Spin-State Variation in Solid State and Solution of Mononuclear Iron(II) 1,4,7-Trimethyl-1,4,7-triazacyclonane Complexes

The series of mononuclear iron(II) complexes with the tridentate macrocycle Me3tacn have been prepared (Me3tacn = 1,4,7-trimethyl-1,4,7-triazacyclonane). A purple, spin-crossover complex [Fe(Me3tacn)(MeCN)3](CF3SO3)2 (1−OTf) forms in acetonitrile but readily loses MeCN ligands to form a colorless high-spin complex
Fe(Me3tacn)(OTf)2 (2). The BPh4- salt of 1 is stable to loss of MeCN and remains purple even under a vacuum.
Methylene chloride solutions of Fe(OTf)2 and Me3tacn afford crystals of [Fe(Me3tacn)(MeCN)2(OTf)](OTf) (3).
Crystallization of 1−OTf in the presence of water affords a colorless high-spin complex, Fe(Me3tacn)(H2O)(CF3SO3)2 (4), that exists as a pair of molecules bridged by hydrogen bonds between the coordinated water and the
two bound triflate anions of the inversion-related partner. The crystallographic parameters are the following.
1−BPh4:  C63H70B2Fe, monoclinic, P21/c, a = 18.360(1) Å, b = 11.761(1) Å, c = 25.754(2) Å, β = 90.72(1)°,
Z = 4. 3:  C16H29Cl2F6FeN5O6S2, triclinic group P1̄, a = 8.500(1) Å, b = 11.421(2) Å, c = 15.677(2) Å, α =
92.23(1)°, β = 94.79(1)°, γ = 97.03(1)°, Z = 2. 4:  C20H18F6FeN4O6S2, monoclinic, P21/n, a = 11.253(3) Å, b
= 12.624(5) Å, c = 14.683(5) Å, β = 94.02(2)°, Z = 4. Variable temperature visible spectra and 1H NMR
spectra of solutions of both 1−OTf and 1−BPh4 exhibit low-spin, high-spin crossover behavior, whereas 2, 3,
and 4 remain high-spin in solution. The extensive role of coordinated triflate as a terminal and/or bridging ligand
as well as a counteranion is demonstrated by variable temperature 19F NMR spectra.