Compressed and Expanded Lattices - Barriers to Spin-State Switching in Mn³⁺ Complexes

Abstract

We report the structural and magnetic properties of two new Mn3+ complex cations in the spin crossover (SCO) [Mn(R-sal2323)]+ series, in lattices with seven different counterions in each case. We investigate the effect on the Mn3+ spin state of appending electron-withdrawing and electron-donating groups on the phenolate donors of the ligand. This was achieved by substitution of the ortho and para positions on the phenolate donors with nitro and methoxy substituents in both possible geometric isomeric forms. Using this design paradigm, the [MnL1]+ (a) and [MnL2]+ (b) complex cations were prepared by complexation of Mn3+ to the hexadentate Schiff base ligands with 3-nitro-5-methoxy-phenolate or 3-methoxy-5-nitro-phenolate substituents, respectively. A clear trend emerges with adoption of the spin triplet form in complexes 1a–7a, with the 3-nitro-5-methoxy-phenolate donors, and spin triplet, spin quintet and thermal SCO in complexes 1b–7b with the 3-methoxy-5-nitro-phenolate ligand isomer. The outcomes are discussed in terms of geometric and steric factors in the 14 new compounds and by a wider analysis of electronic choices of Mn3+ with related ligands by comparison of bond length and angular distortion data of previously reported analogues in the [Mn(R-sal2323)]+ family. The structural and magnetic data published to date suggest a barrier to switching may exist for high spin forms of Mn3+ in those complexes with the longest bond lengths and highest distortion parameters. A barrier to switching from low spin to high spin is less clear but may operate in the seven [Mn(3-NO2-5-OMe-sal2323)]+ complexes 1a–7a reported here which were all low spin in the solid state at room temperature.

Solvent and Dihalide Substituent Effects on Crystal Structure, Spin‐Crossover Behavior and Conductivity of the Cationic Mn(III) Complexes with Electroactive TCNQ Counteranions

“…A further increase the temperature to 373 K reveals an elongation of the MnÀ N bond lengths, corresponding to an increase in the fraction of HS Mn ions in 1, while these values do not reach those of MnÀ N im 2.12 and MnÀ N am 2.23 Å corresponding to the HS Mn(III) ion. [17,18] The strong changes with temperature in the XÀ Mn-X angles are also observed, which is well illustrated by the octahedral distortion parameters (ODPs) (Σ and Θ) shown in Table 1. For complex 1, the ODPs for Mn ions at 100 and 200 K are close and correspond to the LS state of the Mn(III) ion.…”

“…At 100 and 200 K, the average equatorial bond lengths MnÀ N im and MnÀ N am in complex 1 are in the range of that expected for the LS Mn(III) ion. [17,18] At the same time, at 200 K, the MnÀ N am bond lengths are slightly larger than at 100 K, which may indicate the initial point of the spin transition of the manganese(III) ion to the HS state. When the temperature is increased to 300 K, the MnÀ N bond lengths increase slightly, it means that only a small part of the LS Mn ions has been transited to the HS state (Table 1).…”

“…[13][14][15] In the 21st century, however, a class of spin-crossover cationic Mn(III) complexes (d 4 system) with the ligand sal 2 323 (sal 2 323 = N,N'-bis(3-(2-oxy-benzylideneamino)propyl)-ethylenediamine) and its derivatives was discovered, in which one electron is transferred between low-spin (LS, S = 1) and high-spin (HS, S = 2) states. [16][17][18][19] The sal 2 323 ligand is a product of the condensation reaction of two molecules of salicylaldehyde with flexible tetraamine N,N'-bis(3-aminopropyl)ethylenediamine. The abbreviation 323 refers to the number of alkyl linkages in the starting tetraamine.…”

“…[17] The study of the influence of substituents in the phenolate ring, counterions and crystallization solvents on the spin-crossover behavior of the [Mn-(sal 2 323)] + family complexes has led to several important results in recent years. [18,19] Complexes with sharp spin transitions exhibiting significant hysteresis have been synthesized, [20][21][22][23][24][25][26][27] including complexes in which stepwise spin transitions are associated with structural transitions. [25][26][27] In addition to temperature-induced spin transitions, some of these manganese complexes exhibit SCO effects in rather low magnetic fields starting even below 5T.…”

Luminescent Cadmium(II)-Based 12-MC-4 Metallacrown Complex with 2-Methylmercaptobenzohydroxamic Acid Ligand

“…During this process, the lattice DMF molecules present in the crystal lattice might have evaporated and the void spaces in the crystal lattice filled with water molecules from the atmosphere. This is described more often in the literature [27][28][29][30][31].…”