Modulation of Mn3+ Spin State by Guest Molecule Inclusion

Abstract

Spin state preferences for a cationic Mn3+ chelate complex in four different crystal lattices are investigated by crystallography and SQUID magnetometry. The [MnL1]+ complex cation was prepared by complexation of Mn3+ to the Schiff base chelate formed from condensation of 4-methoxysalicylaldehyde and 1,2-bis(3-aminopropylamino)ethane. The cation was crystallized separately with three polyatomic counterions and in one case was found to cocrystallize with a percentage of unreacted 4-methoxysalicylaldehyde starting material. The spin state preferences of the four resultant complexes [MnL1]CF3SO3·xH2O, (1), [MnL1]PF6·xH2O, (2), [MnL1]PF6·xsal·xH2O, (2b), and [MnL1]BPh4, (3), were dependent on their ability to form strong intermolecular interactions. Complexes (1) and (2), which formed hydrogen bonds between [MnL1]+, lattice water and in one case also with counterion, showed an incomplete thermal spin crossover over the temperature range 5–300 K. In contrast, complex (3) with the BPh4−, counterion and no lattice water, was locked into the high spin state over the same temperature range, as was complex (2b), where inclusion of the 4-methoxysalicylaldehyde guest blocked the H-bonding interaction.

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

“…We also suggest that steric effects of this substituent arrangement disfavor packing of both the Λ and Δ enantiomers of the complex cation in the presence of the medium sized counterions reported here: ClO 4 – , BF 4 – , NO 3 – , Br – , and I – . With smaller counterions, including Cl – (compound 6 , structure reported in the Supporting Information), or larger counterions including CF 3 SO 3 – , PF 6 – , and BPh 4 – , the packing pattern is altered, and the crystallization of both enantiomers is observed, as is typical for this type of chiral complex with achiral anions.…”

“…The coordination around the Mn 3+ center is pseudo-octahedral with two trans -phenolate, two cis -amine, and two cis -imine donors, in the same arrangement as reported for related [Mn(R-sal 2 323)] + SCO compounds. 19 , 30 , 31 , 33 , 88 , 105 − 107 , 135 Compounds 1 – 5 provide a good set of samples to study the effect of the counterion on the Mn 3+ spin state, since all five compounds crystallize isotypically.…”

“…Identification of the sign of D is particularly relevant to the current work because, although the majority of known (HS) Mn 3+ complexes display a pronounced axial elongation due to the Jahn–Teller effect, most Mn 3+ SCO complexes appear to have a marked compression in the spin quintet form. 19 , 31 , 33 , 37 , 69 , 88 , 105 − 112 An axially compressed 89 − 100 form may assist the transition to a spin triplet arrangement as the energetic order of orbitals should match that expected in the S = 1 form of the ion ( Figure 1 ). Diffraction alone, however, is not sufficient to discern if this is a genuine compression, but it can be resolved by measuring the sign of D by EPR at high fields.…”

“…Here we use low-temperature multifrequency EPR spectroscopy to establish the magnitude and sign of the axial D parameter in the spin quintet and triplet forms of the [Mn(4-OMe-Sal 2 323] + complex cation 105 when it is crystallized in BF 4 – and Br – lattices, respectively complexes 2 and 4 in Scheme 1 .…”

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

“…In other examples, we have noted a tendency for stabilization of HS Mn 3+ in BPh 4 − salts, possibly due to the absence of counterion-mediated hydrogen bonding to mediate the spin transition. 20,22,23 Perhaps most striking is the effect of appending long alkyl groups onto the ligands in [Mn(Rsal 2 323)] + complexes. In comparable Fe 3+ complexes, this strategy was used to great effect to prepare SCO amphiphiles in a variety of material forms including Langmuir films at the air−water interface, 24 micelles in organic solvents, 11 and template assembly of 1D nanowires.…”