Synthesis of Axial Lanthanide Cyclopentadienyl Single-Molecule Magnets

Abstract

The retention of magnetisation observed for lanthanide single-molecule magnets (Ln SMMs), results from single ion magnetic anisotropy when a Ln cation is placed in a suitable coordination environment. Currently, their potential applications in high density binary data storage are limited by the low temperature conditions required to unlock the desired properties. The introduction chapter gives an overview of synthetic approaches to obtain low coordinate Ln complexes and a summary of how these investigations have led to the best Ln SMMs isolated to date. The remarkable magnetic properties of dysprosocenium complexes, [Dy(CpR)2][WCA] (CpR = C5R5; WCA = weakly coordinating anion), are attributable to their highly axial ligand fields and the rigidity of the ligands, which suppress magnetic relaxation mechanisms. These properties could be improved by increasing the axiality experienced by the cation. This has inspired the investigations herein into the lower limits of steric bulk required for substituted cyclopentadienyl ligands that prohibit equatorial interactions between the Dy centre and other ligands, which would introduce transverse fields and inhibit SMM properties. In Paper one we created the AtomAccess computer program to rapidly screen a range of [Dy(CpR)2]+ cations and predict a heteroleptic system that is at the limit of forming transverse metal-ligand interactions, namely [Dy(Cpttt)(Cp*)]+ (Cpttt = C5H2tBu3-1,2,4, Cp* = C5Me5). We confirmed these predictions experimentally with the crystallization of both the separated ion-pair (SIP) [Dy(Cpttt)(Cp*)][Al{OC(CF3)3}4] and the contact ion-pair (CIP) polymorph [Dy(Cpttt)(Cp*){Al[OC(CF3)3]4-κ-F}]. It was found that the resultant magnetic properties of the SIP were comparable to previously reported dysprosocenium SMMs, whilst the presence of a weak equatorial interaction between the Dy centre and a WCA in the CIP suppressed magnetic properties. In Paper two we investigated the effects of equatorially-bound weak donor ligands on the magnetic properties of the [Dy(Cpttt)(Cp*)]+ cation. The recrystallization of “[{Dy(Cpttt)(Cp*)}{Al[OC(CF3)3]4}]” from a range of halobenzenes afforded the structurally analogous series of mono-haloarene complexes [Dy(Cpttt)(Cp*)(PhX-κ-X)][Al{OC(CF3)3}4] (X = F, Cl, Br) and the related complex [Dy(Cpttt)(Cp*)(C6H4F2-κ2-F,F)][Al{OC(CF3)3}4], presenting the first structurally authenticated examples of halobenzene-bound Ln complexes. Surprisingly, the low temperature magnetic relaxation times of the fluorobenzene and chlorobenzene adducts are markedly enhanced compared to both the SIP and CIP of the {Dy(Cpttt)(Cp*)} motif. In Paper three the methodology from Paper two was expanded to SMMs containing a {Dy(Cp*)2} core to afford the bis-halobenzene complexes [Dy(Cp*)2(PhX-κ-X)2] [Al{OC(CF3)3}4] (X = F, Cl) and [Dy(Cp*)2(C6H4F2-κ2-F,F)(C6H4F2-κ-F)][Al{OC(CF3)3}4]. The effective barrier to reversal of magnetisation measured for the chlorobenzene adduct is the highest for any {Dy(Cp*)2}-containing SMM reported to date and the work also provides new insights into how the desirable solvent-free [Dy(Cp*)2]+ cation could finally be isolated. The final section of the report presents a summary of the main conclusions of the research included herein and puts forward a range of synthetic targets that could lead to the realisation of commercially viable Ln SMMs.

Date of Award	1 Aug 2023
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Richard Winpenny (Supervisor) & David Mills (Supervisor)