Hydrogen-atom and oxygen-atom transfer reactivities of iron(iv)-oxo complexes of quinoline-substituted pentadentate ligands

This is a self-archived version of an original article. This version may differ from the original in pagination and typographic details. Author(s): Title: Year: Version: Copyright: Rights: Rights url: Please cite the original version: CC BY-NC 4.0 https://creativecommons.org/licenses/by-nc/4.0/ Hydrogen-atom and oxygen-atom transfer reactivities of iron(iv)-oxo complexes of quinoline-substituted pentadentate ligands © 2022 Royal Society of Chemistry Published version Munshi, Sandip; Sinha, Arup; Yiga, Solomon; Banerjee, Sridhar; Singh, Reena; Hossain, Md. Kamal; Haukka, Matti; Valiati, Andrei Felipe; Huelsmann, Ricardo Dagnoni; Martendal, Edmar; Peralta, Rosely; Xavier, Fernando; Wendt, Ola F.; Paine, Tapan K.; Nordlander, Ebbe Munshi, S., Sinha, A., Yiga, S., Banerjee, S., Singh, R., Hossain, M. K., Haukka, M., Valiati, A. F., Huelsmann, R. D., Martendal, E., Peralta, R., Xavier, F., Wendt, O. F., Paine, T. K., & Nordlander, E. (2022). Hydrogen-atom and oxygen-atom transfer reactivities of iron(iv)-oxo complexes of quinoline-substituted pentadentate ligands. Dalton Transactions, 51(3), 870-884. https://doi.org/10.1039/d1dt03381f 2022 Dalton Transactions PAPER Cite this: DOI: 10.1039/d1dt03381f Received 6th October 2021, Accepted 8th December 2021 DOI: 10.1039/d1dt03381f rsc.li/dalton Hydrogen-atom and oxygen-atom transfer reactivities of iron(IV)-oxo complexes of quinoline-substituted pentadentate ligands† Sandip Munshi, a Arup Sinha, b,c Solomon Yiga, d,e Sridhar Banerjee, a Reena Singh, b Md. Kamal Hossain, b Matti Haukka, f Andrei Felipe Valiati, g Ricardo Dagnoni Huelsmann, h Edmar Martendal, h Rosely Peralta, g Fernando Xavier, h Ola F. Wendt, * d Tapan K. Paine * a and Ebbe Nordlander * b A series of iron(II) complexes with the general formula [Fe II (L2-Qn)(L)] n+ (n=1,L=F − ,Cl − ;n=2,L= NCMe, H 2 O) have been isolated and characterized. The X-ray crystallographic data reveals that metal– ligand bond distances vary with varying ligand field strengths of the sixth ligand. While the complexes with fluoride, chloride and water as axial ligand are high spin, the acetonitrile-coordinated complex is in a mixed spin state. The steric bulk of the quinoline moieties forces the axial ligands to deviate from the Fe–N axial axis. A higher deviation/tilt is noted for the high spin complexes, while the acetonitrile coordinated complex displays least deviation. This deviation from linearity is slightly less in the analogous low-spin iron(II) complex [Fe II (L1-Qn)(NCMe)] 2+ of the related asymmetric ligand L1-Qn due to the presence of only one sterically demanding quinoline moiety. The two iron(II)-acetonitrile complexes [Fe II (L2-Qn)(NCMe)] 2+ and [Fe II (L1-Qn)(NCMe)] 2+ generate the corresponding iron(IV)-oxo species with higher thermal stability of the species supported by the L1-Qn ligand. The crystallographic and spectroscopic data for [Fe IV (O)(L1-Qn)](ClO 4 ) 2 bear resemblance to other crystallographically characterized S=1 iron(IV)-oxo complexes. The hydrogen atom transfer (HAT) and oxygen atom transfer (OAT) reactivities of both the iron(IV)-oxo complexes were investigated, and a Box–Behnken multivariate optimization of the parameters for catalytic oxidation of cyclohexane by [Fe II (L2-Qn)(NCMe)] 2+ using hydrogen peroxide as the terminal oxidant is presented. An increase in the average Fe–N bond length in [Fe II (L1-Qn)(NCMe)] 2+ is also manifested in higher HAT and OAT rates relative to the other reported complexes of ligands based on the N4Py framework. The results reported here confirm that the steric influence of the ligand environment is of critical importance for the reactivity of iron(IV)-oxo complexes, but additional electronic factors must influence the reactivity of iron-oxo complexes of N4Py derivatives. Introduction High-valent iron-oxo moieties are prevalent in oxygenase enzymes. The presence of a ferryl (Fe IV vO) moiety in the active form –compound I –of the cytochrome P-450 family of heme oxygenases is well established. 1 It has been determined that ferryl units are also formed in reactions catalyzed by nonheme iron oxygenases. Examples include TauD, 2 an α-ketoglutarate dependent oxygenase (hydroxylase). 3 The nonheme Rieske dioxygenases have been proposed to form active oxidation intermediates containing a perferryl (Fe V vO) unit. 4 Over the last decade, a number of relatively stable bio-inspired non-heme ferryl complexes have been characterized, 5 and their reactivities investigated. These complexes have been found to effect a number of different oxidation reactions involving C–H †Electronic supplementary information (ESI) available. CCDC 2045257 and 2113531–2113535. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/d1dt03381f a School of Chemical Sciences, Indian Association for the Cultivation of Science, Kolkata, India. E-mail: [email protected] b Chemical Physics, Department of Chemistry, Lund University, Box 124, SE-221 00 Lund, Sweden. E-mail: Ebbe.Nordla[email protected] c Department of Chemistry, School of Advanced Science, Vellore Institute of Technology, Vellore, India d Center for Analysis and Synthesis, Department of Chemistry, Lund University, Box 124, SE-221 00 Lund, Sweden. E-mail: [email protected] e Department of Chemistry, Makerere University, P. O. Box 7062, Kampala, Uganda f Department of Chemistry, University of Jyväskylä, Box 35, FI-400 14, Jyväskylä, Finland g Department of Chemistry, LABINC, Universidade Federal de Santa Catarina (UFSC), 88040-900 Florianopolis, Santa Catarina, Brazil h Department of Chemistry, Center for Technological Sciences, Universidade do Estado de Santa Catarina (UDESC), 89219-710 Joinville, Santa Catarina, Brazil This journal is © The Royal Society of Chemistry 2022 Dalton Trans. Open Access Article. Published on 07 January 2022. Downloaded on 1/11/2022 10:27:29 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online View Journal bond activation (hydrogen atom transfer, HAT), e.g. dehydrogenation reactions to form arenes, and oxygen atom transfer (OAT), e.g. sulfoxidation reactions, as well as combinations of HAT and OAT, e.g. aliphatic C–H hydroxylation. 6 Non-heme ligands that have been used to stabilize ferryl complexes are typically tetraand pentadentate nitrogen-donor ligands. An overwhelming majority of these synthetic ferryl complexes have S= 1 spin state, as opposed to the ferryl moieties in non-heme metalloenzyme active sites, which are found in the high spin state (S= 2). Shaik and coworkers 7,8 have proposed that high-spin Fe IV vO moieties are inherently more reactive, and that hydrogen atom transfer mediated by many lowspin ferryl complexes proceeds via high-spin transition states (two-state reactivity). Studies on synthetic high spin ferryl complexes have not shown enhanced reactivity relative to low-spin complexes, but there is a quest to synthesize stable high spin Fe IV vO complexes and to investigate their reactivities. 9 Ligand design is of central importance in these investigations; it is desirable to use a ligand that exerts a sufficiently weak ligand field to stabilize a high spin ferryl moiety, yet induces sufficient (thermal) stability to enable facile handling of the complex. Recently, Rasheed et al., 10 Rana et al. 11 and Mukherjee et al. 12 published iron complexes of the ligand N-[di(2-pyridyl) methyl]-N,N-bis(quinolin-2-ylmethyl)methanamine (L2-Qn) 13 (Chart 1). Rasheed et al. synthesized and characterized [Fe IV (O) (L2-Qn)] 2+ (7,vide infra). 10 It was found that this complex was considerably more reactive in hydrogen atom transfer (hydrogen abstraction) reactions than related N5-donor ligands with other nitrogen-containing (-benzimidazolyl, -pyridyl (vN4Py)) substituents. Complex 7is not high spin, but the steric encumbrance exerted by the two quinoline moieties of the pentadentate ligand means that the ligand in the sixth position coordinates in a bent fashion, significantly deviating from the linear axis of an idealized octahedron. This is borne out in the crystal structure of 7, 10 where there is a significant tilt of the iron (IV)vO unit away from the two quinoline entities, giving an N (amine) –Fe–O angle that deviates from linearity by almost 10°; similar deviations have been observed in the crystal structures of [Mn II (OH 2 )(L2-Qn)] 2+ , 14 [Zn II (NCMe)(L2-Qn)] 2+ and [Cu II (NO 3 )(L2-Qn)] + . 13 As a consequence of the steric interaction, the Fe–N distances in the equatorial plane are significantly lengthened in comparison to those found for related N4Py-derived ligands. 10,15–17 The resultant weakened ligand field is expected to lower the energy gap between the low spin and high spin states and provide enhanced reactivity in agreement with the two state reactivity model proposed by Shaik and coworkers (vide supra). Rana et al. 11 also studied the alkane oxidation (hydroxylation) effected by [Fe IV (O)(L2-Qn)] 2+ and obtained results in agreement with the observations made by Rasheed et al. Computational modeling by Rana et al. 11 indicates that the hydroxylation reaction proceeds via an oxygen rebound mechanism, as expected, i.e. initial hydrogen atom abstraction to form an iron(III)-OH intermediate and an alkane radical followed by the rebound of a hydroxyl radical from the intermediate to form a hydroxylated alkane and an iron(II) complex that Chart 1 Ligands and complexes discussed in this work. Paper Dalton Transactions Dalton Trans. This journal is © The Royal Society of Chemistry 2022 Open Access Article. Published on 07 January 2022. Downloaded on 1/11/2022 10:27:29 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online may be (re)oxidized to form the ferryl complex. Furthermore, these authors demonstrated that the hydrogen abstraction capability of [Fe IV (O)(L2-Qn)] 2+ can be utilized for halogenation of sp 3 C–H bonds by a similar rebound mechanism, with the halide complexes [Fe II (X)(L2-Qn)] + (X = Cl − ,Br − ) acting as halogen sources. Computational modeling suggests that [Fe IV (O)(L2-Qn)] 2+ abstracts hydrogen from a suitable alkane substrate to generate [Fe III (OH)(L2-Qn)] 2+ and a substrate radical. Exchange of the hydroxide in the latter species with a halide ligand originating from [Fe II (X)(L2-Qn)] + (X = Cl − ,Br − ) leads to the formation of [Fe III (X)(L2-Qn)] 2+ that undergoes a “halogen rebound”with the substrate radical to form the halogenated product. 11 Independently, we have studied the reactivity of [Fe IV (O) (L2-Qn)] + and the analogous ferryl complex of the closely related ligand L1-Qn (Chart 1). Here we wish to describe the synthesis and characterization of the complexes [Fe II (L2-Qn)(L)] n+ (n=1,L=F − ,Cl − ;n= 2, L = NCMe, H 2 O) and an extended investigation into the HAT and OAT reactivities of [Fe IV (O)(L2-Qn)] 2+ . An optimization of the parameters for catalytic oxidation of cyclohexane by [Fe II (NCMe)(L2Qn)] n+ , using hydrogen peroxide as the ultimate oxidant, is also presented. A multivariate methodology using response surfaces for three variables (Box–Behnken design) was used to simultaneously evaluate reaction time, the relative amounts of substrate (cyclohexane), and oxidant (hydrogen peroxide). This strategy was adopted since it was observed in a previous study that these variables strongly interact in such a catalytic system. 18 Furthermore, the synthesis of the directly related but asymmetric ligand L1-Qn and its iron(II) complex [Fe II (NCMe)(L1-Qn)] 2+ is reported. It has also been possible to prepare [Fe IV (O)(L1-Qn)] 2+ and to determine the crystal structure of [Fe IV (O)(L1-Qn)](ClO 4 ) 2 , and we report a detailed analysis of its reactivity in HAT/OAT reactions. Our results confirm that the steric influence of the ligand environment is of critical importance for the reactivity of iron(IV)vO complexes, but additional electronic factors must influence the reactivity of ferryl complexes of N4Py derivatives. Results and discussion Ligand syntheses The pentadentate bisquinoline ligand N-[di(2-pyridyl)methyl]- N,N-bis(quinolin-2-ylmethyl)methanamine (L2-Qn) was synthesized by the method reported by McMorran and coworkers. 13 The corresponding monoquinoline ligand 1,1-di (pyridin-2-yl)-N-(pyridin-2-ylmethyl)-N-(quinolin-2-ylmethyl) methanamine (L1-Qn) was prepared by the reaction of N-[di(2pyridyl)methyl]-N-(2-pyridylmethyl)methylamine 19 with 2-chloromethyl quinoline hydrochloride in dry acetonitrile in the presence of K 2 CO 3 and KI (Experimental section). Synthesis and characterization of iron(II) complexes of L2-Qn Room temperature treatment of Fe(BF 4 ) 2 with L2-Qn in acetonitrile yielded a light yellow compound. The X-ray analysis revealed the compound to be [Fe II (L2-Qn)(F)](BF 4 )(1·BF 4 ), instead of the anticipated acetonitrile complex; the fluoride ligand apparently originating from tetrafluoroborate. It is likely that the fluoride ion is formed as the result of hydrolysis of the tetrafluoroborate by adventitious water. It may be noted that in related systems, it has been shown that iron(II) species can abstract fluoride from the BF 4 − anion to form a fluoro complex. 20 The molecular structure of 1is shown in Fig. 1. A comparison of pertinent Fe–ligand distances for 1and other complexes reported here is found in Table 1 (vide infra) and relevant crystallographic data for 1and all other crystal structures reported here are summarized in the ESI (Table S1†). The central iron(II) ion is in a distorted octahedral coordination environment. The data indicate that all the Fe–N distances are in the range 2.219(5)–2.249(5) Å, typical for high-spin iron(II) compounds. As discussed by Rasheed et al. 10 (vide supra) the presence of the quinoline moieties in L2-Qn leads to longer Fe–N (quinoline) distances because of the steric bulk of the quinoline moieties and the resultant steric interactions with the axial ligand. Consequently, the equatorial ligand field is decreased and the presence of the axial weak-field fluoride ligand renders complex 1high spin. The increased ionic Fig. 1 Mercury plots of the molecular structures of the cationic complexes 1(a) and 2(b), showing the atom labelling scheme. Thermal ellipsoids are plotted at 30% probability ellipsoids; hydrogen atoms have been omitted for clarity. Dalton Transactions Paper This journal is © The Royal Society of Chemistry 2022 Dalton Trans. Open Access Article. Published on 07 January 2022. Downloaded on 1/11/2022 10:27:29 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online radius in the high spin state results in the iron(II) ion being placed 0.536 Å above the ligand equatorial plane. In order to obtain the corresponding acetonitrile complex, L2-Qn was reacted with Fe(ClO 4 ) 2 in acetonitrile at room temperature. The reaction mixture was kept in an ethyl acetate bath for two days to yield a dark reddish brown crystalline solid. The ESI mass spectrum of this solid revealed the presence of two complexes. A minor fragment at m/z557.68 corresponded to the chloride-coordinated monocationic complex [Fe(L2-Qn) (Cl)](ClO 4 )(2·ClO 4 ), while the major fragment at m/z281.83 corresponded to the dicationic complex [Fe(L2-Qn)(CH 3 CN)] (ClO 4 ) 2 (3·ClO 4 ). Presumably, partial decomposition of the perchlorate ion was the source of the chlorine atom. When the reaction was performed using Fe(ClO 4 ) 2 ·xH 2 O as a starting material, an oxo-bridged diferric complex, [{Fe III (L2-Qn) (H 2 O)} 2 (μ-O)](ClO 4 ) 4 (5,cf. Fig. S1, ESI†), was obtained with no trace of 3. The crystal structure of 2·ClO 4 could be determined and the structure of the cation 2is shown in Fig. 1(b). The crystal structure of [Fe II (L2-Qn)(Cl)]Cl (2·Cl), containing cationic 2, has been reported previously (CCDC 1516453), as has that for the bromide analogue of 2(CCDC 1516421). 11 The relatively long Fe–N distances observed for 2in 2·ClO 4 , which are in the range 2.214(5)–2.252(6) Å, are consistent with the data reported by Maiti and co-workers 11 and are in full agreement with the high-spin electronic configuration in complex 2. In both cases the tilt of the axial chloro ligand is around 19° and the iron atom is 0.556 Å (for complex 2) and 0.542 Å for [Fe II (L2-Qn) (Cl)]Cl above the ligand equatorial plane. As discussed above, this is expected due to the steric interactions imposed by the two bulky quinolyl substituents and the presence of the weakfield chloride ligand. Compound 3·ClO 4 was structurally characterized by single crystal X-ray crystallography. The molecular structure of the cation 3is shown in Fig. 2(a). The structure is similar to that of the “parent”complex [Fe II (N4Py)(CH 3 CN)](ClO 4 ) 2 reported by Que and co-workers. 21 The Fe–N bond lengths are in the range of 1.951(6)–2.103(5) Å which are slightly longer than those reported for low spin iron(II) centres coordinated to similar pyridine based ligands (Table 1). Again, the presence of the relatively bulky and weakly coordinating quinoline moieties is likely to be the reason for the increase in the metal– ligand bond lengths. Indeed, the iron-pyridine nitrogen atoms are 2.019(5) and 2.010(5) Å apart, while those in the N4Py systems are typically less than 2.0 Å. The iron-quinoline nitrogen atom distances are longer, 2.103(5) and 2.092(5) Å. Mitra et al. reported a similar increase in the Fe–N (pyridine) bond distances upon replacement of a pyridyl moiety by an (N-methyl) benzimidazolyl moiety. 16 The iron atom in complex 3sits 0.295 Å above the ligand equatorial plane. The close proximity Table 1 Comparison of Fe–N bond distances (Å) in 1–4and 6with those of related complexes supported by N4Py-type ligands ([Fe II (N4Py) (CH 3 CN)] 2+ , 24 [Fe II (N2Py2B)(CH 3 CN)] 2+ , 16 [Fe II (N4Py Me2 )(CH 3 CN)] 2+ , 17 and [Fe II (N2PyN2Py Me2 )(CH 3 CN)] 2+ (ref. 25) Complex Fe–L axial Fe–N axial Fe–N Py (avg.) Fe–N Qn/BzIm/Me2 (avg.) 11.900(2) 2.248(3) 2.230 2.239 22.3010(18) 2.238(5) 2.219 2.2465 31.951(6) 2.017(5) 2.0145 2.0975 42.059(6) 2.188(6) 2.191 2.200 61.937(2) 1.970(2) 1.961 2.047(18) [Fe II (N4Py)(CH 3 CN)] 2+ 1.915(3) 1.961(3) 1.972 — [Fe II (N2Py2B)(CH 3 CN)] 2+ 1.909(6) 1.980(6) 1.953 1.977 [Fe II (N4Py Me2 )(CH 3 CN)] 2+ 1.889(10) 1.977(8) 1.979 2.044 [Fe II (N2PyN2Py Me2 )(CH 3 CN)] 2+ 1.959(3) 1.990(6) 1.993 2.081 Fig. 2 Mercury plots of the molecular structures of the cationic complexes 3(a) and 4(b), showing the atom labelling scheme. Thermal ellipsoids are plotted at 30% probability ellipsoids; hydrogen atoms have been omitted for clarity. Paper Dalton Transactions Dalton Trans. This journal is © The Royal Society of Chemistry 2022 Open Access Article. Published on 07 January 2022. Downloaded on 1/11/2022 10:27:29 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online of the iron center to the ligand equatorial plane is comparable to that found in the low spin parent complex [Fe II (N4Py) (CH 3 CN)](ClO 4 ) 2 where the iron is placed 0.208 Å above the equatorial plane. The relatively shorter Fe–N distances in the equatorial plane of 3, as compared to other high spin complexes in this study, is indicative of a possible mixture of high and low spin states of the iron ion in the solid state, which was corroborated by solution NMR spectroscopy (vide infra), and the bond parameters associated with complex 3are closer to those expected for a low spin complex of an N4Py-derived ligand. The water-coordinated high spin complex [Fe II (L2-Qn) (OH 2 )](OTf) 2 (OTf = triflate) (4·OTf) was obtained upon reaction of Fe(OTf) 2 ·2CH 3 CN with the ligand in dichloromethane and subsequent crystallization from moist acetonitrile. The Fe–N distances are in the range 2.186(6)–2.201(6) Å, while the oxygen atom of the coordinated water molecule is 2.059(6) Å apart from the iron atom (Table 1). The molecular structure of the cationic moiety is shown in Fig. 2(b). As shown in Table 1, the crystallographic data show the changes in metal–ligand bond distances with varying ligand field strengths of the sixth ligand (fluoride, chloride, water and acetonitrile). Similar to the other high spin complexes considered here, the iron ion is dislocated 0.474 Å above the ligand equatorial plane. In agreement with previous studies (vide supra), the steric effect of the quinoline moieties is reflected in the tilt of the axial ligands from the Fe–N axial axis. The displacement of the iron ion from the ligand equatorial plane places the L axial at the vicinity of the H-8 of the quinoline moiety. The magnitude of the tilt increases with increasing distance of the iron atom from the ligand equatorial plane, and vice versa. Accordingly, a higher deviation/tilt (in the range 16.8° to 19.3°) is observed for the high spin complexes 1,2and 4, whereas the least deviation (11.5°) is observed for 3. The spin states of the complexes are further supported by their room temperature 1 H NMR spectra. For complexes 1,2 and 4, the 1 H NMR spectra display resonances in the region between 120 ppm and −60 ppm, confirming the high spin state. However, for complex 3the high spin and low spin forms were found to be in equilibrium, with the high spin forming a minor component in solution at room temperature. Indeed, variable temperature 1 H NMR spectroscopy reveals that at −40 °C the low spin state is observed for 3(Fig. S2, ESI†). The observed spin states are in complete agreement with the ordering of the four ligands in the spectrochemical series, which gives that the halides are all weak-field ligands and that the σ-donor/π-acceptor ligand acetonitrile is a relative strong field ligand: Br − <Cl − <F − < NCMe < pyridine. Synthesis and characterization of an iron(II) complex of L1-Qn The iron(II) complex [Fe II (L1-Qn)(CH 3 CN)](ClO 4 ) 2 (6) was isolated from the reaction of an equimolar amount of the ligand with Fe(ClO 4 ) 2 ·6H 2 O in dry acetonitrile at room temperature. The ESI-mass spectrum (positive ion mode in acetonitrile) of complex 6exhibits ion peaks at m/z236.50 and 572.02 with the isotope distribution patterns calculated for [Fe II (L1-Qn) (CH 3 CN)] 2+ and {[Fe(L1-Qn)](ClO 4 )} + . In the optical spectrum of 6in acetonitrile, two charge-transfer bands at 375 nm (ε∼ 5800 M −1 cm −1 ) and 460 nm (ε∼5000 M −1 cm −1 ) are observed (Fig. S3, ESI†). These bands may be attributed to metal-toligand charge transfer (MLCT) transitions arising from electron transfer from low-spin iron(II)t 2g orbitals to the π* orbitals of the ligand along with ligand based transitions. 18 The 1 H NMR spectrum of 6in CD 3 CN at 298 K indicates the presence of a low spin iron(II) species in solution. However, variable temperature 1 H NMR experiments showed that some peaks shifted with variation of temperature (Fig. S4, ESI†). This effect was pronounced in the shifts of the diastereotopic methylene protons of the compound, which were found to display an anti-Curie behavior. 22 At 298 K, the 1 H NMR spectrum is first order, indicating a weakly coupled AX spin system, consistent with the presence of a paramagnetic iron ion. At 233 K, one set of methylene protons displays a nearly perfect AB quartet typical of a diamagnetic environment. These observations suggest partial population of the high spin state in solution at room temperature, as observed for 3(vide supra) and [Fe II (N2Py2B)(CH 3 CN)] 2+ . 16 The NMR spectra of these [Fe II (N5donor)(CH 3 CN)] 2+ complexes, in combination with other physical data and reactivity studies, allow us to gauge the relative effective ligand fields of the various related N5-donor ligands (vide infra). The spin state in the solid state was found to be low spin on the basis of its Mössbauer spectrum, which shows isomer shift (δ) and quadrupole splitting (ΔE Q ) values of 0.40 mm s −1 and 0.50 mm s −1 , respectively (Fig. S5, ESI†). Furthermore, the crystal structure of [Fe II (L1-Qn)(CH 3 CN)](BF 4 ) 2 (6·BF 4 ) (Fig. 3) is similar to that of its L2-Qn analogue 3. The complex is chiral but a racemic mixture is formed, and the crystal structure contains two diastereomers related by the inversion center in the monoclinic unit cell; the diastereomer depicted in Fig. 3 displays Sand Cchiralities 23 for the N1 and Fe1 stereogenic centres, respectively. The Fe–N bond distances vary in the range of 2.05–1.96 Å, and are in good agreement with 3and Fig. 3 A Mercury plot of the complex cation of 6·(BF 4 ) 2 with 30% thermal ellipsoids. All hydrogen atoms have been omitted for clarity. Dalton Transactions Paper This journal is © The Royal Society of Chemistry 2022 Dalton Trans. Open Access Article. Published on 07 January 2022. Downloaded on 1/11/2022 10:27:29 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online other reported low-spin iron(II) complexes supported by N4Pytype ligands. 16,17 The axial positions of the pseudo-octahedral ligand environment in 6are occupied by the amine nitrogen (N1) and the nitrogen atom (N6) of the solvent molecule with the N1–Fe1–N6 angle of 171.81(8)°. This deviation from linearity is slightly less than that observed for 3(vide supra), and consistent with the presence of only one sterically demanding quinoline unit in 6, as opposed to two quinoline entities in 3. A comparison of different Fe–N bond distances in 1–4and 6 with those of other related complexes is presented in Table 1. Generation of [Fe IV (O)(L2-Qn)] 2+ (7) Complex 3was reacted with excess (5 equiv.) solid IBX ester (isopropyl 2-iodoxybenzoate ester) in acetonitrile at room temperature to form the pale green cationic complex [Fe IV (O) (L2-Qn)] 2+ (7). The physical data for 7are identical to those determined by Rasheed et al., 10 and Mukherjee et al. 12 Further details on the formation and characterization of 7are found in the ESI.† Generation and characterization of [Fe IV (O)(L1-Qn)] 2+ (8) Given that the steric effect of L2-Qn results in the distorted ferryl complex 7with a resultant weak ligand field and high reactivity, we were interested in assessing whether the asymmetric ligand L1-Qn with only one quinoline donor moiety would exert similar steric influence and thus potentially stabilize an iron(IV)-oxo complex with similar reactivity as that of 7. Upon treatment of 6·(BF 4 ) 2 with three equivalents of IBX ester in acetonitrile at 298 K, a pale green intermediate is formed with a corresponding absorption band at 730 nm (Fig. 4) attributable to ligand field transition of the iron(IV) ion. A comparison of different iron(IV)-oxo species of the related pentadentate ligands is presented in Table 2. The ESI-MS spectrum (positive ion mode) of the green species displays ion peaks at m/z576.3 with the isotope distribution pattern calculated for [Fe(O)(L1-Qn)(BF 4 )] + . The ion peak shifts two mass units higher to m/z578.3 upon treatment with H 218 O with the incorporation of labelled oxygen (40% 18 O incorporation) supporting that the intermediate, [Fe(O)(L1-Qn)] 2+ (8), can exchange its oxygen atom with water (Fig. 4, inset). The half-life of the intermediate species was found to be 50 h at 298 K, demonstrating the relative thermal stability of the species. Complex 8·(BF 4 ) 2 could also be generated by treating an aqueous solution of 6·(BF 4 ) 2 with excess ceric ammonium nitrate (CAN). The iron(IV)-oxo complex thus generated was isolated in crystalline form using excess sodium perchlorate (see Experimental section). The resultant green species 8exhibited the characteristic near-IR absorption at 730 nm (375 M −1 cm −1 ) in acetonitrile. Considering the molar absorption of the complex, an approximate 91% yield of the complex is generated from 6in the reaction with IBX-ester in acetonitrile. Complex 8·(BF 4 ) 2 in CD 3 CN was found to exhibit paramagnetically shifted protons in the 1 H NMR spectrum, confirming the intermediate spin (S= 1) of the iron(IV) ion (Fig. S7, ESI†). The proton resonances of the pyridine and quinoline rings are assigned based on the spectra of previously reported iron(IV)- oxo complexes. 12,16,28 The oxidation state of the iron center of 8was confirmed by zero-field 57 Fe Mössbauer spectroscopy at 293 K. The complex exhibits an isomeric shift (δ) value of −0.05 mm s −1 , and a quadrupole splitting (ΔE Q ) value of 0.66 mm s −1 (Fig. 5). The Mössbauer parameters are quite similar to those reported for related N-ligated S= 1 iron(IV)-oxo complexes (Table 2). 12,19,20 The complex was further characterized by single crystal X-ray diffraction studies. The molecular structure of the cationic complex 8is shown in Fig. 6. Like 6, the complex is chiral with stereogenic centers at the N1 nitrogen and iron ion; two diastereomers are found in the unit cell of the complex. The iron center is coordinated by five nitrogen donors from the ligand and the sixth coordination site is occupied by an oxygen atom. The four pyridine nitrogen donors from the pentadentate ligand form the equatorial plane and the axial posiFig. 4 Formation of 8upon treatment of 6(0.5 mM) with IBX-ester (1.5 mM) in acetonitrile at 298 K; inset: ESI-mass spectrum (positive ion mode in acetonitrile) of (a) 8and (b) 8after treatment with H 218 O. Table 2 Optical spectral properties and stabilities of iron(IV)-oxo complexes ([Fe IV (O)(L))] 2+ supported by different pentadentate ligands based on the N4Py scaffold Ligand (L) λ max ,nm (ε,M −1 cm −1 ) t 1/2 at 298 K Mössbauer parameters Ref. δ (mm s −1 ) ΔE Q (mm s −1 ) L1-Qn 730 (340) 50 h −0.05 0.66 This work N4Py 695 (400) 60 h −0.04 0.93 26 N3Py-(NMB) 708 (400) 40 h −0.03 1.1 16 N2Py2B 725 (380) 2.5 h −0.02 1.34 16 L2-Qn 770 (380) 29 min 0.03 0.56 10, 11 and this work N4Py Me2 740 (220) 14 min 0.05 0.62 27 N2PyN2Py Me2 770 (200) 5.5 min ——17 and 25 Paper Dalton Transactions Dalton Trans. This journal is © The Royal Society of Chemistry 2022 Open Access Article. Published on 07 January 2022. Downloaded on 1/11/2022 10:27:29 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online tions are occupied by the amine nitrogen (N3) and the oxygen atom (O1) with the N3–Fe1–O1 angle of 174.2(2)° (Table 3). The tilt angle (5.8°) lies in between those observed in the iron (IV)-oxo complexes of the N2Py2B and L2-Qn ligand. The Fe1– O1 bond distance of 1.642(5) Å is very close to that reported for other crystallographically characterized S= 1 iron(IV)-oxo complexes. 10 Reactivity studies on [Fe IV (O)(L2-Qn)] 2+ (7) –oxidation of thioethers, alkenes and alkanes Considering the relative instability of [Fe IV (O)(L2-Qn)] 2+ (7), it was expected that 7would show a considerably enhanced reactivity relative to other ferryl complexes of N4Py and its derivatives. This has been confirmed by Rasheed et al., 10 who found that both the oxygen atom transfer (OAT) and the hydrogen atom transfer (HAT) reactivities of 7involve the highest rates of reaction that have thus far been established for a ferryl complex based on the N4Py ligand framework. We have completed a detailed study of the OAT and HAT reactivities of complex 7with an extended set of thioether, alkene and alkane substrates, as well as catalytic oxidation reactions effected by 3/7. These studies are described in detail in the ESI.† From our reactivity experiments on OAT and HAT reactions it is clear that oxo-complex 7reacts at faster rates with different hydrocarbon substrates with respect to previously reported complexes having similar ligand environments; a plot of corrected second-order rate constants for HAT vs. bond dissociation energies is found in Fig. 7 (vide infra). It is evident that the quinoline moiety, being a bulkier donor group, exerts steric effects that increase the average Fe–N bond length to more than 2 Å in the parent complex 3. Although the quinoline entity is a relatively strong donor, the weakened equatorial field that is a consequence of the steric effect results in higher HAT and OAT rates relative to other iron complexes containing ligands of the N4Py framework. 27 Considering the established high reactivity of 7and the steric nature of its origin, and the ability of its precursor 3to function as a (pre)catalyst for alkane and alkene oxidation, we Fig. 5 Zero-field Mössbauer spectrum of the solid sample of complex 8at 293 K. Fig. 6 A Mercury plot of the complex cation of 8with 40% thermal ellipsoids. All hydrogen atoms have been omitted for clarity. Table 3 Selected bond lengths (Å) and angles (°) for complex 8 Fe(1)–O(1) 1.642(5) Fe(1)–N(2) 1.966(5) Fe(1)–N(3) 2.053(6) Fe(1)–N(4) 1.981(5) Fe(1)–N(5) 2.025(5) Fe(1)–N(1) 1.984(6) O(1)–Fe(1)–N(3) 174.2(2) N(4)–Fe(1)–N(1) 164.1(2) N(2)–Fe(1)–N(5) 163.9(3) Fig. 7 Plot of log k’ 2 versus C–H BDEs of different substrates in the oxidation of alkanes by iron(IV)-oxo complexes of different N4Py derivatives. Dalton Transactions Paper This journal is © The Royal Society of Chemistry 2022 Dalton Trans. Open Access Article. Published on 07 January 2022. Downloaded on 1/11/2022 10:27:29 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online were interested in establishing (i) the optimum conditions for catalytic hydroxylation of hydrocarbons, using 3as a catalyst and hydrogen peroxide as the terminal oxidant (ii) to what extent the corresponding complexes of the monoquinoline ligand L1-Qn,i.e. [Fe II (L1-Qn)(CH 3 CN)] 2+ (6) and [Fe IV (O)(L1Qn)] 2+ (8), exhibit reactivities similar to those of 3and 7, and whether their reactivities can be related to steric factors. Multivariate analysis of catalytic cyclohexane oxidation using [Fe II (L2-Qn)(NCMe)](ClO 4 ) 2 (3) as catalyst and H 2 O 2 as oxidant The optimization of experimental conditions using multivariate statistical approaches such as the Box–Behnken design, a response surface methodology, can be used to study several variables of interest simultaneously. 29 This mathematical approach, besides reducing the number of experiments to be performed, generates mathematical models and allows the evaluation of the significance of each factor under study and the interactions between them. 30 As it had been demonstrated spectroscopically that complex 3can generate the Fe IV O species 7, and that 3is catalytically active upon oxidation of organic substrates (vide supra and ESI†), we decided to pursue an optimization of the variables affecting conversion of cyclohexane into cyclohexanol and cyclohexanone. A Box–Behnken experimental design according to Table S5 (ESI†) was performed which generated response surfaces depicted in Fig. S9– S14 (ESI).†Analysis of the response surfaces allowed us to summarize the optimized conditions for the conversion of cyclohexane into cyclohexanol and cyclohexanone, as well as the percent selectivity for the formation of cyclohexanol (Table 4). The amount of catalyst is the most significant difference between the conversions of cyclohexane (CyH) into cyclohexanol (CyOH) or cyclohexanone (CyO) (Fig. S9 and S10, ESI†). This implicates the catalyst as an active species in the initial step to form CyOH as well as in (over)oxidation of CyOH into CyO. Another significant difference can be observed by the strong interaction between the amount of catalyst and peroxide (Fig. S10,†middle and bottom). At lower peroxide concentration, the increase in catalyst concentration does not lead to any significant increase in the amount of CyO formed. On the other hand, at the highest level of peroxide tested, there is a significant increase in CyO formation as the amount of catalyst is increased. This corroborates that the species formed by the interaction between catalyst and peroxide is crucial for the formation of the products, especially CyO. Finally, the results in terms of percent conversion of CyOH, CyO and the percent selectivity for the formation of CyOH can be found in Table 5. Reactivity studies on [Fe IV (O)(L1-Qn)] 2+ (8) Hydrogen atom transfer. The HAT activity of 8was examined in the oxidation of hydrocarbons having C–H bond dissociation energies ranging from 81 to 99.3 kcal mol −1 . Under pseudo-first-order reaction conditions, the characteristic 730 nm band (vide supra) was found to decay rapidly in the presence of triphenylmethane, ethylbenzene, toluene, 2,3dimethyl butane and cyclohexane. The second-order rate constant (k 2 ) values (cf. Fig. S15, ESI†) are listed in Table 6. The k 2 value for cyclohexane oxidation by 8is almost 20 times higher than that of [Fe IV (O)(N4Py)] 2+ and three times higher than that of [Fe IV (O)(N3Py-(NMB))] 2+ complex. A comparison of rate constants for alkane oxidation by similar complexes is shown in Table 6. Complex 8is efficient in the C–H bond cleavage of aliphatic substrates and shows a linear correlation between log k 2 ′and the C–H BDE of the substrates (vide infra,k 2 ′is the second order rate constant corrected for number of “abstractable” hydrogens). The primary kinetic isotope effect (KIE) for the oxidation of toluene and toluene-d 8 was found to be ≈19, a value supporting that of a metal-based oxidant (Fig. S16, ESI†). This KIE value is very similar to that of the N4Py system, but higher than the value observed with the iron(IV)-oxo complexes of the N4Py Me2 and N3Py-(NMB) ligands. 16,17 Such a large KIE value implies a tunneling mechanism in H atom abstraction, as has been proposed for C–H bond activation by Fe IV vO species. The linear correlation of k 2 ′and the observed large kinetic isotope effect indicate that the reactions of 8with the substrates take place via the transfer of a hydrogen atom in a rate-determining step, as observed for other related Fe(IV)-oxo complexes. 31,32 The oxidation of cyclohexane by 8affords cyclohexanol and cyclohexanone in 12% (TON = 0.37) and 6% (TON = 0.19) yield, respectively, with an alcohol/ketone ratio (A/K) of 1.9. It may be noted that a very poor A/K ratio (0.5) for cyclohexane oxidation was obtained by the [Fe IV (O)(N4Py)] 2+ complex. Furthermore, 8was found to oxidize adamantane with a C3/C2 normalized selectivity of 15.5. The reactivities of 7and 8in HAT reactions are compared to those of [Fe IV (O)(N4Py)] 2+ and [Fe IV (O)(N2Py2B)] 2+ in Fig. 7. Table 4 Summary of optimized conditions found by multivariate analysis for the formation of cyclohexane, cyclohexanone, and cyclohexanol selectivity, for the catalytic oxidation of cyclohexane by hydrogen peroxide promoted by [Fe II (L2-Qn)(NCMe)] 2+ (3) Catalyst, mol% Hydrogen peroxide a Reaction time (h) Cyclohexanol (CyOH) 1.55–3.00 5 2–6 Cyclohexanone (CyO) 3.00 5 2–6 Selectivity CyOH 0.1 0.2 2–6 a Molar ratio of excess in relation to cyclohexane, used as substrate. Table 5 Maximized conversion values obtained for cyclohexanol and cyclohexanone on basis of previously optimized conditions (cf. Table 4) Optimized condition % CyOH % CyO %Selectivity for CyOH a Cyclohexanol (CyOH) 14 7 66 Cyclohexanone (CyO) 10 12 45 Selectivity CyOH 3 0.2 90 a The selectivity for CyOH was calculated by 100×%CyOH/(%CyOH + %CyO). Paper Dalton Transactions Dalton Trans. 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