Facile Interchange of 3d and 4f Ions in Single‐Molecule Magnets: Stepwise Assembly of [Mn₄], [Mn₃Ln] and [Mn₂Ln₂] Cages within Calix[4]arene Scaffolds

Abstract

The central Mn(II) ions in a series of calix[4]arene-stabilised butterflies can be sequentially replaced with Ln(III) ions, maintaining the structural integrity of the molecule but transforming its magnetic properties. The replacement of Mn(II) for Gd(III) allows for the examination of the transferability of spin-Hamiltonian parameters within the family as well as permitting their reliable determination. The introduction of the 4f ions results in weaker intramolecular magnetic exchange, an increase in the number of low-lying excited states, and an increase in magnetisation relaxation, highlighting the importance of exchange over single-ion anisotropy for the observation of SMM behaviour in this family of complexes. The presence of the [TM(II/III) (TBC[4])(OH)(solvent)] metalloligand (TM=transition metal, TBC=p-tBu-calix[4]arene) suggests that magnetic calix[n]arene building blocks can be employed to encapsulate a range of different "guests" within structurally robust "hosts".

A New Family of 3d–4f Bis‐Calix[4]arene‐Supported Clusters

“…Inspection of the structures of 1 (Figure ) and 3 (Figure ) shows there are no direct, ligand‐mediated TM⋅⋅⋅TM (TM=transition metal) magnetic exchange pathways, only TM⋅⋅⋅Ln and Ln⋅⋅⋅Ln (Ln=lanthanide metal) contacts, both of which would be expected to be very weak. Indeed, previous measurements of calix[ n ]arene‐based TM II/III –Gd III clusters reveal exchange constants that are just a fraction of a wavenumber, with the shape of the χ M T versus T curve for 3 being very similar to that observed for the C[4]‐supported [Mn III 2 Gd III 2 ] species, where J Mn–Gd =+0.075 and J Gd–Gd =−0.006 cm −1 …”

“…In this regard, it should be noted that the free ligand exists as an up-down antiparallel arrangement of C [4] ( Figure S1), andi nversion gives rise to thesen ew metal ion bindings ites between their polyphenolic lower-rims. As is the case for [Mn III 2 Mn II 2 (OH) 2 Figure 1D and 1E), [17] the main difference being the interchange of two Mn II for Ln III ions;t his behaviori saclose parallel to that described above for C [4], [10] further suggesting that methylene bridge linkingi saviable route to the logical extension of metal ion bindingr ules/ behaviors for multi-component systems. We recently showedt hat, as in C[4]-supported clusterc hemistry,v ariation in reaction stoichiometry has adramatic effect on the assembly of bis-C [ 2 ]r elatives,respectively.…”

“…As is the case for [Mn III 2 Mn II 2 (OH) 2 (C[4]) 2 ], the Mn II/III ions in the bis‐C[4]‐supported cluster are linked through bridging phenolates and hydroxides. The second cluster to emerge from this exploratory chemistry was a [Mn III 4 Mn II 2 Gd III 2 (bis‐C[4]) 2 ] species that is a structural relative of the aforementioned Mn 8 cluster (compare Figure 1 D and E), the main difference being the interchange of two Mn II for Ln III ions; this behavior is a close parallel to that described above for C[4], further suggesting that methylene bridge linking is a viable route to the logical extension of metal ion binding rules/ behaviors for multi‐component systems. We recently showed that, as in C[4]‐supported cluster chemistry, variation in reaction stoichiometry has a dramatic effect on the assembly of bis‐C[4]‐supported clusters.…”

“…Indeed, previous measurements of calix[n]arene-based TM II/III -Gd III clusters reveal exchange constants that are justafraction of aw avenumber,w ith the shape of the c M T versus T curve for 3 being very similart ot hat observed for the C[4]-supported [Mn III 2 Gd III 2 ]s pecies, where J Mn-Gd =+0.075 and J Gd-Gd = À0.006 cm À1 . [10] Analysis of the magnetic data of compound 2 is further complicated by the presence of the central Fe3 ion which is disordered over two positions.W hile reported Fe III -Gd III exchange constantsi nC [4]-supported cages are of the same magnitude as described above, [14c] Fe III -O-Fe III exchange can be large, thoughs trongly dependent on the Fe III -O-Fe III angle, with the strongestA Fe xchange observed for al inear moiety and becoming less AF as the angle decreases. [20] The Fe III -O-Fe III angles in 2 (Fe1-O9-Fe3;F e3-O10-Fe2) are approximately 1308 (with m 4 -O9, m 4 -O10a lso bridging to two Gd ions), and inspection of Figure5 suggests little evidence for the presence of strong TM···TM exchange.…”

“…We recently found that it is possible to replace one or both of the body Mn II ions in the aforementioned cluster with Ln III ions through variation in the metal salt, stoichiometries and reaction conditions employed; this afforded structurally related butterflies having {Mn III 2 Mn II Ln III (OH) 2 } and {Mn III 2 Ln III 2 (OH) 2 } cores, respectively . Analogous reactions carried out in the absence of TM II/III ions afforded C[4]‐supported Ln III 6 octahedra, and a series of Mn III 4 Ln III 4 clusters (Figure B) were also obtained when using both TM and Ln ions .…”