Relationship between Hydrogen-Atom Transfer Driving Force and Reaction Rates for an Oxomanganese(IV) Adduct

Abstract

Hydrogen atom transfer (HAT) reactions by high-valent metal-oxo intermediates are important in both biological and synthetic systems. While the HAT reactivity of Fe-oxo adducts has been extensively investigated, studies of analogous Mn-oxo systems are less common. There are several recent reports of Mn-oxo complexes, supported by neutral pentadentate ligands, capable of cleaving strong C-H bonds at rates approaching those of analogous Fe-oxo species. In this study, we provide a thorough analysis of the HAT reactivity of one of these Mn-oxo complexes, [Mn(O)(2pyN2Q)], which is supported by an N5 ligand with equatorial pyridine and quinoline donors. This complex is able to oxidize the strong C-H bonds of cyclohexane with rates exceeding those of Fe-oxo complexes with similar ligands. In the presence of excess oxidant (iodosobenzene), cyclohexane oxidation by [Mn(O)(2pyN2Q)] is catalytic, albeit with modest turnover numbers. Because the rate of cyclohexane oxidation by [Mn(O)(2pyN2Q)] was faster than that predicted by a previously published Bells-Evans-Polanyi correlation, we expanded the scope of this relationship by determining HAT reaction rates for substrates with bond dissociation energies spanning 20 kcal/mol. This extensive analysis showed the expected correlation between reaction rate and the strength of the substrate C-H bond, albeit with a shallow slope. The implications of this result with regard to Mn-oxo and Fe-oxo reactivity are discussed.

MnIV-Oxo complex of a bis(benzimidazolyl)-containing N5 ligand reveals different reactivity trends for MnIV-oxo than FeIV-oxo species

“…This slight influence of solvent on the rate is similar to that observed for [Mn IV (O)N4py] 2+ , 26 and reinforces other reports of modest enhancements for HAT reaction rates in TFE . 34 The rate of DHA oxidation by [Mn IV (O)(2pyN2B)] 2+ in 1:1 TFE:CH 2 Cl 2 is two-fold slower than that observed for [Mn IV (O)( DMM N4py)] 2+ (k 2 = 0.34 2M −1 s −1 in 1:1 TFE:CH 2 Cl 2 ), which had shown the slowest rate of DHA oxidation for Mn IV -oxo complexes of N4py and its derivatives (Table 2). 6,26 To further compare the HAT reactivity of We also probed the reactivity of [Mn IV (O)(2pyN2B)] 2+ with thioanisole.…”

Regulating the Basicity of Metal–Oxido Complexes with a Single Hydrogen Bond and Its Effect on C–H Bond Cleavage

“…7–13 Similarly, there are several examples of synthetic Mn IV/V –oxido complexes that have the ability to functionalize substrates via C–H bond activation. 6,14–20 These reactions are often thermodynamically challenging because C–H bonds have relatively large bond dissociation free energies (BDFEs) that can approach 100 kcal/mol. Because of this difficulty, it is usually thought that reagents for C–H bond functionalization have to be strong oxidants; this concept is particularly relevant to M–oxido species in which the 4+ formal oxidation state at the metal center is common.…”

Ligand Influence on Structural Properties and Reactivity of Bis(μ‐oxo)dimanganese(III,IV) Species and Comparison of Reactivity with Terminal MnIV‐oxo Complexes

“…As high‐valent manganese‐oxo species are often proposed as intermediates in C−H bond activation, olefin epoxidation, and oxygenation of hydrocarbons, one aim of such studies is the development of C−H functionalization catalysts for synthetic transformations ,. It is now well known that high‐valent manganese‐oxo species have a broad spectrum of reactivity, with some complexes showing slow rates of reaction with activated C−H bonds, and others capable of cleaving the strong C−H bonds in cyclohexane ,,,,,. In a recent study, our group systematically investigated the reactivities of manganese(IV)‐oxo species with the series of pentadentate N5 ligands shown in Figure A (N4py= N , N ‐bis(2‐pyridylmethyl)‐ N ‐bis(2‐pyridyl)methylamine, DMM N4py= N , N ‐bis(4‐methoxy‐3,5‐dimethyl‐2‐pyridylmethyl)‐ N ‐bis(2‐pyridyl)methylamine, and 2pyN2Q=bis(2‐pyridyl)‐ N , N ‐bis(2‐quinolylmethyl)methanamine) .…”