The quest for highly axial Dy SMMs.
Nowadays, the standard approach for Ln SMM design revolves around monometallic complexes and thus relies on knowledge of coordination chemistry to maximize the anisotropy of the Ln cation. Because of the core-like nature of the 4f orbitals, Ln coordination chemistry is dominated by electrostatic interactions and only a small crystal field (CF) splitting is observed for the 4f orbitals, leading to an essentially unquenched orbital moment. Here, 4f-electron-electron repulsion and spin-orbit coupling dominates the electronic structure of Ln(III) ions,[10] and later 4f ions such as Tb(III), Dy(III), Ho(III) and Er(III) have the largest magnetic moments and thus have been the main target for the design of high-performance SMMs. In the case of Dy(III) (6H15/2), the largest projection of the ground-state total angular momentum (mJ = ± 15/2) has an oblate-shaped (pancake-like) 4f electron density, whilst the smallest projection (m J = ± 1/2) has a prolate (cigar-like) shape. Thus an axial ligand field stabilizes the most magnetic states and destabilizes the least magnetic ones, thus setting an obvious target for coordination chemists: a two-coordinate Dy(III) complex.
Molecular f-element chemistry and magnetism had long been focal areas of research at The University of Manchester, on which this account focuses,[15] and where Mills started a lectureship in 2012, and Chilton commenced his PhD studies with Winpenny and McInnes soon after. Our personal journey started with the work of Chiltonet al. in 2013, who reported a simple electrostatic model for facile prediction of the magnetic anisotropy axis of monometallic Dy(III) SMMs.[16] The synthesis group led by Mills was working on low-coordinate Ln(II) complexes and isolated a near-linear two-coordinate Sm(II) complex, [Sm{N(SiiPr3)2}2], in 2014.[17] Chilton and Winpenny proposed that this complex could be a blueprint for an idealized highly axial [E–Dy–E]+ species (E = alkyl, amide) which calculations indicated could lead to values ofU eff approaching 2000 K;[17,18] at the time, the largestU eff to date was 938 K, reported by Coronado and co-workers in a Tb phthalocyanide derivative.[19]Soon after our proposal, our colleagues at Manchester and others further afield reported highly-axial Dy(III) SMMs with coordination numbers of 6 or 7 with U eff values of ~1000 K.[13] The ball was firmly back in the synthetic chemists’ court to prepare Dy(III) SMMs with lower coordination numbers, and the Mills Group, together with the fellow awardees of the aforementioned EPSRC grant, were presented with the challenge of delivering such species. Two synthetic strategies for the target complex [Dy{N(SiiPr3)2}2]+(1 ) were devised (Scheme 1). One started with the synthesis of the analogous Dy(II) complex [Dy{N(SiiPr3)2}2] (Scheme 1, Route A), followed by direct oxidation to a trivalent complex cationic complex, 1 , (using e.g. AgBPh4) or via a Dy(III) halide intermediate followed by halide abstraction to form 1 .