COMMUNICATION First example of 3d/Dy coordination clusters as catalysts in a domino reaction

Three isoskeletal tetranuclear coordination clusters with general formula [M2Dy III 2L4(EtOH)6](ClO4)2·2(EtOH), (M = Co, 1; M= Ni, 2) and [Ni2Dy III 2L4Cl2(CH3CN)2]·2(CH3CN) (3), have been synthesized and characterized. These air-stable compounds, and in particular 3, display efficient homogeneous catalytic behavior towards the room temperature synthesis of trans-4,5diaminocyclopent-2-enones from 2-furaldehyde and primary or secondary amines under a non-inert atmosphere. In the last two decades interest in the construction of polynuclear 3d/4f Coordination Clusters (CCs) was driven by their aesthetically pleasing structures, however the interest burgeoned due to the inartistic magnetic properties that such species display. The synthesis of such species can be aptly described by the term “Serendipitous Self Assembly” [8] but, in recent years, tremendous attention has been given on the selection of the appropriate organic ligands to develop the syntheses of such CCs. Apart from the employment of an heptanuclear CeMn6 CC for the oxidation of benzyl alcohol to benzaldehyde, less attention has been given to the catalytic properties of such heteronuclear compounds. For instance, dysprosium triflate, due to its mild nature, has proven to be an excellent catalyst for reactions where both nitrogen and oxygen functionalities are present. Of particular interest is its ability to retain catalytic activity in the presence of Lewis-basic nitrogen groups, allowing for its use in transformations involving unprotected amines. We recently initiated a project aiming to develop dual catalytic CCs and to achieve this goal, compounds that retain their topology into the solution are required. 3d/Dy CCs possessing a defect dicubane topology derived from (E)-2-(2hydroxy-3-methoxybenzylidene-amino)phenol (H2L), pose as ideal candidates to serve this purpose. H2L supports a defect dicubane topology bearing two divalent 3d ions and two trivalent 4f ions in the absence of any additional bridging atom. Indeed, the combination in open air of H2L with Dy(OTf)3 and Co(ClO4)2·6H2O or Ni(ClO4)2·6H2O in the presence of Et3N in EtOH afforded air-stable tetranuclear defect dicubane compounds with general formula [M2Dy2L4(EtOH)6](ClO4)2·2(EtOH) (M = Co, 1; M = Ni, 2) (Figure 1). Each 3d center has an octahedral geometry and each Dy center has square antiprismatic geometry. Two different coordination modes can be found for the organic ligand (Scheme S1), occupying five and six vertices for the 3d and the Dy centers, respectively. The remaining vertices are occupied by one (3d) and two (Dy) ethanol molecules. To further confirm the identity of compounds 1 and 2 in solution, we performed a broad electrospray ionization mass (ESI-MS) spectrometry study. For 1, we observed two peaks in the MS (positive ion mode) at 1506.9719 m/z and at 736.0393 m/z which perfectly correspond to two fragments, [Co2Dy2L4(ClO4)-H] and [Co2Dy2L4(MeOH)2-H], respectively (Figs S1-S2). Similarly, for 2, peaks at 1520.0169 and 744.55 correspond to {[Ni2Dy2L4(MeOH)2(EtOH)]+2H} and {[Ni2Dy2L4(MeOH)(EtOH)]+2H} fragments (Figs S3-S4), respectively. Therefore, in both cases, the ESI-MS data indicate that the dicationic [M2Dy2L4] cores remain intact. Figure 1. Molecular structure of 2 (1 is isostructural with Co atoms in the Ni positions). Colour code: Ni, green; Dy, light blue; O, red; N, blue; C, black; Cl, purple. Hydrogen atoms are omitted for clarity With the CCs in hand, the next step was to identify a challenging catalytic reaction. Batey and co-workers reported on the use of Dy(OTf)3 as a Lewis acid catalyst for the domino condensation/ring-opening/electrocyclization of secondary amines and 2-furaldehyde, leading to the synthesis of exclusively trans-4,5-diaminocyclopent-2-enones. This is a remarkably atom-efficient reaction with only 1 equivalent of water generated as a sub-product. The authors postulated that the reaction proceeds through a deprotonated Stenhouse salt intermediate, although they showed that the lanthanide is not involved in the cyclization step (Scheme 1). Primary amines required the use of more expensive Sc(OTf)3 to produce the corresponding products, albeit in considerably lower yields. [a] K. Griffiths, Dr. C. W. D. Gallop, Dr. A. Abdul-Sada, Dr. A. Vargas, Dr. O. Navarro, Dr. G. E. Kostakis Department of Chemistry, School of Life Sciences, University of Sussex, Brighton BN1 9QJ, United Kingdom E-mail: [email protected], Supporting information for this article is given via a link at the end of the document.