Crystallographic Detection of the Spin State in FeIII Complexes

We report a single example of thermal spin crossover in a series of FeIII complexes, [FeIII(R-sal2323)]+, which typically stabilize the low-spin (S = 1/2) state. Single-crystal X-ray diffraction analysis of 53 such complexes with varying “R” groups, charge-balancing anions, and/or lattice solvation confirms bond lengths in line with an S = 1/2 ground state, with only the [FeIII(4-OMe-sal2323)]NO3 complex (1a) exhibiting longer bond lengths associated with a percentage of the spin sextet form at room temperature. Structural distortion parameters are investigated for the series. A magnetic susceptibility measurement of 1a reveals a gradual, incomplete transition, with T1/2 = 265 K in the solid state, while Evans method NMR reveals that the sample persists in the low-spin form in solution at room temperature. Computational analysis of the spin state preferences for the cations [FeIII(4-OMe-sal2323)]+ and [FeIII(sal2323)]+ confirmed the energetic preference for the spin doublet form in both, and the thermal spin crossover in complex 1a is therefore attributed to perturbation of the crystal packing on warming.


S1.11 Synthesis of [Fe III (3-OMe-sal2323)]PF6·H2O (2c)
3-Methoxysalicylaldehyde (304 mg, 2 mmol) and 1,2-bis(3-aminopropylamino)ethane (174 mg, 1 mmol) were briefly stirred in IMS and acetonitrile (1:1, 20 mL), a yellow color was immediately observed. To this solution iron(III) chloride (162 mg, 1 mmol) and ammonium hexafluorophosphate (163 mg, 1 mmol) was added, a deep purple color was observed. The solution was briefly stirred to dissolve all solids, and subsequently filtered. Crystals were obtained through slow evaporation of solvent over a week. Elemental analysis, calculated for C24H34N4O5F6PFe, Theory % were briefly stirred in methanol (10 mL), a yellow color was immediately observed. To this solution iron(III) nitrate nonahydrate (40.4 mg, 0.1 mmol) was added, a deep red color was observed. The solution was briefly stirred to dissolve all solids, and subsequently filtered. Crystals were obtained through slow evaporation of solvent over a week.
Too few crystals were obtained to carry out elemental analysis on the bulk sample. were briefly stirred in methanol (10 mL), a yellow color was immediately observed. To this solution iron(II) tetrafluoroborate hexahydrate (33.8 mg, 0.1 mmol) was added, a deep red color was observed. The solution was briefly stirred to dissolve all solids, and subsequently filtered. Crystals were obtained through slow evaporation of solvent over a week. Too few crystals were obtained to carry out elemental analysis on the bulk sample. were briefly stirred in methanol (10 mL), a yellow color was immediately observed. To this solution iron(II) perchlorate hexahydrate (36.3 mg, 0.1 mmol) was added, a deep red color was observed. The solution was briefly stirred to dissolve all solids, and subsequently filtered. Crystals were obtained through slow evaporation of solvent over a week. Too few crystals were obtained to carry out elemental analysis on the bulk sample.
The solution was briefly stirred to dissolve all solids, and subsequently filtered. Crystals were obtained through slow evaporation of solvent over a week. Too few crystals were obtained to carry out elemental analysis on the bulk sample.

S1.18 Synthesis of [Fe III (3-OEt-sal2323)]BF4·0.32H2O (5b)
3-Ethoxysalicylaldehyde (33.2 mg, 0.2 mmol) and 1,2-bis(3-aminopropylamino)ethane (17.4 mg, 0.1 mmol) were briefly stirred in methanol (10 mL), a yellow color was immediately observed. To this solution iron(II) tetrafluoroborate hexahydrate (33.8 mg, 0.1 mmol) was added, a deep red color was observed. The solution was briefly stirred to dissolve all solids, and subsequently filtered. Crystals were obtained through slow evaporation of solvent over a week. Too few crystals were obtained to carry out elemental analysis on the bulk sample. were briefly stirred in ethanol (5 mL), a yellow color was immediately observed. To this solution iron(III) chloride (12.7 mg, 0.1 mmol) and sodium hexafluorophosphate (27.6 mg, 0.15 mmol) was added, a deep purple color was observed. The solution was refluxed at 110 °C for 3 hours, and subsequently filtered. The solvent was removed by rotary evaporation. Recrystallisation of the crude solid in the minimum acetonitrile/ethanol (1:1) yielded crystals suitable for SCXRD. Too few crystals were obtained to carry out elemental analysis on the bulk sample.
Too few crystals were obtained to carry out elemental analysis on the bulk sample.
Too few crystals were obtained to carry out elemental analysis on the bulk sample.

S4 Quantum Chemistry Calculations
The simulated spectra for 1a and 1a·S from the single crystal XRD data are provided in red and grey respectively.