Spectroscopic properties and reactivity of a mononuclear oxomanganese( IV ) complex †

A non-porphyrinic, mononuclear oxomanganese( IV ) complex was generated at room temperature and characterized by spectroscopic methods. The Mn IV QQQ O adduct is capable of activating C–H bonds by a H-atom transfer mechanism and is more reactive in this regard than most Mn IV QQQ O species. High-valent oxomanganese adducts are suggested as active oxidants for synthetic and biological manganese catalysts, including those involved in textile and paper bleaching with H 2 O 2 and oxygen evolution from water. 1 Oxomanganese( V ) adducts with S = 0 and 1 spin states have been reported, and these invariably feature strongly electron-donating, anionic ligands. 2 Oxomanganese( IV ) complexes with neutral, non-porphyrinic ligands are comparatively less common. 3 Detailed studies of substrate oxidation exist for a limited number of complexes. 3 a – c ,4 In addition, few Mn IV Q O complexes have been characterized by Mn K-edge X-ray absorption spectroscopy (XAS), 3 b ,5 a technique featuring prominently in the study of Mn enzymes 1 b and biominerals. 6 In this report, we describe the spectroscopic properties and oxidative reactivity of an oxomanganese( IV ) complex supported by the neutral, pentaden-tateN4pyligand( N , N -bis(2-pyridylmethyl)- N -bis(2-pyridyl)methylamine). This Mn IV Q O compound is more reactive than the majority of Mn IV Q O complexes for C–H bond oxidation.

Spectroscopic properties and reactivity of a mononuclear oxomanganese(IV) complex † Domenick F. Leto, Rena Ingram, Victor W. Day and Timothy A. Jackson* A non-porphyrinic, mononuclear oxomanganese(IV) complex was generated at room temperature and characterized by spectroscopic methods.The Mn IV Q Q QO adduct is capable of activating C-H bonds by a H-atom transfer mechanism and is more reactive in this regard than most Mn IV Q Q QO species.
High-valent oxomanganese adducts are suggested as active oxidants for synthetic and biological manganese catalysts, including those involved in textile and paper bleaching with H 2 O 2 and oxygen evolution from water. 1 Oxomanganese(V) adducts with S = 0 and 1 spin states have been reported, and these invariably feature strongly electron-donating, anionic ligands. 2 Oxomanganese(IV) complexes with neutral, nonporphyrinic ligands are comparatively less common. 3Detailed studies of substrate oxidation exist for a limited number of complexes.3a-c,4 In addition, few Mn IV QO complexes have been characterized by Mn K-edge X-ray absorption spectroscopy (XAS), 3b,5 a technique featuring prominently in the study of Mn enzymes 1b and biominerals. 6In this report, we describe the spectroscopic properties and oxidative reactivity of an oxomanganese(IV) complex supported by the neutral, pentadentate N4py ligand (N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine).This Mn IV QO compound is more reactive than the majority of Mn IV QO complexes for C-H bond oxidation.
The manganese(II) complex [Mn II (N4py)] 2+ (1) was generated as the triflate salt.The X-ray diffraction (XRD) structure exhibits a distorted octahedral Mn II center with pentadentate N4py and monodentate triflate ligands (Fig. 1A).The Mn-ligand bond lengths are 2.1 to 2.3 Å.The Mn K-edge XAS spectrum of a frozen aqueous sample of 1(OTf) 2 displays a pre-edge feature at 6540.6 eV and an edge at 6547.3 eV.The EXAFS data are best fit with 1 O at 2.09 Å, 5 N at 2.26 Å, and 3 C at 3.00 Å, in excellent agreement with the XRD structure (cf.Tables S4 and S6; ESI †).
The addition of excess PhIO (2.5 equiv.) to 1 in CF 3 CH 2 OH at 298 K led to the formation of a green species (2), with a broad electronic absorption band at 950 nm and weaker features at 600 and 450 nm (Fig. 1B).At 298 K, the formation of 2 finished in B10 minutes, and 2 showed a half-life of 30 minutes.The absorption features of 2 are very similar to those of other non-porphyrinic Mn IV QO complexes in tetragonal, six-coordinate environments, which display broad nearinfrared bands from B1040-825 nm and weaker features at higher energies.3a,b,7 The perpendicular mode X-band EPR spectrum of 2 is typical of a mononuclear, S = 3/2 Mn IV ion (Fig. S4; ESI †).3a,b,d,8 Hyperfine coupling for the g eff = 5.76 feature is B76 G, in good agreement with that observed for other Mn IV QO complexes.As we have been unable to grow crystals of 2, its molecular structure was investigated by Mn K-edge XAS.The edge energy of 2 (6550.8eV) is blue-shifted over 3 eV relative to that of 1, as expected for the higher oxidation state of Mn (Fig. 2).The edge energy of 2 is within 1 eV of those reported for Mn IV QO complexes supported by salen and porphyrin ligands (6549.9-6551.2eV; Table S3; ESI †). 5 The pre-edge peak of 2 at 6541.9 eV is significantly more intense than that of 1 (Fig. 2, inset), consistent with a large deviation from centrosymmetry.
The Fourier transform (R 0 space) of the EXAFS data of 2 exhibits a prominent, sharp peak at R 0 E 1.5 Å with less prominent peaks at R 0 E 1.9, 2.2, and 2.8 Å (Fig. 1C).The first coordination sphere of 2 is fit well with two or three shells of N/O atoms at distances (r) of 1.69, 2.00, and 2.24 Å (Table S4; ESI †).The shell at 1.69 Å, which corresponds to the oxo ligand, is much shorter than the Mn II -O (solvent H 2 O) distance of 2.09 Å observed for 1.The remaining first coordination sphere can be fit with either a single shell of 5 nitrogen scatterers at 1.99 Å or two shells of nitrogen scatterers at 2.00 Å (4 N atoms) and 2.24 Å (1 N atom), representing the nitrogen atoms of the pentadentate N4py ligand.The fit with two shells of N scatterers affords lower GOF and Debye-Waller values than the fit with just one shell of five N scatterers.Fits including outer-sphere features reveal two MnÁÁÁC shells at 2.82 and 2.97 Å (3 and 5 C atoms, respectively).
Metric parameters from the EXAFS data of 2 are in good agreement with a DFT-computed structure (Fig. 1A, right).This structure has a MnQO bond of 1.673 Å (cf. the EXAFS distance of 1.69 Å).The equatorial nitrogens in the DFT-optimized structure have an average Mn-N distance of 2.024 Å, while the trans amine has a longer distance of 2.138 Å, consistent with the two shells of Mn-N scatterers at 2.00 and 2.24 Å.
The ability of 2 to activate C-H bonds was investigated using dihydroanthracene (DHA), diphenylmethane (DPM), ethylbenzene (EtBz), and toluene (Tol), which span a B10 kcal mol À1 range of C-H bond strengths.For each substrate, the addition of an excess to 2 at 298 K under an Ar atmosphere led to (i) a disappearance in the optical bands of 2, (ii) formation of a new species, 3, with bands at 460 and 630 nm, and (iii) the appearance of an isosbestic point at 714 nm (Fig. 3A).The decay of 2 and the formation of 3 occurred with the same rate, both following pseudo-first order behaviour to at least four half-lives (see ESI †).Second-order rate constants (k 2 0 , corrected for the number of reactive C-H bonds) determined for all substrates revealed a linear relationship between log(k 2 0 ) and substrate bond dissociation enthalpies (BDEs), with a slope of 0.35 (Fig. 3B).Such behaviour is highly suggestive of a rate-limiting H-atom transfer., where a H-atom tunnelling mechanism was also implicated. 10 The reaction of 2 with DHA proceeded with a second-order rate constant (k 2 0 = 3.6 M À1 s À1 ) 2-3 orders of magnitude larger than those observed at similar temperatures for nearly all other non-porphyrinic Mn IV QO complexes (Table S8; ESI †).3a,4a,9,11 An Eyring analysis of DHA activation from 35 to À5 1C, reveal DH ‡ and DS ‡ of 9 AE 0.8 kcal mol À1 and À35 AE 3 cal mol À1 K, respectively (Table S9; ESI †).These parameters yield a DG ‡ (at 25 1C) comparable to that of [Mn IV (O)(H 3 buea)] À but 2 kcal mol À1 smaller than those observed for other Mn IV QO complexes, 3a,4a,b consistent with the greater reactivity of 2.
The reaction of 2 with DHA yielded 0.56(8) equiv. of anthracene per equiv. of 2. A final Mn oxidation state of 2.7(15) was determined by iodometric titration.This product distribution is consistent with the generation of anthracene by reaction of 1 equiv.DHA with 2 equiv.Mn IV QO rather than two successive H-atom transfers with a single Mn IV QO centre.Thus, 2 acts as a one-electron oxidant, which has been observed for other Mn IV QO compounds.2c,3a,b,4c Such reactivity is consistent with DFT studies by Shaik and Nam that have shown a second H-atom transfer between the nascent organic radical and Mn III -OH centre to be less favorable than diffusion of the organic radical from the Mn III -OH adduct. 12To the best of our knowledge, two-electron oxidation of DHA by a Mn IV QO has only been observed for [Mn IV (O) 2 (Me 2 EBC)] + and [Mn IV (O)(OH 2 )(BQCN)] 2+ .3c,4c While the iodometric product analysis gives an average Mn oxidation state following the reaction of 2 with DHA, the nature of the Mn-based products can be better defined on the basis of EPR, electronic absorption, and ESI-MS data.Perpendicular-mode EPR spectra of the product solution showed the strong Mn IV QO signals replaced by very weak signals.Broad features over a large field range and a sharp multiline signal at g E 2 are respectively attributed to mononuclear Mn II and binuclear species (Fig. S5; ESI †).Corresponding parallel-mode EPR spectra are silent.This does not preclude the presence of mononuclear Mn III species, as favourable Mn III zero-field splitting parameters and high-quality glasses are often required to observe the weak six-line signals of mononuclear Mn III centres in X-band experiments.The optical absorption features of product 3 are quite similar to those of [Mn III (OCH 2 CF 3 )(Bn-TPEN)] 2+ (Bn-TPEN = N-benzyl-N,N 0 ,N 0 -tris-(2-pyridylmethyl)-1,2-diaminoethane), which was the dominant Mn product when [Mn IV (O)(Bn-TPEN)] 2+ was reacted with hydrocarbons.3b In addition, the dominant molecular ion peak in ESI-MS data of 3 is at m/z 620.1289, consistent with [Mn III (OCH 2 CF 3 )(N4py)](OCH 2 CF 3 ) + (m/z calc.620.1293).Thus, we propose a mononuclear Mn III species as the dominant, but not sole, Mn-based product when 2 reacts with DHA. 13 The chemical reactivity of 2 is similar to that of [Mn IV (O)-(Bn-TPEN)] 2+ .3b Both N4py and Bn-TPEN are N5 aminopyridyl ligands that also support highly reactive Fe IV QO complexes. 10or the Mn IV QO adducts, previous DFT computations predicted that 2 has a larger barrier for H-atom abstraction from cyclohexane than [Mn IV (O)(Bn-TPEN)] 2+ . 12Although the addition of a large excess (400-600 equiv.) of cyclohexane increases the decay rate of 2, the reaction does not show pseudo-first order behaviour.In contrast, [Mn IV (O)(Bn-TPEN)] 2+ reacts with cyclohexane at 25 1C.Thus, we are unable to determine a k 2 value to provide a quantitative comparison of reactivity using cyclohexane.However, a comparison can be made using EtBz, with which both compounds react at 25 1C in CF 3 CH 2 OH.In the reaction with EtBz, [Mn IV (O)(Bn-TPEN)] 2+ (1 mM) shows a rate constant five-fold larger than that of 2 (2 mM): k 2 0 = 2.7 Â 10 À2 and 5.7 Â 10 À3 M À1 s À1 , respectively.
Thus, while 2 is dramatically more reactive towards C-H bonds than most Mn IV QO adducts, it is less reactive than [Mn IV (O)(Bn-TPEN)] 2+ .This trend holds for the corresponding Fe IV QO adducts; i.e., [Fe IV (O)(Bn-TPEN)] 2+ is more reactive towards C-H bonds. 10he origin of the high reactivity of 2 towards C-H bonds is currently unclear.Cyclic voltammetry studies of 2 show a Mn III/IV reduction potential (E 1/2 ) B700 mV higher than those of other Mn IV QO complexes (ESI †).3a,4a,9 Thus, 2 is a significantly more effective one-electron oxidant.Notably the E 1/2 of 2 is similar to those of other dicationic Mn IV complexes, 3a,9 suggesting that the increase in E 1/2 is attributed to the +2 total charge of 2 versus the +1 and À1 charges of other Mn IV QO adducts.3a,4a However, rates of H-atom transfer reactions, which are strongly correlated to thermodynamic driving force, depend not only on the reduction potential of the oxidant, but also on the basicity of the metalhydroxo product. 14Both the Mn III/IV reduction potential and the pK a of the Mn III -OH complex, which for this system is unknown, are necessary for a thermodynamic analysis.While we cannot comment at present on the driving force for C-H bond activation by 2, we note that many other Mn IV QO adducts have sterically demanding supporting ligands that shield the oxo.In contrast, the oxo ligand in 2 is well-exposed to substrate (Fig. 1A).Reduced steric clash with substrate could contribute to the relatively high reactivity of 2. Future work is needed to determine the role ligand sterics, solvent effects, and thermodynamic driving force play in influencing the H-atom transfer reactivity of 2, and to explore further why Mn IV QO adducts such as 2 may eschew standard rebound or desaturation mechanisms for C-H activation.
This work was supported by the US NSF (CHE-1056470 to T.A.J.; CHE-1004897 for R.I.; CHE-0946883 and CHE-0079282 supported instrument purchases).XAS experiments were supported by the Center for Synchrotron Biosciences grant, P30-EB-009998, from the National Institute of Biomedical Imaging and Bioengineering (NIBIB).We thank Dr Erik Farquhar at NSLS for outstanding support of our XAS experiments.
other Mn IV QO adducts (3.1-8).2c,3a,4a,9Activationenergies(E)and Arrhenius prefactors (A) determined from the reaction of 2 with DHA and d 4 -DHA from 35 to À5 1C in 1:1 CF 3 CH 2 OH : CH 2 Cl 2 provide evidence for H-atom tunnelling (TableS10; ESI †).Specifically, 3aIn support, reactions of 2 with deuterated-DHA (d 4 -DHA) reveal a kinetic isotope effect (KIE) of 11.2, which is larger than that observed for DHA oxidation by the difference in activation energies for DHA and d 4 -DHA (E D À E H ) is greater than the difference in zero-point energies of the C-H and C-D bonds (3.6 and 1.26 kcal mol À1 , respectively); and the ratio of Arrhenius prefactors (A H /A D = 0.02) is much less than 0.7, and comparable to that of [Fe IV (O)(N4py)] 2+