Steric Quenching of Mn(III) Thermal Spin Crossover: Dilution of Spin Centers in Immobilized Solutions

Komala Pandurangan,Aizuddin Sultan,Brendan Gildea,Paulo N Martinho,Tia E Keyes,Helge Müller-Bunz,Anthony B Carter,Tibebe Lemma,Shang Shi,Grace G Morgan

doi:10.3390/magnetochemistry8010008

Komala Pandurangan, Aizuddin Sultan + Show 8 more

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https://doi.org/10.3390/magnetochemistry8010008

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Abstract

Structural and magnetic properties of a new spin crossover complex [Mn(4,6-diOMe-sal2323)]+ in lattices with ClO4−, (1), NO3−, (2), BF4−, (3), CF3SO3−, (4), and Cl− (5) counterions are reported. Comparison with the magnetostructural properties of the C6, C12, C18 and C22 alkylated analogues of the ClO4− salt of [Mn(4,6-diOMe-sal2323)]+ demonstrates that alkylation effectively switches off the thermal spin crossover pathway and the amphiphilic complexes are all high spin. The spin crossover quenching in the amphiphiles is further probed by magnetic, structural and Raman spectroscopic studies of the PF6− salts of the C6, C12 and C18 complexes of a related complex [Mn(3-OMe-sal2323)]+ which confirm a preference for the high spin state in all cases. Structural analysis is used to rationalize the choice of the spin quintet form in the seven amphiphilic complexes and to highlight the non-accessibility of the smaller spin triplet form of the ion more generally in dilute environments. We suggest that lattice pressure is a requirement to stabilize the spin triplet form of Mn3+ as the low spin form is not known to exist in solution.

Highlights

Cooperative effects in spin crossover complexes are well-known to influence thermal evolution profile and many approaches to modulate a spin transition from gradual to abrupt have been reported
In the synthesis of complexes [1,2,3,4,5], an appropriate manganese source was added in a one-pot reaction, where complexes [1, 2], and 5, were produced using the relevant manganese salt, while preparation of complexes 3 and 4 utilized a salt metathesis procedure starting with hydrated manganese(II) chloride
Analysis of the structural and magnetic data of twelve new Mn(III) complexes reveals the importance of packing and lattice strain in modulating the choice of spin state and accessibility of thermal spin state switching

Summary

Introduction

Cooperative effects in spin crossover complexes are well-known to influence thermal evolution profile and many approaches to modulate a spin transition from gradual to abrupt have been reported These include targeted crystal engineering through the inclusion of specific anions, guest molecules or ligand substituents [1–6] with 6-coordinate complexes of Mn(III) [4,7–12], Fe(III) [13–16], Fe(II) [17–25], and Co(II) [26–29], i.e., the full range of d4 –d7 configurations for which thermal spin state switching is possible. Intermolecular interactions such as hydrogen-bonding [30–32] and π-π stacking [33–37] have proven to be very effective mechanisms to connect SCO-active sites in crystalline samples, as thermal differences in the intermolecular interaction can change the internal lattice pressure across a temperature gradient.

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