NMR and Mössbauer Studies Reveal a Temperature-Dependent Switch from S = 1 to 2 in a Nonheme Oxoiron(IV) Complex with Faster C-H Bond Cleavage Rates

S = 2 Fe^IV═O centers generated in the active sites of nonheme iron oxygenases cleave substrate C-H bonds at rates significantly faster than most known synthetic Fe^IV═O complexes. Unlike the majority of the latter, which are S = 1 complexes, [Fe^IV(O)(tris(2-quinolylmethyl)amine)(MeCN)]²⁺ (3) is a rare example of a synthetic S = 2 Fe^IV═O complex that cleaves C-H bonds 1000-fold faster than the related [Fe^IV(O)(tris(pyridyl-2-methyl)amine)(MeCN)]²⁺ complex (0). To rationalize this significant difference, a systematic comparison of properties has been carried out on 0 and 3 as well as related complexes 1 and 2 with mixed pyridine (Py)/quinoline (Q) ligation. Interestingly, 2 with a 2-Q-1-Py donor combination cleaves C-H bonds at 233 K with rates approaching those of 3, even though Mössbauer analysis reveals 2 to be S = 1 at 4 K. At 233 K however, 2 becomes S = 2, as shown by its ¹H NMR spectrum. These results demonstrate a unique temperature-dependent spin-state transition from triplet to quintet in oxoiron(IV) chemistry that gives rise to the high C-H bond cleaving reactivity observed for 2.