Homochiral Mn³⁺ Spin-Crossover Complexes: A Structural and Spectroscopic Study

Abstract

Structural, magnetic, and spectroscopic data on a Mn 3+ spin-crossover complex with Schiff base ligand 4-OMe-Sal 2 323, isolated in crystal lattices with five different counteranions, are reported. Complexes of [Mn(4-OMe-Sal 2 323)]X where X = ClO 4 – ( 1 ), BF 4 – ( 2 ), NO 3 – ( 3 ), Br – ( 4 ), and I – ( 5 ) crystallize isotypically in the chiral orthorhombic space group P 2 1 2 1 2 with a range of spin state preferences for the [Mn(4-OMe-Sal 2 323)] + complex cation over the temperature range 5–300 K. Complexes 1 and 2 are high-spin, complex 4 undergoes a gradual and complete thermal spin crossover, while complexes 3 and 5 show stepped crossovers with different ratios of spin triplet and quintet forms in the intermediate temperature range. High-field electron paramagnetic resonance was used to measure the zero-field splitting parameters associated with the spin triplet and quintet states at temperatures below 10 K for complexes 4 and 2 with respective values: D S =1 = +23.38(1) cm –1 , E S =1 = +2.79(1) cm –1 , and D S =2 = +6.9(3) cm –1 , with a distribution of E parameters for the S = 2 state. Solid-state circular dichroism (CD) spectra on high-spin complex 1 at room temperature reveal a 2:1 ratio of enantiomers in the chiral conglomerate, and solution CD measurements on the same sample in methanol show that it is stable toward racemization. Solid-state UV–vis absorption spectra on high-spin complex 1 and mixed S = 1/ S = 2 sample 5 reveal different intensities at higher energies, in line with the different electronic composition. The statistical prevalence of homochiral crystallization of [Mn(4-OMe-Sal 2 323)] + in five lattices with different achiral counterions suggests that the chirality may be directed by the 4-OMe-Sal 2 323 ligand.

Spin-Crossover and Slow Magnetic Relaxation Behavior in Hexachlororhenate(IV) Salts of Mn(III) Complexes [Mn(5-Hal-sal2323)]2[ReCl6] (Hal = Cl, Br)

“…Upon cooling below 100 K, the χ m T values for these compounds continue to slowly decrease at first, and then, around 40-30 K, they decrease abruptly to reach 1.99 ( 1 ) and 1.80 cm 3 K/mol ( 2 ) at 2 K. The abrupt decrease in the χ m T values is mainly due to the strong magnetic anisotropy, which are observed in Re IV - and [Mn III (sal 2 323)]-based systems [ 31 , 32 , 33 , 34 , 35 ]. These χ m T values at 2.0 K are comparable to the sum of the χ m T values for the isolated Re(IV) complexes with halide ligands (approx.…”

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 best-fit ZFS parameter D for complex 1 is 15.9(3) cm −1 and is comparable to those of the other low-spin ( S =1) Mn( iii ) complexes with a MnN 4 O 2 coordination environment (∼20 cm −1 ). 23,40–42…”

“…As the temperature is lowered, the monoclinic phase shows a gradual transition from a high-spin state to a low-spin state, while the triclinic phase remains in a high-spin state down to T = 22 K, where, most likely, there is an abrupt and not complete spin transition between HS and LS states. This spin transition is difficult to diagnose due to the competition with the zero-field splitting effects, which are typically observed below 25 K. 23 Nevertheless, the d( χ m T) /d T vs. T plot shows a maximum centered at 22 K (see the inset to Fig. S5c, ESI†), which represents the T 1/2 temperature for spin transition between HS and LS states of Mn( iii ) ions.…”

“…S5b and c, ESI†) and are in good agreement with the D values for low-spin ( 2 ) and high-spin ( 3 ) Mn( iii ) complexes. 13,23,40–42 In the case of complex 3 , this indicates that if the spin transition occurs at 22 K, then it is incomplete and most of the complex remains in the high-spin state.…”

“…Current research on this family of Mn( iii ) SCO compounds is aimed at tuning the Mn( iii ) spin state by varying the substituents on the phenolate ring, the anion, and the co-crystallized solvent molecules. 9–26 The development of these approaches has recently led to the synthesis of tetraphenylborates of Mn( iii ) complexes with a sal 2 323 ligand containing dihalide (Cl, Br) substituents on the phenolate ring: [Mn(3,5-diCl-sal 2 323)]BPh 4 ( 4 ), [Mn(3,5-Br,Cl-sal 2 323)]BPh 4 ( 5 ) and [Mn(3,5-diBr-sal 2 323)]BPh 4 ( 6 ) which showed unusual structural and magnetic properties. 27–30 All three complexes are isostructural, but have fundamentally different properties.…”