Spectroscopic and Computational Investigations of a Mononuclear Manganese(IV)-Oxo Complex Reveal Electronic Structure Contributions to Reactivity

Abstract

The mononuclear Mn(IV)-oxo complex [Mn(O)(N4py)], where N4py is the pentadentate ligand N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine, has been proposed to attack C-H bonds by an excited-state reactivity pattern [ Cho, K.-B.; Shaik, S.; Nam, W. J. Phys. Chem. Lett. 2012 , 3 , 2851 - 2856 (DOI: 10.1021/jz301241z )]. In this model, a E excited state is utilized to provide a lower-energy barrier for hydrogen-atom transfer. This proposal is intriguing, as it offers both a rationale for the relatively high hydrogen-atom-transfer reactivity of [Mn(O)(N4py)] and a guideline for creating more reactive complexes through ligand modification. Here we employ a combination of electronic absorption and variable-temperature magnetic circular dichroism (MCD) spectroscopy to experimentally evaluate this excited-state reactivity model. Using these spectroscopic methods, in conjunction with time-dependent density functional theory (TD-DFT) and complete-active space self-consistent-field calculations (CASSCF), we define the ligand-field and charge-transfer excited states of [Mn(O)(N4py)]. Through a graphical analysis of the signs of the experimental C-term MCD signals, we unambiguously assign a low-energy MCD feature of [Mn(O)(N4py)] as the E excited state predicted to be involved in hydrogen-atom-transfer reactivity. The CASSCF calculations predict enhanced Mn-oxyl character on the excited-state E surface, consistent with previous DFT calculations. Potential-energy surfaces, developed using the CASSCF methods, are used to determine how the energies and wave functions of the ground and excited states evolved as a function of Mn═O distance. The unique insights into ground- and excited-state electronic structure offered by these spectroscopic and computational studies are harmonized with a thermodynamic model of hydrogen-atom-transfer reactivity, which predicts a correlation between transition-state barriers and driving force.

Structural Characterization of a Series of N5‐Ligated MnIV‐Oxo Species

“…Our previously reported TD-DFT computations for [Mn IV (O)(N4py)] 2+ + predicted electronic absorption bands near7 90 and4 75 nm, with more intense features at wavelengths below 400 nm. [26] These results were in reasonable agreement with the experimental electronic absorption spectrum of [Mn IV (O)(N4py)] 2+ + , which containsaprominentf eature near 950 nm, aw eakb and at approximately 600 nm, and intense features below 400 nm. [26, 10a] The TD-DFTm ethodp redicts the electronic transitions to be too high in energy,a nd the computed band intensities were also about threefold highert han those observed experimentally.…”

“…[32] Electronic absorption spectra for Mn IV -hydroxo complexes were calculated using both TD-DFT and CASSCF/NEVPT2 methods, using levels of theory described previously. [26] Frequency calculations for all Mn IV -oxo species lacking second-sphere TFE molecules, and [Mn IV (O)(2pyN2Q)] 2+ + ·(TFE) 2 showedn oi maginary frequencies, ensuring that these structures represent true minima.T he [Mn IV (O)(N4py)] 2+ + ·(TFE) 2 and [Mn IV (O)( DMM N4py)] 2+ + ·(TFE) 2 structures each showedo ne small imaginary frequency( À7.30 and À25.18 cm À1 ,r espectively), associated with rotationalm odes of the methyl groups of TFE and the para-methoxy-pyridyl moieties, respectively.A ttemptst oe liminate these imaginary modes through the use of ad enser integration grid or tighter convergence criteria wereu nsuccessful. Cartesian coordinates for all DFT-optimized modelsa re included as Supporting Information (TablesS8-S32).…”

“…[26, 10a] The TD-DFTm ethodp redicts the electronic transitions to be too high in energy,a nd the computed band intensities were also about threefold highert han those observed experimentally. [26] The TD-DFT computed electronic absorption spectra for [Mn IV (OH)(N4py)] 3+ + ,[ Mn IV (OH)(N4py)] + + ·(OTf À ), and [Mn IV (OH)(N4py)] 3+ + ·(TFE) each show prominent features at 425, 440, and 473 nm, respectively (Figure 7). The spectra no longer contain the 950 nm near-IRf eature characteristic of [Mn IV (O)(N4py)] 2+ + .T hese spectral changes are consistentw ith those observed by Nam, Fukuzumi, and co-workersu pon the addition of triflic acid to [Mn IV (O)(N4py)] 2+ + .…”

Equatorial Ligand Perturbations Influence the Reactivity of Manganese(IV)‐Oxo Complexes

“…[8] We recently identified the 4 E excited state of 2-N4py using magnetic circular dichroism spectroscopy. [9] This state arises from a Mn IV e (d xz , d yz ) → b 1 (d x 2 -y 2 ) one-electron excitation and gives rise to a weak electronic absorption band at 10 500 cm −1 . As the Mn IV b 1 (d x 2 -y 2 ) acceptor MO is Mn-N4py σ-antibonding, the 4 E energy, and hence HAT and OAT reactivity, should be sensitive to perturbations in the equatorial ligand field.…”

“…Moreover, these correlations pertain to Mn IV -oxo species in tetragonal environments with N5 supporting ligands. Although, we have recently noted that [Mn IV (O)(OH)(Me 2 EBC)] + , which is a more sluggish oxidant, [4a] has a fairly high energy 4 E excited state, [9] the applicability of these correlations to Mn IV -oxo complexes with anionic ligands, O-donor ligands, and/or trigonal coordination environments is an open question. [4b, 16] It is also unclear what effect, if any, the steric interactions between the oxo and quinoline ligands in 2-2pyN2Q have on reactivity.…”