Abstract
AbstractMechanically chelating ligands have untapped potential for the engineering of metal ion properties. Here we demonstrate this principle in the context of CoII‐based single‐ion magnets. Using multi‐frequency EPR, susceptibility and magnetization measurements we found that these complexes show some of the highest zero field splittings reported for five‐coordinate CoII complexes to date. The predictable coordination behaviour of the interlocked ligands allowed the magnetic properties of their CoII complexes to be evaluated computationally a priori and our combined experimental and theoretical approach enabled us to rationalize the observed trends. The predictable magnetic behaviour of the rotaxane CoII complexes demonstrates that interlocked ligands offer a new strategy to design metal complexes with interesting functionality.
Highlights
It should be noted that the rotaxane framework enforces the formation of a pseudo-heteroleptic complex and prevents binding of additional ligands, to give a predictable, if relatively rare, 5 coordinate all-neutral N-donor distorted square-based pyramidal[38] binding mode, which is not observed with the non-interlocked ligands
Two independent structures with different bond lengths (RMSD = 0.12, Table S1) were observed in the asymmetric unit of [Co(1)](ClO4)2, suggesting that high spin (HS) and low spin (LS) configurations co-exist in the solid state
Having validated our de novo computational approach in the case of [Co(1)](ClO4)2, we modelled complexes based on interlocked ligands containing other readily available macrocycle components,[39] one of which (2) is more rigid and the other (3) contains a potentially weakly coordinating ether unit near to the bipyridine ligand (Figure 1 A). [Co(2)]2+ was predicted to display an all-neutral N, 5-coordinate environment similar to that of [Co(1)](ClO4)2 but in this case, p-Co and p-p interactions result in distortion of the sbpy geometry and a twisting of the macrocycle relative to the axle (Figure 1 B)
Summary
Interlocked molecules[1,2,3,4,5] contain cavities within which donor atoms can be positioned to bind metal ions.[6,7,8,9,10,11,12,13] some of the first observations of the properties of the mechanical bond, including its ability to kinetically stabilize metal complexes,[6, 14,15,16,17,18] were made by Sauvage and co-workers over 30 years ago.[19,20,21] More recently,[22] we demonstrated that rotaxane-based ligands can be used to produce complexes the non-interlocked equivalent of which are inaccessible, including examples reminiscent of the distorted “entatic states” of metalloproteins,[23, 24] suggesting that interlocked ligands could allow engineering of the properties of metal [*] Dr M. The ability to engineer the fundamental properties of metal ions by controlling their coordination environment using the mechanical bond has not been demonstrated in prototypical functional systems. Since their discovery in 1991,[28] single-molecule magnets (SMMs) have received significant attention due to their potential applications in spintronics, data storage and quantum computing.[29] The slow relaxation of applied magnetization that defines SMMs is typically dictated by an energy barrier (U) which arises from the magnetic anisotropy, characterized by the zero-field splitting term (D), of a nonzero spin ground state. 16:03:39 Uhr plexes, we apply computational approaches for the accurate correlation of magnetic anisotropy[34, 35] and the electronic structure of CoII ions[31, 32, 36] to demonstrate that the geometry of the complexes formed can be predicted with sufficient precision to identify interesting magnetic properties a priori
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