Manganese(IV)-oxo complexes are often invoked as intermediates in Mn-catalyzed C-H bond activation reactions. While many synthetic MnIV-oxo species are mild oxidants, other members of this class can attack strong C-H bonds. The basis for these reactivity differences is not well understood. Here we describe a series of MnIV-oxo complexes with N5 pentadentate ligands that modulate the equatorial ligand field of the MnIV center, as assessed by electronic absorption, electron paramagnetic resonance, and Mn K-edge X-ray absorption methods. Kinetic experiments show dramatic rate variations in hydrogen-atom and oxygen-atom transfer reactions, with faster rates corresponding to weaker equatorial ligand fields. For these MnIV-oxo complexes, the rate enhancements are correlated with both i) the energy of a low-lying 4E excited state, which has been postulated to be involved in a two-state reactivity model, and ii) the MnIII/IV reduction potentials.

Investigations of a MnIV-oxo adduct supported by an N5 ligand with mixed pyridyl and benzimidazolyl ligation uncovers distinct reactivity trends for MnIV-oxo and FeIV-oxo adducts at parity of coordination sphere.

Analysis of extendedX -ray absorption fine structure (EXAFS) data for the Mn IV -oxo complexes [Mn IV (O)( DMM N4py)] 2+ + ,[ Mn IV (O)(2pyN2B)] 2+ + ,a nd [Mn IV (O)(2pyN2Q)] 2+ + ( DMM N4py = N,N-bis(4-methoxy-3,5-dimethyl-2-pyridylmethyl)-N-bis(2-pyridyl)methylamine; 2pyN2B = (N-bis(1-methyl-2-benzimidazolyl)methyl-N-(bis-2pyridylmethyl)amine, and 2pyN2Q = N,N-bis(2-pyridyl)-N,Nbis(2-quinolylmethyl)methanamine) afforded Mn=Oa nd MnÀNb ond lengths.T he Mn=Od istances for [Mn IV (O)( DMM N4py)] 2+ + and [Mn IV (O)(2pyN2B)] 2+ + are 1.72 and 1.70 ,r espectively.I nc ontrast, the Mn=Od istance for [Mn IV (O)(2pyN2Q)] 2+ + was significantly longer( 1.76 ). We attribute this long distance to sample heterogeneity, which is reasonable given the reduced stability of [Mn IV (O)(2pyN2Q)] 2+ + .T he Mn=Od istances for [Mn IV (O)( DMM N4py)] 2+ + and [Mn IV (O)(2pyN2B)] 2+ + could only be well-reproduced using DFT-derived modelst hat included strong hydrogen-bonds between second-sphere solvent 2,2,2-trifluoroethanol molecules and the oxo ligand. These results suggesta ni mportantr olef or the 2,2,2-trifluoroethanol solvent in stabilizing Mn IV -oxo adducts. The DFT methods were extended to investigate the structure of the putative [Mn IV (O)(N4py)] 2+ + ·(HOTf) 2 adduct. These computations suggest that aM n IV -hydroxos pecies is most consistentw ith the availablee xperimental data.[a] Dr.

High valent metal-oxo intermediates are versatile oxidants known to facilitate both oxygen atom transfer (OAT) and hydrogen atom transfer (HAT) reactions in nature.