Co-3-(R,R)-dpen]OTs selectively, as confirmed by 1H NMR spectraand X-ray structure analysis. DFT computation can be used toexplain the origin of stereoselectivity. The cis-b-CoIII complexexhibits strong CD signals, which were closely simulated byelectronic TD-DFT calculations, due to the induced chirality. Themetal-centered helical chirality was shown to be retained whenthe chiral diamine was replaced by an achiral bpy ligand. Thiswork demonstrates chirality transfer from ligands to stereo-dynamic metal complexes and the asymmetric coordinationchemistry of CoIII-salen-type complexes.

We are grateful to KAIST (HRHRP), the National ResearchFoundation of Korea (2011-0014197), and the Institute for BasicScience (IBS) for supporting this research. We thank Dr AlanJ. Lough (University of Toronto) for X-ray analysis.

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9 Two OH groups of 1 significantly enhance the imine formation. See:H. Kim, S. M. So, C. P.-H. Yen, E. Vinhato, A. J. Lough, J.-I. Hong,H.-J. Kim and J. Chin, Angew. Chem., Int. Ed., 2008, 47, 8657.

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12 The Gaussian 09 was used for all calculations. We used B3LYP/6-31G(d,p) basis for C, H, N and O and LANL2DZ for Co. TD-SCFmethod was used for electronic CD simulation and CD spectra weregenerated using the program SpecDis v. 1.61.

13 (a) W. Kim, S. M. So, L. Chagal, A. J. Lough, B. M. Kim and J. Chin,J. Org. Chem., 2006, 71, 8966; (b) W. Kim, C. Chun, H. Kim,A. J. Lough, B. M. Kim and J. Chin, Bull. Korean Chem. Soc., 2007,28, 133.

14 Circular Dichroism: Principles and Applications, ed. K. Nakanishi,N. Berova and R. W. Woody, Wiley-VCH, New York, 2nd edn, 2000.

Fig. 4 Asymmetric coordination chemistry of CoIII complexes.

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