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

= 2 Fe═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═O complexes. Unlike the majority of the latter, which are = 1 complexes, [Fe(O)(tris(2-quinolylmethyl)amine)(MeCN)] () is a rare example of a synthetic = 2 Fe═O complex that cleaves C-H bonds 1000-fold faster than the related [Fe(O)(tris(pyridyl-2-methyl)amine)(MeCN)] complex (). To rationalize this significant difference, a systematic comparison of properties has been carried out on and as well as related complexes and with mixed pyridine (Py)/quinoline (Q) ligation. Interestingly, with a 2-Q-1-Py donor combination cleaves C-H bonds at 233 K with rates approaching those of , even though Mössbauer analysis reveals to be = 1 at 4 K. At 233 K however, becomes = 2, as shown by its H 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 .