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(CF3-SO3)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)(CF3-SO3)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, P2(1)/c, a = 18.360(1) A, b = 11.761(1) A, c = 25.754(2) A, beta = 90.72(1) degrees, Z = 4. 3: C16H29Cl2F6FeN5O6S2, triclinic group P1, a = 8.500(1) A, b = 11.421(2) A, c = 15.677(2) A, alpha = 92.23(1) degrees, beta = 94.79(1) degrees, gamma = 97.03(1) degrees, Z = 2. 4: C20H18F6FeN4O6S2, monoclinic, P2(1)/n, a = 11.253(3) A, b = 12.624(5) A, c = 14.683(5) A, beta = 94.02(2) degrees, 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.