Spin-crossover phenomena of the mononuclear Mn(III) complex tuned by metal dithiolene counteranions

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

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.

Push and Pull Effect of Methoxy and Nitro Groups Modifies the Spin-State Switching Temperature in Fe(III) Complexes

We have explored the impact of electron-donating (methoxy) and electron-withdrawing (nitro) substituents on SalEen ligand based spin crossover (SCO) behavior of Fe(III) complexes. Thus, 3-X-substituted SalEen ligands were employed to prepare [Fe(3-X-SalEen)₂]·NCSe, where X = OMe (1), H (2), and NO₂ (3) (3-X-SalEen is the condensation product of 3-substituted salicylaldehyde and N-ethylethylenediamine). The characteristic spin transition temperature (T _1/2) is shown to shift to a lower temperature when an electron-donating substituent (OMe) is used and to a higher temperature when an electron-withdrawing substituent (NO₂) is used. We used experimental and theoretical methods to determine the reasons for this behavior. The solid-state magnetic data revealed the transition temperatures for complexes 1, 2, and 3 to be 219, 251, and 366 K, respectively. The solution-state magnetic data also support this trend in T _1/2 values. UV-vis spectra analysis indicates that there is greater delocalization in the π-manifold of the ligand when the nitro group is the substituent. Theoretical studies through density functional theory methods suggest the methoxy substituent decreases the energy gap between the t_2g and e_g orbitals (explaining the lower T _1/2 value), while the nitro substituent increases the energy gap between the t_2g and e_g orbitals and thus increases the T _1/2 value.

FeII spin crossover complexes containing N4O2 donor ligands

Spin crossover (SCO) is one of the most studied magnetic bistable phenomena because of its application in the field of multifunctional magnetic materials. Fe^II complexes in a N₆ coordination environment have been the most well-studied in terms of their SCO behaviour. Other coordination environments, notably the N₄O₂ coordination environment, has also been quite effective in inducing SCO behaviour in the corresponding Fe^II complexes. This review deals with such systems. The three ligand families that are discussed are: Jager type ligands, hydrazone based ligands and tridentate ligands having salicylaldehyde derivatives. These ligands allow the assembly of both mononuclear and multinuclear complexes that exhibit cooperative spin transitions. Also, Fe^II complexes obtained from some of these ligands are multifunctional and exhibit a coupling of optical and magnetic properties. Most of the Fe^II complexes obtained from these families of ligands are charge neutral which allows easy surface deposition. Further, modulation of these ligand families allows a fine tuning of the ligand field strength which results in varying SCO behavior. In addition some of the Fe^II complexes derived from these ligands exhibit a light induced excited spin state trapping (LIESST) effect. All of the above aspects are reviewed in this review.

Impact of counter anions on spin-state switching of manganese(III) complexes containing an azobenzene ligand

Four mononuclear manganese(III) complexes coordinated with photo-active hexadentate azobenzene ligands, [Mn(5azo-sal₂-323)](X) (X = Cl, 1; X = BF₄, 2; X = ClO₄, 3; X = PF₆, 4), were prepared. The impact of various counter anions on the stabilization and switching of the spin state of the manganese(III) center was explored through detailed magneto-structural investigation using variable temperature single-crystal X-ray diffraction, magnetic, spectroscopic, and spectroelectrochemical studies, along with theoretical calculations. All four complexes consisted of an isostructural monocationic distorted octahedral MnN₄O₂ coordination environment offered by the hexadentate ligand and Cl^-, BF₄^-, ClO₄^-, and PF₆^- as counter anions respectively. Complex 1 with a spherical Cl^- counter anion showed a reversible and gradual spin-state switching between low-spin (LS) (S = 1) and high-spin (HS) (S = 2) states above 400 K, where non-covalent cation-anion interactions played a significant role in stabilizing the LS state. While, irrespective of the shape of the counter anion, complexes 2-4 remained in the HS state throughout the measured temperature range of 300-2 K, where strong π-π interaction between the azobenzene motifs among cationic units played a substantial role in stabilizing the HS state. Furthermore, magnetic data analyses revealed significantly large zero-field splitting in the S = 1 state for 1 (D = 19.4 cm^-1, E/D = 0.008) in comparison with that in the S = 2 state for 2-4 (D = 3.99-4.97 cm^-1, E/D = 0.002-0.195). Spectroelectrochemical investigations revealed the quasi-reversible reduction and oxidation of the manganese(III) center to manganese(II) and manganese(IV), respectively. A detailed theoretical calculation at the DFT and CASSCF level of theory was carried out to better understand the magneto-structural correlation.

The effect of fluorine substituents on the crystal structure and spin crossover behavior of the cation [MnIII(3,5-diHal-sal2323)]+ complex family with BPh4 anions

The effect of fluorine substituents on the structure and magnetic behavior of [Mn(R′,R′′-sal2323)]BPh4 complexes was studied and compared it with that of chlorine and bromine substituents.

A facile biomimetic catalytic activity through hydrogen atom abstraction by the secondary coordination sphere in manganese(III) complexes

This paper describes the synthesis and structural characterization of four new manganese(iii) complexes (1-4) derived from N3O donor Schiff base ligands and their biomimetic catalytic activities related to catechol oxidase and phenoxazinone synthase. X-ray crystallography reveals that the Schiff bases coordinate the metal centre in a tridentate fashion, leaving the pendant tertiary amine nitrogen atom either protonated or free to balance the charge of the system, and these pendant triamines participate in strong hydrogen bonding interactions in the solid state. The hydrogen bonding ability of the pendant triamines at the second coordination sphere plays a crucial role in the substrate recognition and the stability of the complex-substrate intermediates. The effect of substitution at the phenolate ring towards the redox potential of the metal centre and the catalytic activity of these complexes has been observed. Detailed kinetic studies further disclose the deuterium kinetic isotope effect in which the transfer of the proton along the hydrogen bond from the substrates to the pendant triamine group at the secondary coordination sphere occurs at the key step in the catalytic reaction. The present reactivity nicely resembles the biochemical reactivities in the natural system in which a concerted electron and proton transfer to different species is usually observed. Remarkably, although some sort of influence of the secondary coordination sphere on catalytic activity has been reported mimicking the function of these metalloenzymes, such a direct participation of the secondary coordination sphere, particularly in modelling phenoxazinone synthase, has not been observed to date.

Structure-function correlations in mononuclear manganese(iii) spin crossover systems with a big conjugated hexadentate Schiff-base ligand

This paper reports the syntheses, crystal structures and magnetic properties of spin crossover (SCO) salts of formulae [Mn(naphth-sal-N-1,5,8,12)]SbF6 (1), [Mn(naphth-sal-N-1,5,8,12)]AsF6 (2), [Mn(naphth-sal-N-1,5,8,12)]PF6·1/2CH3OH (3) and [Mn(naphth-sal-N-1,5,8,12)]ClO4 (4), where (naphth-sal-N-1,5,8,12)2- (2,2'-((1E,14E)-2,6,10,14-tetraazapentadeca-1,14-diene-1,15-diyl)diphenolate) is a big conjugated hexadentate Schiff-base ligand. Magnetic susceptibility measurements demonstrated that complexes 1 and 2 showed a gradual one-step SCO between the high-spin (HS, S = 2) and low-spin (LS, S = 1) states without thermal hysteresis. The transition temperatures T1/2 of the SbF6 (1) and AsF6 (2) salts estimated from the magnetic susceptibility measurements are 164 and 171 K, respectively. The existence of the crystal solvent of complex 3 changes the supramolecular packing, leading to close ππ stacking interactions between the phenyl groups of the ligands. These close stacking interactions hinder the flexibility of the whole ligand, precluding the spin transformation of complex 3 and leading to its stabilization in the HS state in the temperature range of 2-300 K. For 4, crystal structure analysis indicates that the reduction in the anion size leads to close contacts between the naphthalene rings. These C-Hπ interactions provide a means for preventing the spin crossover occurring at low temperatures.

Structural studies of a manganese(iii) complex with spin-crossover and thermochromic properties

A new Mn^III complex exhibits a gradual spin crossover phenomenon with single crystal thermochromism. A comparative structural study was conducted using octahedral deviation parameters.

The molecular and supramolecular aspects in mononuclear manganese(iii) Schiff-base spin crossover complexes

A cooperative SCO process with a hysteresis width of 18 K is the largest one reported in d⁴ SCO chemistry.

Structural insights into the counterion effects on the manganese(iii) spin crossover system with hexadentate Schiff-base ligands

The close stacking and arrangement preclude the spin transformation of the Mn(iii) Schiff-base cations.