Effects of Solvation on the Spin State of Iron(III) in 2,8,12,18-Tetrabutyl-3,7,13,17-tetramethyl-5,10-diazaporphyrinatoiron(III) Chloride

The chloroiron(III) complex of 2,8,12,18-tetrabutyl-3,7,13,17-tetramethyl-5,10-diazaporphyrin, [(Cl)FeMBDAP], was prepared and studied by X-ray crystallography and by solution 1H NMR and UV−vis measurements. In the crystal structure of hemisolvate [(Cl)FeMBDAP] × 0.5CHCl3, two nonequivalent [(Cl)FeMBDAP] units containing Fe1 and Fe2 are arranged in π-dimers with considerable overlap on their concave sides. Axial chloride bonded to Fe2 is solvated by hydrogen bonding with CHCl3. Parameters of the coordination pyramid have typical values for the spin-mixed (S = 3/2 ÷ 5/2) FeIII complexes in the case of Fe1 and are characteristic for the pure intermediate-spin state for Fe2 (displacement from the (NPyr)4 planes − 0.385 and 0.290 Å and the average NPyr−Fe bond lengths −1.992 and 1.954 Å for Fe1 and Fe2, respectively). Effective magnetic moments in CHCl3 and CH2Cl2 capable of specific solvation of chloride by hydrogen bonding (4.5−4.6 μB at 298 K) are indicative about mixed intermediate/high-spin state S = 3/2 ÷ 5/2, with the S = 3/2 contribution increasing upon lowering of the temperature (4.02 μB in CD2Cl2 at 193 K). In nonsolvating CCl4, C6D6, and THF-d8, the μeff values are consistent with the predominantly high-spin state at ambient temperature (5.5−5.75 μB at 298 K) and almost pure S = 5/2 state at low temperature (ca. 5.9 μB in THF-d8 below 270 K). Downfield isotropic shifts from 35 to 50 ppm are observed for α-alkyl protons and upfield shifts from −5 to −15 ppm for meso-CH protons, which is characteristic for the presence of the intermediate-spin state. The splitting of signals of the diastereotopic α-CH2 protons is increased with growth of the S = 3/2 state contribution from 1.5 to 4 ppm in nonsolvating to 11 ppm in specifically solvating solvents at 298 K and further to 31 ppm at 193 K (in CD2Cl2). In the presence of DMSO addition and in methanol solution, the single CH2 signal is observed at 25−28 ppm, and the meso-CH resonance is also shifted downfield to ca. 30 ppm, indicating the formation of six-coordinated complexes [(DMSO)2FeMBDAP]+ and [(MeOH)2FeMBDAP]+, the latter having the μeff value of 4.92 μB at 291 K is a spin-mixed species. The electron spin resonance spectra recorded at 77 K indicates that in frozen glasses in CD2Cl2 and THF molecules in the high-spin state (g⊥ ∼ 6) and the predominantly intermediate-spin state (g⊥ ∼ 4.2−4.3) coexist together.