Synthesis and Electronic Structure Diversity of Pyridine(diimine)iron Tetrazene Complexes

A series of pyridine(diimine)iron tetrazene compounds, (PDI)Fe[(NR)NN(NR)] [PDI = 2,6-(ArN = CMe)₂C₅H₃N; Ar = 2,6-ⁱPr₂C₆H₃] has been prepared either by the addition of 2 equiv of an organic azide, RN₃, to the corresponding iron bis(dinitrogen) compound, (PDI)Fe(N₂)₂ or by the addition of azide to the iron imide derivatives, (PDI)FeNR. The electronic structures of these compounds were determined using a combination of metrical parameters from X-ray diffraction, solution and solid-state magnetic measurements, zero-field ⁵⁷Fe Mössbauer and ¹H NMR spectroscopies, and density functional theory calculations. The overall electronic structure of the iron tetrazene compounds is sensitive to the nature of the tetrazene nitrogen substituent with three distinct classes of compounds identified: (i) overall diamagnetic ( S = 0) compounds arising from intermediate-spin iron(II) centers ( S_Fe = 1) engaged in antiferromagnetic coupling with both pyridine(diimine) and tetrazene radical anions ( S_PDI = -¹/₂ and S_tetrazene = -¹/₂; R = 2-adamantyl, cyclooctyl, benzyl); (ii) overall S = 1 compounds best described as intermediate-spin iron(III) ( S_Fe = ³/₂) derivatives engaged in antiferromagnetic coupling with a pyridine(diimine) radical anion ( S_PDI = -¹/₂; R = 3,5-Me₂C₆H₃, 4-MeC₆H₄); (iii) overall S = 2 compounds best described as high-spin iron(III) centers ( S_Fe = ⁵/₂) engaged in antiferromagnetic coupling to a pyridine(diimine) radical anion ( S_PDI = -¹/₂; R = 1-adamantyl). For both the intermediate- and high-spin ferric cases, the tetrazene ligand adopts the closed-shell, dianionic form, [N₄R₂]^2-. For the case where R = SiMe₃, spin-crossover behavior is observed, arising from a spin-state change from intermediate- to high-spin iron(III).