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 FeIVO centers generated in the active sites of nonheme iron oxygenases cleave substrate C–H bonds at rates significantly faster than most known synthetic FeIVO complexes. Unlike the majority of the latter, which are S = 1 complexes, [FeIV(O)­(tris­(2-quinolylmethyl)­amine)­(MeCN)]2+ (3) is a rare example of a synthetic S = 2 FeIVO complex that cleaves C–H bonds 1000-fold faster than the related [FeIV(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.