Xiang-Cui Huanga,
Hui-Fang Wang*a and
Jian-Ping Lang*ab
aCollege of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, People's Republic of China. E-mail: wanghf@suda.edu.cn; jplang@suda.edu.cn
bState Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, Shanghai 200032, People's Republic of China. E-mail: jplang@suda.edu.cn
First published on 22nd August 2016
By taking a Mn–dioxygen complex supported by a zwitterionic calix[4]arene ligand ([MnL(O2)(H2O)](PF6)2; H4L = [5,11,17,23-tetrakis(trimethylammonium)-25,26,27,28-tetrahydroxycalix[4]arene], CSD refcode: CAGVAJ) as a model system, we investigated the geometric and electronic structures of [MnL(O2)(H2O)]2+ through density functional theory (DFT) calculations without and with the presence of additional waters. It was found that the ground-state structure of [MnL(O2)(H2O)]2+ possesses a typical bent end-on binding mode (Mn–O–O angle at around 130° and dioxygen bond length at ∼1.32 Å) within a sextet state (S = 5/2) irrespective of calculated methodologies. The experimently proposed linear end-on structure (Mn–O–O angle at ∼180°) and the side-on structure (Mn–O–O angle ∼75°) are shown to be transition-state structures in the process of dioxygen flipping from one bent-end-on structure to another. By the inclusion of water solvation effect via both the explicit water-cluster model ([MnL(O2)(H2O)]2+–nH2O, n = 1, 4, 6, 8) and implicit CPCM solvation model, a good correlation was concluded: the more additional waters and the more contracted Oe square benefit the larger Mn–O–O angle, indicating a water solvation effect and a ligand size effect. Inspired by the relationship of linear end-on mode with bent end-on mode and the consistence of the projected dioxygen bond length of [MnL(O2)(H2O)]2+–8H2O in the CPCM model along the Mn–O bond direction (dO1–O2(p) = 1.240 Å) with the experimental dioxygen bond length (1.249 Å), we concluded that the linear Mn–O–O arrangement fitted by experimentalists should be a result of the flipping motion of dioxygen rotating about the Mn–O bond in solid-state structures of the [MnL(O2)(H2O)](PF6)2 complex based on analyses of the crystallographic data. Natural bond orbital (NBO) analyses show the sextet [MnL(O2)(H2O)]2+–nH2O (n = 0, 1, 4, 6, 8) have a Mn(III)-superoxo nature, consistent with experimental observations. The differences in the bonding structures of the three binding modes were discussed by the second-order perturbation energies ΔEi→j from the superoxo lone pair (LP) orbitals to Mn(III) 3d orbitals in β electron space. It can also give clues about the instability of the linear end-on binding mode and the increased Mn–O–O angle in the presence of water solvents.
Worth mentioning, a confusing binding mode of dioxygen was reported in a Mn–dioxygen complex [MnL(O2)(H2O)](PF6)2 (H4L = [5,11,17,23-tetrakis(trimethylammonium)-25,26,27,28-tetrahydroxycalix[4]arene]) by Liu et al. recently.21 It shows a linear Mn–O–O unit (Mn–O–O angle is 180°) enclosed by a NMe3+-substituted calix[4]arene ligand, and the distal O atom is surrounded by several water molecules distributing at top of the ‘bowl’ (Fig. 1). The CIF of this complex is enclosed as ESI† and the view of its cationic structure [MnL(O2)(H2O)]2+ with 50% thermal ellipsoids is shown in Fig. S1 (ESI†). Liu et al. assigned it as a Mn(III)-superoxo complex due to the typical signal for a superoxo radical observed in the electron paramagnetic resonance (EPR) spectrum of [MnL(O2)(H2O)](PF6)2 (a rhombic signal with g values of 2.0037 (293 K) and 2.0036 (110 K)). As far as we knew, such a linear coordination mode was seldom found in other structurally-defined dioxygen complexes except in some theoretical models.22,23 Most of the reported Mn–dioxygen complexes are established as side-on MnIII/IV-peroxo species24–26 and a few examples are found as η1 bent end-on hydroperoxo (Mn(III)–OOH27) and alkylperoxo (Mn(III)–OOR28) compounds. Few Mn-superoxo complex has been reported. Thirdly, M–O2 adducts are frequently supported by tetradentate nitrogenous ligands like porphyrin ligands,29 aminopyridyl ligands,30 and TMC ligands,3,31 but very few with calix[n]arene ligands. Thus, it intrigues us to investigate the geometric and electronic structures of [MnL(O2)(H2O)]2+ from density functional theory (DFT) calculations.
In the past years, DFT calculations have given reliable descriptions about geometric and electronic structures of synthesized metal–O2 complexes, making good supplementary for spectroscopic observations.19,32,33 Schatz et al.34 and Donoghue et al.33 detected copper(II)-superoxo complexes with various spectroscopic methods including UV/vis, EPR and nuclear magnetic resonance (NMR) spectroscopy, and determined the dioxygen bound in η1 bent end-on fashions based on DFT calculations. Zapata-Rivera et al.35,36 calculated the potential energy surfaces (PESs) involved in the complete intermolecular O2 transfer between TMC ligated Ni(II) and Mn(III) complexes. The calculated results supported the experimentally proposed two-step mechanism and the formation of intermediate was in agreement with kinetic studies. Therefore, in the present work, aimed at clarifying the most favourable binding mode of [MnL(O2)(H2O)]2+, different configurations of [MnL(O2)(H2O)]2+ have been investigated in combination of three different binding modes (bent end-on, linear end-on and side-on binding modes) with four different overall spin states (S = 7/2, 5/2, 3/2, 1/2) using DFT calculations. Water solvation effect was considered by combining an explicit water-cluster model and an implicit water solvation model. Influences of differently sized calix[4]arene ligands were explored and the Mn–O2 bond electronic structure was identified from NBO calculations. Through our calculations, a reasonable understanding on the experimentally fitted linear Mn–O–O unit was achieved and the water solvation effect and ligand size effect on Mn–O–O angle were clarified. It would shed light on controllable synthesis of more bent or more end-on Mn–dioxygen complexes through solvent polarity perturbations and ligand size adjustment.
Employing the most suitable functional and basis set level, we then compared energies and structures of [MnL(H2O)(O2)]2+ within different binding modes and different spin states. All possible spin states were considered for each binding mode, including octet state (S = 7/2), sextet (S = 5/2), quartet (S = 3/2) and doublet (S = 1/2). Optimization of octet state shows O2 flies away from Mn center with the nearest Mn–O distance more than 4.5 Å, implying hardly any ferromagnetically coupling interaction between the triplet dioxygen and sextet Mn(II). Based on the most stable configuration, we further studied the water solvation effect on the binding mode of [MnL(H2O)(O2)]2+ by combining an explicit water cluster model and an implicit conductor-like polarizable continuum model (CPCM) encoded in Gaussian software. The calculated results show that inclusion of the explicit water solvent molecules has significant influences on Mn–O–O bond angle. The details are elaborated in the following sections.
None structural or symmetry constraints were imposed during optimizations. Most structures were converged at default convergences, while the linear end-on configurations were located at tight convergences within ultrafine integration grids (grid = 99770). Analytical frequencies were calculated to verify local minima or transition states. Vibration frequencies reported here are scaled by 0.946.46 In some cases, different structures have been located for the same binding mode within the same spin state, e.g., the bent end-on mode within S = 5/2 has more than one equilibrium structures (like S1 and S2 shown in Fig. S2 in this work†), only structures with the lowest electronic energies were discussed in this work. NBO calculations47 were done using NBO 3.0. NBO orbital contour plot were generated by Gview 5.0.
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Fig. 2 Optimized structures of [MnL(H2O)(O2)]2+ in (a) bent end-on, (b) linear end-on and (c) side-on configurations. Only structural parameters of S = 5/2 are shown here and those of the quartet and doublet can be found in Table 1. |
Bent end-on | Linear end-ona | Side-on | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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S = 5/2 | S = 3/2 | S = 1/2 | S = 5/2 | S = 3/2b | S = 5/2 | S = 3/2 | S = 1/2 | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
a Location of the doublet linear end-on configuration failed and the predicated relative energy should be higher than 16.6 kcal mol−1.b The structure is converged at default convergences.c dMn–Oe is the averaged distances of dMn–Oe1, dMn–Oe2, dMn–Oe3, dMn–Oe4.d dOe–Oe is the averaged distances of Oe1–Oe3 and Oe2-Oe4, which is considered as a size measurement of the Oe square at the lower rim of the calix[4]arene cone and will be elaborated in main text. | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
ΔG | 0.00 | 1.80 | 15.6 | 4.2 | 4.7 | 9.5 | 14.2 | 40.4 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
vimg/cm−1 | None | None | None | 153.7i | 153.7i, 146.4i | 186.6i | 263.3i | 317.7i | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
vO1–O2/cm−1 | 1131.6(1124.0) | 1136.9 | 1143.0 | 1134.5 | 1160.9 | 1133.1 | 1130.8 | 1085.2 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
∠MnO2O1 | 129.3°(180.0°) | 131.7° | 125.0° | 178.9° | 179.5° | 75.0° | 75.2° | 79.7° | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
∠O2MnOw | 170.6°(180.0°) | 170.3° | 172.7° | 173.4° | 173.5° | 168.2° | 169.5° | 164.5° | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
dO1–O2/Å | 1.321(1.249) | 1.322 | 1.307 | 1.320 | 1.318 | 1.321 | 1.325 | 1.337 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
dMn–O1/Å | 3.248(3.693) | 3.201 | 2.875 | 3.492 | 3.426 | 2.467 | 2.572 | 2.208 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
dMn–O2/Å | 2.245(2.444) | 2.165 | 1.919 | 2.173 | 2.108 | 2.454 | 2.569 | 2.013 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
dMn–Ow/Å | 2.335(2.210) | 2.328 | 2.081 | 2.298 | 2.308 | 2.325 | 2.286 | 2.104 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
dMn–Oe/Åc | 1.928(1.887) | 1.933 | 1.935 | 1.934 | 1.939 | 1.937 | 1.926 | 1.941 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
dOe–Oe/Åd | 3.858(3.773) | 3.864 | 3.865 | 3.867 | 3.874 | 3.859 | 3.848 | 3.856 |
The vibration modes of the imaginary frequencies for the linear end-on and side-on configurations are indicated by the green arrows in Fig. 2b and c. They correspond to the motion of dioxygen ligand over Mn center, either flipping between right and left or swing up and down, respectively. It indicates that the linear end-on and side-on configurations should be transients during the dioxygen flipping from one equilibrium bent end-on configuration to another. To confirm this assumption, we performed optimizations on the linear end-on and side-on configurations by exerting very small displacements along the vibration mode of the imaginary frequency. It was found both the linear end-on configuration and side-on configuration thermodynamically converged into two bent end-on structures. The relevance of these three binding structures can be described by PES as shown in ESI (Fig. S2†).
To answer these questions, we investigated water solvation effect on the equilibrium structure of [MnL(O2)(H2O)]2+. Based on the optimized bent end-on [MnL(O2)(H2O)]2+ complex of S = 5/2, we progressively added additional waters to solvate the dioxygen moiety in the calix[4]arene ‘bowl’, building a set of solvated complexes [MnL(O2)(H2O)]2+–nH2O (n = 1, 4, 6, 8). As starting structures for each [MnL(O2)(H2O)]2+–nH2O complex, the additional waters were intentionally arranged in different patterns with the most hydrogen bonds being formed. Then they were optimized to local minima. Only structure with the lowest electronic energy was chosen as our target structure, which was found to possess the shortest hydrogen bonds between dioxygen moiety and additional waters. The optimized [MnL(O2)(H2O)]2+–nH2O structures are listed in Fig. 3, as well as some key structural parameters and H-bonding distances. The other structural parameters are given in ESI (Table S3†). It is seen that the distal O1 atom is hydrogen-bonded to additional waters with H-bonding distances in a range of 1.61–2.07 Å. In MnL(O2)(H2O)]2+–8H2O complex (Fig. 3d), the explicit additional waters tend to form a water-cubic cluster by H-bonding to each other. With the number of additional waters increasing from 1 to 8, the Mn–O2–O1 angle (∠MnO2O1) increases gradually from 132.1° to 162.6°, getting closer to a linear angle; the Mn–O2 distance elongates from 2.366 Å to 2.552 Å, becoming less deviated from the experimental distance (2.444 Å) than that of n = 0 (2.245 Å). The dOe–Oe decreases from 3.853 Å to 3.826 Å, getting nearer to the X-ray distance (3.773 Å). All these improved bond distances finally lead to reduced RMSDs of [MnL(O2)(H2O)]2+–8H2O (0.066 in Table S3†) as compared to that of [MnL(O2)(H2O)]2+ (0.118 in Table S1†). It implies that additional waters are necessary in describing the structure of [MnL(O2)(H2O)]2+ from a theoretical model. Besides, the shortest H-bonding distance in each [MnL(O2)(H2O)]2+–nH2O complex is generally located at the back face of the O1 atom (Fig. 3). It should be the right reason for the enlarged Mn–O2–O1 angle. Attempts with even more additional waters (e.g., n = 10 or 12) have been done, but none equilibrium structures with Mn–O2–O1 angle larger than 162.6° were located. It indicates that a linear Mn–O2 unit could not be anticipated even if more additional waters are included. Otherwise, the optimized structure of [MnL(O2)(H2O)]2+–8H2O at B3LYP-D3/B1 level (Table S3†) show that the empirical dispersion correction benefit good bond length description (RMSD = 0.045) as expected,43 while predicts more bent bond angles than B3LYP functional (e.g. 156.4° vs. 162.2° for Mn–O2–O1 angle, and 172.9° vs. 174.9° for O2–Mn–Ow angle).
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Fig. 3 Optimized structures of sextet [MnL(O2)(H2O)]2+–nH2O (n = 1, 4, 6, 8) complexes under B3LYP/B1 method. The H-bonds are shown in light-blue dot lines. The key structural parameters are labeled in bigger-size numbers and main H-bond distances (in Å) in smaller-size numbers. Data in parentheses correspond to results at B3LYP/B2 level in CPCM model (see B3LYP/B2* in Table S4†). dO1–O2(p) is the projected distances of dO1–O2 along the Mn–O2 bond direction. |
Subsequently, we investigated the basis set effect and the bulk water solvation effect on geometry of [MnL(O2)(H2O)]2+–8H2O. We re-optimized structure of [MnL(O2)(H2O)]2+–8H2O under different basis set levels and in an implicit water solvation model, CPCM model, respectively. The calculated results are shown in ESI (Table S4†). It shows that polarizations on H atoms, especially on those of the additional waters, benefit a larger Mn–O2–O1 angle (e.g. the Mn–O2–O1 angle of [MnL(O2)(H2O)]2+–8H2O increases to 164.6° under B3LYP/B2 method and 168.6° under B3LYP/B8 method as in Table S4†). It suggests that the polarization effect of additional waters is important on the bonding angle of Mn–O2 unit. While, the bulk solvation effect via CPCM model, makes the Mn–O2–O1 angle slightly reduced, e.g., the Mn–O2–O1 angle of [MnL(O2)(H2O)]2+–8H2O decreases from 162.6° in vacuum (B3LYP/B1) to 159.6° in CPCM model (B3LYP/B2*) (Fig. 3d and Table S4†). Whereas, the Mn–O2 distance in CPCM model (2.467 Å under B3LYP/B2* level in Table S4†) gets more closed to the experimental distance (2.444 Å), resulting in a smaller RMSD (0.073 in Table S4†). However, the calculated Mn–O2–O1 angle (∼160°) and dO1–O2 (∼1.32 Å) are still the two most deviated parameters from the X-ray results (180.0° and 1.249 Å, respectively). While, such deviations can be easily reconciled if we take into consideration the projected distances of dO1–O2 (dO1–O2(p) in Fig. 3) along the Mn–O2 bond directions (the Mn–O2 bond is nearly vertical to the mean equatorial plane of Oe square). We noticed that within CPCM model (B3LYP/B2*), the dO1–O2(p) of [MnL(O2)(H2O)]2+–8H2O is 1.240 Å, approximately equal to the X-ray distance of dioxygen bond (1.249 Å). The other key bond lengths like dMn–O2 and dMn–Oe under B3LYP/B2* method also coincide well with the crystallographic results with deviation less than 0.05 Å (see data in parentheses in Fig. 3 and Table S4†). Such good consistencies suggest two possibilities: (i) the proposed linear Mn–O2 unit in [MnL(O2)(H2O)](PF6)2 complex would result from superposition of disordered Mn–O2 moieties rotating about the Mn–O2 bond in equilibrium bent end-on configuration, similar to the observed dioxygen disorder in solid-state structures of cobalt–dioxygen and iron–dioxygen complexes ligated by porphrins.53–55 Li and co-workers have treated such disorder as the dynamics of dioxygen rotation around the M–O bond based on detailed analyses about the solid-state structures of oxycobalt and oxyiron picket fence porphyrins under different temperatures.5,55 (ii) When the diffraction data of [MnL(O2)(H2O)](PF6)2 were collected, the dioxygen ligand would be in a constant flipping motion around the Mn–O bond as indicated by the vibration mode of the imaginary frequency of the linear end-on structure shown in Fig. 2b. The resulting averaged electron density of O atoms would then misguide experimentalists with a linear Mn–O2–O1 arrangement as Liu et al. made.21 By examination on the crystallographic data of [MnL(O2)(H2O)](PF6)2, we inclined to the latter possibility. The proofs are: (1) the O1, O2, Mn and Ow atoms are ordinary thermal ellipsoids, with little sign of normal disorder (Fig. S1, ESI†), even if the crystallographic data were collected at −50 °C; (2) the thermal factors of O1, O2, Mn and Ow atoms are, respectively, 0.052(2) e/Å2, 0.045(2) e/Å2, 0.0433(5) e/Å2 and 0.043(2) e/Å2. They are generally larger than those of Oe atoms (0.0296(8) e/Å2) and C atoms of phenyl rings (0.016–0.026 e/Å2) (see CIF, ESI†), implying an amplitude librational motion of these atoms around their equilibrium positions. Also, the larger thermal factor of O1 atom than that of O2 atom indicates a larger amplitude librational motion of O1 than O2. (3) The calculated flipping barrier between the bent end-on structure and the liner end-on structure would be not more than 4.20 kcal mol−1 as indicated by the PES in Fig. S2 (ESI†) and thus can be easily conquered at −50 °C where the crystallographic data were collected.
Presumably, the hydrophobic repulsion between the additional waters and the phenyl rings would expand the upper rim of the calix[4]arene (CN square), thus result in a contracted Oe square at the lower rim due to the rigidity of phenyl rings. Such a contracted Oe square would finally lead to a increased Mn–O2–O1 angle because of the constriction effect of the ‘bowl’ bottom on the Mn–O2 unit. Therefore, the more additional waters would induce the stronger hydrophobic repulsion, thus the more expanded CN squares and more contracted Oe squares, finally leading to the larger Mn–O2–O1 angles. To certify this supposition, we modified the calix[4]arene ligand of [MnL(O2)(H2O)]2+ by substituting the NMe3+ groups with four more bulky groups like NEt3+ (Et = ethyl) and N(t-Bu)3+ (t-Bu = tert-butyl), producing two complexes [MnLEt(O2)(H2O)]2+ (NEt3+-appended complex) and [MnLt-Bu(O2)(H2O)]2+ (N(t-Bu)3+-appended complex). Evidenced from the enlarged dCN–CN and reduced dOe–Oe in [MnLEt(O2)(H2O)]2+ and [MnLt-Bu(O2)(H2O)]2+ complexes as tabulated in Table 2, we see the more bulky groups at the upper-rim of the cone directly lead to more expanded upper rims and thus more contracted lower rims. As expected, the larger Mn–O2–O1 angles are obtained in [MnLEt(O2)(H2O)]2+ and [MnLt–Bu(O2)(H2O)]2+ as shown in Table 2 (131.1° and 133.4°, respectively). Additionally, we directly enlarged the Oe square (dOe–Oe) of the NMe3+-substituted calix[4]arene (i.e., [MnL(O2)(H2O)]2+) by replacing the four connecting methylene (–CH2–) groups with four –S– groups ([MnL−S–(O2)(H2O)]2+ in Table 2 and Fig. S4†). Then we got a decreased Mn–O2–O1 angle at 120.5° expectedly. Therefore, we can conclude that calix[4]arene ligand with a shrinked bottom benefits a more end-on Mn–O2 unit and that with an expanded bottom favors a more bent Mn–O2 unit, indicating a ligand size effect. The supporting ligand ring-size effect has been also reported by Nam and coworkers on the metal dioxygen complexes supported by TMC ligands.3,14 Whereas, such a ligand size effect is less important than the water solvation effect on modulating Mn–O2–O1 angle. The evident proof can be found in Table 2. From [MnL−S–(O2)(H2O)]2+ complex to [MnLt-Bu(O2)(H2O)]2+ complex, the dOe–Oe decreases by ∼0.13 Å (from 3.975 Å to 3.848 Å) and the Mn–O2–O1 angle increases by ∼13° (from 120.5° to 133.4°). By contrast, from [MnL(O2)(H2O)]2+ to [MnL(O2)(H2O)]2+–8H2O, the dOe–Oe decreases by only 0.05 Å (from ∼3.87 Å to ∼3.82 Å, see Fig. 2 and 3), the Mn–O2–O1 angle increases by about 33.3° (129.3° vs. 162.6°).
NMe3+-appended a | NEt3+-appended | N(t-Bu)3+-appended | –S– connecting | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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a Calculated structural parameters of [MnL(O2)(H2O)]2+ at B3LYP/B1 level.b One of the dCN–CN in NEt3+-substituted calix[4]arene is longer than NMe3+-substituted calix[4]arene. | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
dCN–CN/Å | 8.922 | 8.916b | 9.164 | 9.691 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
dOe–Oe/Å | 3.858 | 3.854 | 3.848 | 3.975 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
∠MnO2O1 | 129.3° | 131.1° | 133.4° | 120.5° |
Would any other polar solvents benefit a larger Mn–O2–O1 angle? To answer this question, we further optimized the structures of [MnL(O2)(H2O)]2+ and [MnL(O2)(H2O)]2+–8H2O in CPCM model with different solvents. The chosen solvents include water (ε = 78.36), methanol (ε = 32.61), chloroform (ε = 4.71) and benzene (ε = 2.27). The calculated Mn–O2–O1 angles and dOe–Oe are exhibited in Table 3 and the Cartesian coordinates of the optimized structures in different solvents are enclosed in ESI.† From Table 3, we can see that when the polarity of the solvent (indicated by the dielectric constant ε) increases in the order: vacuum < benzene < chloroform < methanol < water, the Mn–O2–O1 angle as well as the dOe–Oe of [MnL(O2)(H2O)]2+ in solvents increase, suggesting the more polarized solvent favors larger Mn–O2–O1 angle. It is consistent with the suggestion that the bulk polarization of the solvent can affect the binding modes of metal–O2 complexes.56 On the contrary, the Mn–O2–O1 angle of [MnL(O2)(H2O)]2+–8H2O complex increases as the solvent polarity decreases (Table 3). Such a result can be explained by the deeper insertion of the water-cubic cluster into the calix[4]arene bowl in a less polarized solvent (e.g. chloroform and benzene) due to the strengthened water-phobic interaction between the additional water-cluster and the implicit water-insoluble solvents. The evidence is that the vertical distances from the Mn center to the water-cubic cluster decrease in the following order: water > methanol > chloroform > benzene > vacuum (see the Cartesian coordinates listed in ESI†). The deeper insertion of the water-cubic cluster in a less polarized solvent is assumed to make a larger steric effect with the calix[4]arene cone, thus the reduced dOe–Oe and increased Mn–O2–O1 angle (Table 3). It confirms that the ligand size effect induced by the explicit additional water molecules is maintained in non-water solvents. Moreover, the deep insertion of the water-cubic cluster also leads to enhanced hydrogen bonding interaction between the dioxygen ligand with additional water molecules in [MnL(O2)(H2O)]2+–8H2O complex in non-water solvents evidenced from the shortened H-bond distances (see Cartesian coordinates in ESI†). All above results indicate that the bending degree of the M–O2 unit in metal–dioxygen complexes can be adjusted through solvent polarity perturbations and ligand size modulations.
Solvent (ε) | [MnL(O2)(H2O)]2+ | [MnL(O2)(H2O)]2+–8H2O | ||
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∠MnO2O1 | dOe–Oe/Å | ∠MnO2O1 | dOe–Oe/Å | |
Water (ε = 78.36) | 133.3° | 3.874 | 153.1° | 3.843 |
Methanol (ε = 32.61) | 133.1° | 3.872 | 153.5° | 3.843 |
Chloroform (ε = 4.71) | 132.7° | 3.865 | 155.1° | 3.840 |
Benzene (ε = 2.27) | 131.8° | 3.859 | 158.6° | 3.834 |
Vacuum (ε = 1) | 129.3° | 3.858 | 162.6° | 3.826 |
n = 0 | n = 1 | n = 4 | n = 6 | n = 8 | n = 8 (CPCM) | |
---|---|---|---|---|---|---|
QMn/e | 1.15 | 1.18 | 1.21 | 1.21 | 1.24 | 1.22 |
QO1/e | −0.32 | −0.37 | −0.44 | −0.44 | −0.43 | −0.41 |
QO2/e | −0.37 | −0.34 | −0.32 | −0.31 | −0.31 | −0.30 |
To estimate the stabilizing effect of charge delocalization from superoxo to Mn(III) in β space (similar charge delocalization in α space is negligible since the referred 3d orbitals is occupied), we listed in Fig. 5 the dominant second-order perturbation energies ΔEi→j from superoxo LP orbitals to Mn(III) unoccupied 3d orbitals in β space for bent end-on [MnL(O2)(H2O)]2+–nH2O complexes. The larger ΔEi→j indicates the more tendency for electrons delocalization from superoxo to Mn(III), thus the more intensive stabilizing interaction. It was found that there are two important stabilization energies for bent end-on structures (Fig. 5 and S5†): one corresponds to charge delocalization from the pip(O2) orbital to Mn(III) 3dz2 orbital (pip(O2) → 3dz2); the other one corresponds to charge delocalization from pip(O1) orbitals to the σ anti-bonding orbitals of the nearest additional water () (none for n = 0). pip(O2) → 3dz2 charge delocalization should be a σ-donation. Seen from Fig. 5, ΔEi→j (pip(O2) → 3dz2) is quite large for [MnL(O2)(H2O)]2+ (i.e., n = 0, 41.47 kcal mol−1), corresponding to a σ-bonding interaction of superoxo
with Mn 3dz2 as described in other work.15,36,58 The corresponding molecular orbital (
) is depicted in ESI (Fig. S6†). While, this interaction gets greatly reduced as number of additional waters increases, e.g., from 41.47 kcal mol−1 with n = 0 to 1.08 kcal mol−1 with n = 8. It is possibly because of the elongated Mn–O2 distances in the presence of water as shown in Fig. 3 and the enlarged Mn–O2–O1 angle that would make pip(O2) orbital more horizontal, thus unfavourable charge delocalization to Mn 3dz2 orbital. The
stabilization energy in the presence of water suggests strong H-bonding interactions between O1 atom with additional waters. Moreover, the dominant
donation are always trans to the bending direction of dioxygen (Fig. 5 and S5†). It should be the exact reason for the enlarged Mn–O2–O1 angles in the presence of water.
![]() | ||
Fig. 5 NBOs of superoxo donors (left) and Mn(III) acceptors (right) for sextet [MnL(O2)(H2O)]2+–nH2O (n = 1) complex, as well as second-order perturbation energies (ΔEi→j in kcal mol−1) for n = 0, 1, 4, 6, 8 in β space. All hydrogen atoms are omitted for clarity. NBOs for [MnL(O2)(H2O)]2+–nH2O (n = 0, 4, 6, 8) are similar to n = 1 and can be found in ESI.† |
The stabilizing interactions between superoxo and Mn(III) center in linear end-on and side-on [MnL(O2)(H2O)]2+ structures can also be interpreted based on ΔEi→j (Fig. S5, ESI†), although a general understanding on the bonding interaction of different configurated M–O2 complexes have been achieved in many other work.13,15,57 We found that the pip(O2) → 3dz2 charge delocalization is largely decreased in the linear end-on (6.94 kcal mol−1) and side-on configurations (2.39 kcal mol−1) as compared to the bent end-on structure, because of the unmatched orbital symmetry of pip(O2) with 3dz2 in these two binding modes. Instead, a π-type delocalization between pip(O2) and 3dxz orbital (pip(O2) → 3dxz), becomes dominant in the linear end-on structure (Fig. 5b) (about 8.63 kcal mol−1). It must cause the linear end-on structure less stable than the bent end-on structure; for side-on structure,ΔEi→j shows comparable stabilizing interactions between pip orbitals of O1, O2 atoms with Mn 3dxz orbital. It should correspond to the σ bonding interaction of the π* orbital of dioxygen with metal 3d orbital (e.g., 3dxz or else) as described elsewhere13,15,36
The deviation of DFT calculated dO1–O2 (1.321 Å) from the X-ray distance (1.249 Å) was reconciled by the projected distance of dO1–O2 on the Mn–O bond direction of [MnL(O2) (H2O)]2+–8H2O within CPCM model (1.240 Å). Together with analyses on thermal ellipsoids of O atoms and the transformation energies from a bent end-on structure to a linear end-on structure (PES in Fig. S2, ESI†), this reconciliation gave clues about the dioxygen flipping motion around Mn–O bond in the solid state structure of [MnL(O2)(H2O)](PF6)2 complex when the diffraction data were collected. It thus misguided the experimentalists with a linear Mn–O–O arrangement. Otherwise, a good correlation is concluded that the more additional waters and the smaller Oe squares would benefit a larger Mn–O–O angle, indicating a water solvation effect and a ligand size effect. These results shed light on controllable synthesis of more end-on or more bent M–O2 complexes through solvent polarity perturbations and supporting ligand size modifications. Whereas, the ligand size effect is shown to be less important than the water solvation effect. The physical origin of the water solvation effect can attribute to the strongest H-bonding interaction locating at the back face of the distal O atom in each [MnL(O2)(H2O)]2+–nH2O (n = 1, 4, 6, 8) complex (see Fig. 3 and 5). NBO analyses show [MnL(O2)(H2O)]2+ has a Mn(III)-superoxo nature, consistent with experimental results. The presence of additional waters would not affect the Mn(III)-superoxo nature. The second-order perturbation energies ΔEi→j from superoxo LP orbitals to Mn(III) unoccupied 3d orbitals in β space gave explanations about the instability of linear end-on mode as compared to the bent end-on mode and why the presence of water benefits a more end-on Mn–O–O angle.
Footnote |
† Electronic supplementary information (ESI) available: Test calculations using different functional and different basis set levels, optimized structural parameters of [MnL(O2)(H2O)]2+–nH2O (n = 1, 4, 6, 8) under different basis sets, PES of three binding configurations, NBO orbital interactions of [MnL(O2)(H2O)]2+–nH2O (n = 1, 4, 6, 8) and Catesian coordinates for all structures described in this work. See DOI: 10.1039/c6ra11199h |
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