Two new pentadentate {N5} donor ligands based on the N4Py (N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine) framework have been synthesized, viz. [N-(1-methyl-2-benzimidazolyl)methyl-N-(2-pyridyl)methyl-N-(bis-2-pyridyl methyl)amine] (L(1)) and [N-bis(1-methyl-2-benzimidazolyl)methyl-N-(bis-2-pyridylmethyl)amine] (L(2)), where one or two pyridyl arms of N4Py have been replaced by corresponding (N-methyl)benzimidazolyl-containing arms. The complexes [Fe(II)(CH3CN)(L)](2+) (L = L(1) (1); L(2) (2)) were synthesized, and reaction of these ferrous complexes with iodosylbenzene led to the formation of the ferryl complexes [Fe(IV)(O)(L)](2+) (L = L(1) (3); L(2) (4)), which were characterized by UV-vis spectroscopy, high resolution mass spectrometry, and Mössbauer spectroscopy. Complexes 3 and 4 are relatively stable with half-lives at room temperature of 40 h (L = L(1)) and 2.5 h (L = L(2)). The redox potentials of 1 and 2, as well as the visible spectra of 3 and 4, indicate that the ligand field weakens as ligand pyridyl substituents are progressively substituted by (N-methyl)benzimidazolyl moieties. The reactivities of 3 and 4 in hydrogen-atom transfer (HAT) and oxygen-atom transfer (OAT) reactions show that both complexes exhibit enhanced reactivities when compared to the analogous N4Py complex ([Fe(IV)(O)(N4Py)](2+)), and that the normalized HAT rates increase by approximately 1 order of magnitude for each replacement of a pyridyl moiety; i.e., [Fe(IV)(O)(L(2))](2+) exhibits the highest rates. The second-order HAT rate constants can be directly related to the substrate C-H bond dissociation energies. Computational modeling of the HAT reactions indicates that the reaction proceeds via a high spin transition state.

A m o n o n u c l e a r i r o n ( I I ) c o m p l e x [Fe II (N4Py Me2 )(OTf)](OTf)(1), supported by a new pentadentate ligand, bis(6-methylpyridin-2-yl)-N,N-bis((pyridin-2yl)methyl)methanamine (N4Py Me2 ), has been isolated and characterized. Introduction of methyl groups in the 6-position of two pyridine rings makes the N4Py Me2 a weaker field ligand compared to the parent N4Py ligand. Complex 1 is high-spin in the solid state and converts to [Fe II (N4Py Me2 )(CH 3 CN)]-(OTf) 2 (1a) in acetonitrile solution. The iron(II) complex in acetonitrile displays temperature-dependent spin-crossover behavior over a wide range of temperature. In its reaction with m-CPBA or oxone in acetonitrile at −10 °C, the iron(II) complex converts to an iron(IV)-oxo species, [Fe IV (O)(N4Py Me2 )] 2+ (2). Complex 2 exhibits the Mossbauer parameters δ = 0.05 mm/s and ΔE Q = 0.62 mm/s, typical of N-ligated S = 1 iron(IV)-oxo species. The iron(IV)-oxo complex has a half-life of only 14 min at 25 °C and is reactive toward oxygen-atom-transfer and hydrogen-atom-transfer (HAT) reactions. Compared to the parent complex [Fe IV (O)(N4Py)] 2+ , 2 is more reactive in oxidizing thioanisole and oxygenates the C−H bonds of aliphatic substrates including that of cyclohexane. The enhanced reactivity of 2 toward cyclohexane results from the involvement of the S = 2 transition state in the HAT pathway and a lower triplet-quintet splitting compared to [Fe IV (O)(N4Py)] 2+ , as supported by DFT calculations.The second-order rate constants for HAT by 2 is well correlated with the C−H bond dissociation energies of aliphatic substrates. Surprisingly, the slope of this correlation is different from that of [Fe IV (O)(N4Py)] 2+ , and 2 is more reactive only in the case of strong C−H bonds (>86 kcal/mol), but less reactive in the case of weaker C−H bonds. Using oxone as the oxidant, the iron(II) complex displays catalytic oxidations of substrates with low activity but with good selectivity.

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Two new pyrazolate‐based ligands, N′‐[1‐(3‐acetyl‐4‐methyl‐1H‐pyrazol‐5‐yl)ethylidene]‐2‐(hydroxyimino)propanehydrazide (L1) and 5‐[(E)‐1‐(2‐{(E)‐2‐(hydroxyimino}propanoyl}hydrazono)ethyl]‐4‐methyl‐1H‐pyrazole‐3‐carboxylic acid (L2), were synthesized and studied for zinc(II) complexation. A set of pH‐dependent UV/Vis measurements has been performed to determine the complex formation properties of L2. According to the calculations, in solution, L2 forms variously protonated mononuclear (ZnII/L2 = 1:1) and dinuclear (ZnII/L2 = 2:1) complexes. The reaction of the deprotonated ligands with hydrated ZnII salts and slow diffusion of ammonia into the reaction mixtures gave mononuclear [Zn(L1‐2H)(NH3)2]·DMF (1) and trinuclear μ‐pyrazolato‐bridged [Zn3(L2‐3H)2(NH3)5]·4H2O (3). In both complexes, the zinc ions are in the same distorted trigonal‐bipyramidal environment, coordinated to two nitrogen atoms of the ammonia and one oxygen and two nitrogen atoms of the pyrazolate and hydrazide groups. The molecular structures of all of the ligands and complexes have been elucidated by X‐ray crystallography.