Three-Dimensional (3-D) Ferromagnetic Network of Mn₁₂ Single-Molecule Magnets: Subtle Environmental Effects and Switching to Antiferromagnetic

Abstract

A new member of the Mn family of single-molecule magnets (SMMs) has been prepared and found to be the first of this family to give a 3-D ferromagnetic network. [MnO(OCCH-p-F)(HO)] (2) was prepared by carboxylate substitution on the acetate derivative with p-F-benzoic acid and crystallizes as 2·8MeCN in space group I4̅2m with extensive formation of intermolecular C-H···F hydrogen-bonding. The latter leads to a combination of ferromagnetic (F) and antiferromagnetic (AF) interactions and an overall F network that gives a χT value at low T that is abnormally high for an S = 10 ground state. 2·8MeCN undergoes solvent loss under vacuum to 2, with a decrease in unit-cell volume of 17%, primarily due to a 13% decrease in the c-axis. The χT vs T plot for 2 indicates a switch to a net AF network. Exposure to air causes hydration to 2·3HO, a concomitant increase in unit cell volume, and a switch back to a F network. The same conversion of 2·8MeCN to 2·3HO can also be accomplished in one step rather than two steps, by leaving crystals of the former exposed to air at ambient temperature and pressure for 10 days, giving the same magnetic plots. Interestingly, the desolvation/solvation processes cause Jahn-Teller isomerism to occur, but the ratio of the faster-relaxing isomer to the normal slowly relaxing one does not change monotonically. Single-crystal micro-SQUID studies on 2·8MeCN show the expected magnetization hysteresis loops for a SMM and a small exchange-bias from the intermolecular interactions that is unexpectedly AF. Since the micro-SQUID study only identifies interactions along the easy-axis (z-axis) of the crystal, this is readily rationalized as due to the J components of the intermolecular interactions in 2·8MeCN being net AF. The combined results offer useful insights into the degree of sensitivity of the magnetic properties to small environmental perturbations.

[Mn14] “Structural Analogues” of Well‐Known [Mn12] Single‐Molecule Magnets

“…These six µ 3 ‐O 2– and two additional µ 3 ‐O 2– ligands bridging exclusively Mn III ions connect the ten Mn III ions of the outer ring forming the [Mn III 10 (µ 3 ‐O) 8 ] 14+ structural core. This ring is similar to the [Mn III 8 ] outer ring appearing in known [Mn 12 ] complexes,, where the most obvious difference between them is the presence of two additional Mn III ions each of which is connected through a µ 3 ‐O 2– ion to two Mn III ions forming a [Mn III 3 (µ 3 ‐O)] 7+ triangle. Two η 1 :η 1 :µ‐EtCO 2 – ligands and one η 1 :η 1 :µ‐EtCO 2 – /one alkoxide µ‐RO – arm of a η 2 :η 2 :µ 3 pd 2– ligand bridge two of the edges of the triangle.…”

“…Interestingly, the [Mn III 10 Mn IV 4 (µ 3 ‐O) 12 ] 22+ core is similar to the structural cores of the “flat [Mn 12 ]” and “normal [Mn 12 ]” complexes (Figure ). The core of “normal [Mn 12 ]” complexes consisting of an outer [Mn III 8 (µ 3 ‐O) 8 ] 8+ ring surrounding a central [Mn IV 4 (µ 3 ‐O) 4 ] 8+ cube has been shown to be quite solid since it is not affected from the significant variations in its peripheral ligation in the over 50 analogues reported to date , . On the other hand the core of “flat [Mn 12 ]” complexes consisting of a [Mn III 8 (µ 3 ‐O) 8 ] 8+ ring surrounding a [Mn IV 4 ] defective dicubane has been observed only in three examples, namely (NBu n 4 ) 2 [Mn III 8 Mn IV 4 O 12 (MeO) 2 (PhCO 2 ) 16 (H 2 O) 2 ] ( 4 ),, [Mn III 8 Mn IV 4 O 10 (MeO) 3 (OH)(C 6 H 3 F 2 CO 2 ) 16 (MeOH) 2 ] ( 5 ), and [Mn III 8 Mn IV 4 O 10 (MeO) 4 (Bu t CO 2 ) 16 (MeOH) 2 ] ( 6 ) .…”

“…This exciting discovery attracted the interest of scientists from diverse areas, , and created the need for the synthesis of (i) more [Mn 12 (µ 3 ‐O) 12 (RCO 2 ) 16 (H 2 O) 4 ] ([Mn 12 ]) derivatives and (ii) new structural types of Mn complexes to better study and understand physical phenomena related to single‐molecule magnetism and also to obtain SMMs with enhanced properties. As a result various [Mn 12 ] derivatives have been prepared in their neutral, one‐, two‐ or three‐electron reduced versions with a variety of carboxylate, mixed carboxylate, and mixed carboxylate/non‐carboxylate ligands , . The synthesis of these compounds involved targeted structural modifications to preformed [Mn 12 ] clusters .…”

“…The synthesis of these compounds involved targeted structural modifications to preformed [Mn 12 ] clusters . In addition, in all such modifications, the [Mn III 8 Mn IV 4 (µ 3 ‐O) 12 ] 16+ (in the case of the neutral analogues) core consisting of a [Mn III 8 (µ 3 ‐O) 8 ] 8+ outer ring surrounding an inner [Mn IV 4 (µ 3 ‐O) 4 ] 8+ cube remains essentially the same , . However, there is a family of [Mn 12 ] analogues in which the core is slightly changed.…”

“…Interestingly, although [Mn 12 ] chemistry has been exhaustively investigated over the last 25 years, it still attracts significant interest . In particular, there are still reports on the synthesis of new [Mn 12 ] derivatives and also on the utilization of [Mn 12 ] analogues as building‐blocks for the construction of oligomers or multidimensional coordination polymers and as catalysts , . In addition, there is significant interest in the incorporation of [Mn 12 ] derivatives into the pores of metal organic frameworks and other types of porous materials .…”