New Trendy Magnetic C-Scorpionate Iron Catalyst and Its Performance towards Cyclohexane Oxidation

For the first time, a magnetic C-scorpionate catalyst was prepared from the iron(II) complex [FeCl2{κ-HC(pz)3}] (pz = pyrazol-1-yl) and ferrite, using the sustainable mechanochemical synthetic procedure. Its catalytic activity for the cyclohexane oxidation with tert-butyl hydroperoxide (TBHP) was evaluated in different conditions, namely under microwave irradiation and under the effect of an external magnetic field. The use of such magnetic conditions significantly shifted the catalyst alcohol/ketone selectivity, thus revealing a promising, easy new protocol for tuning selectivity in important catalytic processes.


Introduction
The recovery and reuse of catalysts is still a challenge in the development of sustainable chemical processes.Loading the catalyst onto a magnetic material has recently emerged as a new synthetic procedure addressing this problem with significance in green chemistry [1][2][3][4][5][6].
The catalyst preparation is itself a very important issue to consider in the design of sustainable processes.In this regard, mechanochemistry, particularly dry ball milling synthesis, is appealing as it eliminates the need for solvents and drastically reduces the energy input [7][8][9].
Among the current relevant industrial oxidative processes, the well-established homogeneous cyclohexane oxidation, as part of the large scale (~4 million ton/year) Nylon 6 production, presents several drawbacks related to the catalyst selectivity (which is only able to generate 5-12% yields to assure a selectivity of ca.80-85% [10]) and requires urgent improvement [10][11][12].
Therefore, in pursuit of our interest in the sustainable oxidation of alkanes, the main objectives of the present study consisted of the development of an improved catalytic process for the oxidation of cyclohexane by taking advantage of the unique properties of the magnetized C-scorpionate iron(II) catalyst (e.g., its recyclability and reuse by easy magnetic separation) prepared by an eco-friendly route.Moreover, we also aimed at an evaluation of the effect of an external magnetic field on the catalytic performance of our new magnetic material.
To our knowledge, this is the first time that the successful preparation of a magnetic scorpionate catalyst and the use of an external magnetic field for tuning the oxidation of alkanes has been reported.
Three samples were analysed by XPS: The C-scorpionate iron complex ([FeCl 2 {κ 3 -HC(pz) 3 }], 1); the ferrite sample (Fe 3 O 4 ); and the mixture [FeCl 2 {κ 3 -HC(pz) 3 }] + Fe 3 O 4 (1:1, wt.%, 2). Figure 1 shows the photoelectron C 1s and Fe 2p XPS regions and the valence band of the mixture compared with the sum of [FeCl 2 {κ 3 -HC(pz) 3 }] and Fe 3 O 4 valence bands. of cyclohexane by taking advantage of the unique properties of the magnetized C-scorpionate iron(II) catalyst (e.g., its recyclability and reuse by easy magnetic separation) prepared by an eco-friendly route.Moreover, we also aimed at an evaluation of the effect of an external magnetic field on the catalytic performance of our new magnetic material.
To our knowledge, this is the first time that the successful preparation of a magnetic scorpionate catalyst and the use of an external magnetic field for tuning the oxidation of alkanes has been reported.
Three samples were analysed by XPS: The C-scorpionate iron complex ([FeCl2{κ 3 -HC(pz)3}], 1); the ferrite sample (Fe3O4); and the mixture [FeCl2{κ 3 -HC(pz)3}] + Fe3O4 (1:1, wt.%, 2). Figure 1   The iron(II) complex C 1s region (Figure 1a) was fitted with two main peaks, assigned to sp 2 carbon atoms (-C=) and carbon singly bound to nitrogen (C-N), plus two extra peaks from some aliphatic carbon (grey curve) and one peak centered at higher binding energies, characteristic of carbon atoms in very electronegative neighborhoods such as carboxyl and/or carboxylate groups.After mixing with Fe3O4, C 1s fitting is qualitatively very similar to that of the complex [FeCl2{κ 3 -HC(pz)3}], although the relative amount of aliphatic carbon has increased.Such carbonaceous contamination mainly results from Fe3O4, as attested by the carbon detected in the ferrite sample.Fe 2p regions (Figure 1b) were fitted with several doublets, but only the most intense are shown.Centered at a lower binding energy, the complex 1 has a component, Fe 2p3/2, centered at 709.4 ± 0.2 eV, which is typical of Fe 2+ .As expected, this component binding energy increases in the presence of Fe3O4 to 710.7 ± 0.2 eV, attesting that Fe 2p is a sum of Fe 2+ and Fe 3+ ions [21].Fe 2p3/2 in Fe3O4 is centered at 711.5 ± 0.2 eV.Roughly between 715 and 723 eV and above 729 eV, multiplet structures arising from spin-spin coupling (between unpaired core electron and the unpaired electrons in the outer shell) are detected.
XPS analysis shows that the as-synthesised iron(II) complex [FeCl2{κ 3 -HC(pz)3}] has the expected Fe/N ratio (0.17, Table 1); however, the atomic ratio N/Cl = 4.0 (against 3.0 for the anticipated stoichiometry) is compatible with some chloride loss or chloride atoms shared between complexes.In fact, from the quantitative analysis of C 1s fitted peaks, the experimental Csp 2 /C-N atomic ratio (computed from the areas of the main peaks identified in Figure 1a) is close to the one predicted for the as-synthesized [FeCl2{κ 3 -HC(pz)3}] (~1.2), which means that the detected O-C=O groups must be from carboxylated iron, which could promote some dimerization.
When mixed with Fe3O4, complex 1 seems to lose nitrogen atoms, as shown by the ratio N/Cl (N and Cl only exist in the organometallic compound) <3.0.Also, the large ratio C/N = 2.7 quantified in the mixture is compatible with nitrogen depletion, although the formation of some carbonaceous contamination after milling cannot be discarded, as attested by C 1s regions of Fe3O4 and the mixture (Figure 1a).Moreover, when comparing the mixture valence band (VB) with the sum of individual valence bands (VBComplex + VBFe3O4) (Figure 1c), it is clear that the shoulders detected in the sum spectrum, around 18 eV and 13 eV assigned to N 2s and C-N, respectively, have a lower intensity in the mixture, favouring the nitrogen depletion hypothesis.The iron(II) complex C 1s region (Figure 1a) was fitted with two main peaks, assigned to sp 2 carbon atoms (-C=) and carbon singly bound to nitrogen (C-N), plus two extra peaks from some aliphatic carbon (grey curve) and one peak centered at higher binding energies, characteristic of carbon atoms in very electronegative neighborhoods such as carboxyl and/or carboxylate groups.After mixing with Fe 3 O 4 , C 1s fitting is qualitatively very similar to that of the complex [FeCl 2 {κ 3 -HC(pz) 3 }], although the relative amount of aliphatic carbon has increased.Such carbonaceous contamination mainly results from Fe 3 O 4 , as attested by the carbon detected in the ferrite sample.Fe 2p regions (Figure 1b) were fitted with several doublets, but only the most intense are shown.Centered at a lower binding energy, the complex 1 has a component, Fe 2p 3/2 , centered at 709.4 ± 0.2 eV, which is typical of Fe 2+ .As expected, this component binding energy increases in the presence of Fe 3 O 4 to 710.7 ± 0.2 eV, attesting that Fe 2p is a sum of Fe 2+ and Fe 3+ ions [21].Fe 2p 3/2 in Fe 3 O 4 is centered at 711.5 ± 0.2 eV.Roughly between 715 and 723 eV and above 729 eV, multiplet structures arising from spin-spin coupling (between unpaired core electron and the unpaired electrons in the outer shell) are detected.
XPS analysis shows that the as-synthesised iron(II) complex [FeCl 2 {κ 3 -HC(pz) 3 }] has the expected Fe/N ratio (0.17, Table 1); however, the atomic ratio N/Cl = 4.0 (against 3.0 for the anticipated stoichiometry) is compatible with some chloride loss or chloride atoms shared between complexes.In fact, from the quantitative analysis of C 1s fitted peaks, the experimental C sp 2 /C-N atomic ratio (computed from the areas of the main peaks identified in Figure 1a) is close to the one predicted for the as-synthesized [FeCl 2 {κ 3 -HC(pz) 3 }] (~1.2), which means that the detected O-C=O groups must be from carboxylated iron, which could promote some dimerization.When mixed with Fe 3 O 4 , complex 1 seems to lose nitrogen atoms, as shown by the ratio N/Cl (N and Cl only exist in the organometallic compound) <3.0.Also, the large ratio C/N = 2.7 quantified in the mixture is compatible with nitrogen depletion, although the formation of some carbonaceous contamination after milling cannot be discarded, as attested by C 1s regions of Fe 3 O 4 and the mixture (Figure 1a).Moreover, when comparing the mixture valence band (VB) with the sum of individual valence bands (VB Complex + VB Fe3O4 ) (Figure 1c), it is clear that the shoulders detected in the sum spectrum, around 18 eV and 13 eV assigned to N 2s and C-N, respectively, have a lower intensity in the mixture, favouring the nitrogen depletion hypothesis.In order to evaluate the morphology of the [FeCl 2 {κ 3 -HC(pz) 3 }] + Fe 3 O 4 (1:1) material ( 2), characterization by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) was performed (Figure S1, Supplementary Materials).The mixture exhibits a similar morphology to the [FeCl 2 {κ 3 -HC(pz) 3 }] before ball milling.Although the milling speed (510 rpm) is already considered high power ball milling, the duration of the mixing (1 h) was not enough to dramatically decrease the size of the compounds.The mixture of both compounds is observed in both images (Figure S1a,b), confirming that ball milling is an efficient method for performing solvent-free mixtures.
The thermal stability of mixture 2 was evaluated by TGA (Figure S2, Supplementary Materials).The weight loss percentage is insignificant in the temperature range to be used in the catalytic experiments.Moreover, as expected, at the end of the analysis, the mixture maintained its magnetic character.
The magnetic mixture 2 was used as a catalyst for the oxidation of cyclohexane to KA oil [cyclohexanol (A, CyOH) and cyclohexanone (K, CyO)], with TBHP (70% aqueous solution) in the conventional solvent CH 3 CN (Scheme 1).Two different protocols were used (i) microwave (MW) assisted and (ii) under the effect of an external magnetic field.In order to evaluate the morphology of the [FeCl2{κ 3 -HC(pz)3}] + Fe3O4 (1:1) material ( 2), characterization by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) was performed (Figure S1, Supplementary Materials).The mixture exhibits a similar morphology to the [FeCl2{κ 3 -HC(pz)3}] before ball milling.Although the milling speed (510 rpm) is already considered high power ball milling, the duration of the mixing (1 h) was not enough to dramatically decrease the size of the compounds.The mixture of both compounds is observed in both images (Figure S1a,b), confirming that ball milling is an efficient method for performing solvent-free mixtures.
The thermal stability of mixture 2 was evaluated by TGA (Figure S2, Supplementary Materials).The weight loss percentage is insignificant in the temperature range to be used in the catalytic experiments.Moreover, as expected, at the end of the analysis, the mixture maintained its magnetic character.
The magnetic mixture 2 was used as a catalyst for the oxidation of cyclohexane to KA oil [cyclohexanol (A, CyOH) and cyclohexanone (K, CyO)], with TBHP (70% aqueous solution) in the conventional solvent CH3CN (Scheme 1).Two different protocols were used (i) microwave (MW) assisted and (ii) under the effect of an external magnetic field.

(i) Microwave (MW) assisted oxidation of cyclohexane
The performance of the magnetic C-scorpionate catalyst 2 was evaluated in terms of products yield, turnover number (TON) or frequency (TOF, h −1 ), and selectivity as a function of the reaction time and the presence of a co-catalyst (e.g., pyrazinecarboxylic acid, Hpca) (Table 2).
The best catalytic conditions were found when using 2, the oxidant tert-butyl hydroperoxide (t-BuOOH, TBHP, 70% aqueous solution) and the solvent acetonitrile (MeCN), in acidic medium (pyrazine carboxylic acid, Hpca) and under microwave (MW) irradiation at 50 °C during 6 h (Table 2).The use of different acidic additives (entries 5 and 6, Table 2) or of hydrogen peroxide (entry 12, Table 2) instead of TBHP did not improve the catalytic system.Importantly, KA oil (cyclohexanol and cyclohexanone mixture) was the only product detected by GC analysis, revealing the high selectivity of the catalytic oxidative system under the above reaction conditions.
The yield values achieved (30%, entry 3, Table 2) with the magnetic mixture 2 are considerably higher than those provided by the current industrial process (homogeneous cobalt catalyst, dioxygen at 150 °C [10]) and comparable to those previously obtained when 1 was submitted to microwave irradiation (total yield of 33%) in the presence of Hpca [22].The ferrite (Fe3O4) or the FeCl2•2H2O salt

(i) Microwave (MW) assisted oxidation of cyclohexane
The performance of the magnetic C-scorpionate catalyst 2 was evaluated in terms of products yield, turnover number (TON) or frequency (TOF, h −1 ), and selectivity as a function of the reaction time and the presence of a co-catalyst (e.g., pyrazinecarboxylic acid, Hpca) (Table 2).
The best catalytic conditions were found when using 2, the oxidant tert-butyl hydroperoxide (t-BuOOH, TBHP, 70% aqueous solution) and the solvent acetonitrile (MeCN), in acidic medium (pyrazine carboxylic acid, Hpca) and under microwave (MW) irradiation at 50 • C during 6 h (Table 2).The use of different acidic additives (entries 5 and 6, Table 2) or of hydrogen peroxide (entry 12, Table 2) instead of TBHP did not improve the catalytic system.Importantly, KA oil (cyclohexanol and cyclohexanone mixture) was the only product detected by GC analysis, revealing the high selectivity of the catalytic oxidative system under the above reaction conditions.
The yield values achieved (30%, entry 3, Table 2) with the magnetic mixture 2 are considerably higher than those provided by the current industrial process (homogeneous cobalt catalyst, dioxygen at 150 • C [10]) and comparable to those previously obtained when 1 was submitted to microwave irradiation (total yield of 33%) in the presence of Hpca [22].The ferrite (Fe 3 O 4 ) or the FeCl 2 •2H 2 O salt exhibited significantly lower catalytic activity (entries 9 and 14, respectively, Table 2) than mixture 2, suggesting the favorable involvement of the scorpionate ligands in the metal-assisted steps [13] (Equations ( 1) and ( 2)) of this oxidation reaction that led to the radical species (e.g., t BuO • ) able to generate the cyclohexyl radical by H-abstraction.Control experiments in the absence of 2, with t-BuOOH, confirm the crucial role of 2 to efficiently catalyse the oxidation of cyclohexane.
(ii) Cyclohexane oxidation under the effect of an external magnetic field The performance of the magnetic C-scorpionate catalyst 2 was also tested using the above reaction conditions but under an external magnetic field [neodymium magnet(s)] at 35 • C, instead of the MW irradiation (Figure 2).
As shown in Table 3, with this new optimized procedure (magnetic 2 under an external magnetic field), 100% selectivity towards cyclohexanol formation is obtained (entries 1-6, Table 3).Only for longer reaction times, the incipient formation of cyclohexanone is detected (entry 3, Table 3).However, the characteristics (e.g., magnetization, strength, or configuration) of the applied magnetic field can significantly influence the alcohol/ketone selectivity (compare entries 3, 7, and 8, Table 3, corresponding to the different external NdFeB magnetic fields depicted in Figure 2a-c, respectively).Such effects are now the target of our further studies.Nevertheless, the selectivity outcome from the reaction performed under the same conditions except not submitted to an external magnetic field is completely different (entry 9, Table 3), leading to a ca.1:1 cyclohexanol:cyclohexanone mixture.As shown in Table 3, with this new optimized procedure (magnetic 2 under an external magnetic field), 100% selectivity towards cyclohexanol formation is obtained (entries 1-6, Table 3).Only for longer reaction times, the incipient formation of cyclohexanone is detected (entry 3, Table 3).However, the characteristics (e.g., magnetization, strength, or configuration) of the applied magnetic field can significantly influence the alcohol/ketone selectivity (compare entries 3, 7, and 8, Table 3, corresponding to the different external NdFeB magnetic fields depicted in Figure 2a-c, respectively).Such effects are now the target of our further studies.Nevertheless, the selectivity outcome from the reaction performed under the same conditions except not submitted to an external magnetic field is completely different (entry 9, Table 3), leading to a ca.1:1 cyclohexanol:cyclohexanone mixture.Recently, the possibility of tuning the selectivity towards alcohol or ketone formation by choosing the appropriate solvent was previously found [22,24] for 1: in supercritical CO 2 and in the ionic liquids [bmim]X [X = N(CN) 2 , [BF 4 ], [PF 6 ] or FAP] (bmim = 1-butyl-3-methylimidazolium; FAP = tris(pentafluoroethyl)trifluorophosphate).Such solvent selectivity was also found for the cyclohexane oxidation catalyzed by ferrocene [25] or a Cu(II) complex [26] in some of the above ionic liquids.However, to date, there is no report concerning the application of an external magnetic field in a procedure for the selective oxidation of cyclohexane.
The use of an external field to tune selectivity is a new concept than can potentially change how the catalysts are perceived, since the orientation of a catalyst is not a usual parameter in the optimization of catalytic oxidative conditions.
Another advantage of our magnetic catalytic system is the easy separation of the catalyst at the end of the reaction [using either method (i) or (ii)] by the easy and convenient application of an external magnetic field, and its recyclability.
The stability of 2 was tested in terms of its re-usability in consecutive cycles (see Section 3.3) for both oxidative methods (microwave-assisted and under an external magnetic field) and is depicted in Figure 3.
of Figure 2c.i Without an external magnetic field.
Recently, the possibility of tuning the selectivity towards alcohol or ketone formation by choosing the appropriate solvent was previously found [22,24] for 1: in supercritical CO2 and in the ionic liquids [bmim]X [X = N(CN)2, [BF4], [PF6] or FAP] (bmim = 1-butyl-3-methylimidazolium; FAP = tris(pentafluoroethyl)trifluorophosphate).Such solvent selectivity was also found for the cyclohexane oxidation catalyzed by ferrocene [25] or a Cu(II) complex [26] in some of the above ionic liquids.However, to date, there is no report concerning the application of an external magnetic field in a procedure for the selective oxidation of cyclohexane.
The use of an external field to tune selectivity is a new concept than can potentially change how the catalysts are perceived, since the orientation of a catalyst is not a usual parameter in the optimization of catalytic oxidative conditions.
Another advantage of our magnetic catalytic system is the easy separation of the catalyst at the end of the reaction [using either method (i) or (ii)] by the easy and convenient application of an external magnetic field, and its recyclability.
The stability of 2 was tested in terms of its re-usability in consecutive cycles (see Section 3.3) for both oxidative methods (microwave-assisted and under an external magnetic field) and is depicted in Figure 3.Under microwave (MW) irradiation at 50 °C during 6 h, the recycling studies showed a loss of 40% in the activity relative to the first run, which is probably due to mixture decomposition under the oxidant conditions used.Under the external magnetic field, similar behavior is observed, but the loss in activity is smaller (17%).The UV/vis analysis of the corresponding supernatant phases (using MW irradiation or a magnetic field) after the first cycle revealed leaching of 2 in both catalytic methods, being more extensive for the first one.This could account for the observed loss of activity in the subsequent recycling cycles (Figure 3).

Materials and Methods
All the reagents and solvents were purchased from Sigma-Aldrich (Munich, Germany) and used as received.The scorpionate complex 1 [18] and the magnetic ferrite [27] were synthesized and characterized according to methods described in the literature.Under microwave (MW) irradiation at 50 • C during 6 h, the recycling studies showed a loss of 40% in the activity relative to the first run, which is probably due to mixture decomposition under the oxidant conditions used.Under the external magnetic field, similar behavior is observed, but the loss in activity is smaller (17%).The UV/vis analysis of the corresponding supernatant phases (using MW irradiation or a magnetic field) after the first cycle revealed leaching of 2 in both catalytic methods, being more extensive for the first one.This could account for the observed loss of activity in the subsequent recycling cycles (Figure 3).

Materials and Methods
All the reagents and solvents were purchased from Sigma-Aldrich (Munich, Germany) and used as received.The scorpionate complex 1 [18] and the magnetic ferrite [27] were synthesized and characterized according to methods described in the literature.

Experimental Section
TEM measurements were performed on a Transmission Electron Microscope Hitachi 8100 with a ThermoNoran light elements EDX detector and digital image acquisition (Hitachi, Tokyo, Japan).Morphology and distribution of metal composites were characterized using a scanning electron microscope (SEM, EDS) JEOL 7001F with an Oxford light elements EDX detector and EBSD detector (JEOL, Tokyo, Japan).
A dual anode XSAM800 spectrometer was used for X-ray Photoelectron Spectroscopy (XPS) characterization (KRATOS, Manchester, UK).Spectra were obtained using non-monochromatic Al Kα X-radiation (hν = 1486.6eV) produced with a power of 120 W, in FAT mode, high magnification, pass energy of 20 eV, and at TOA = 45 • .Data collection and processing details are described elsewhere [28].The charge shifts were corrected using the aliphatic carbon binding energy centered at 285 eV as the reference.A fine readjustment of the mixture and ferrite charge shift corrections was performed by matching O 2s and C 2s features.The sensitivity factors were: 0.278 for C 1s, 0.78 for O 1s, 0.891 for Cl 2p, 0.477 for N 1s, and 2.957 for Fe 2p.
The microwave used is an Anton Paar, monowave 300, with a G10 borosilicate glass reactor (Anton Paar GmbH, Graz, Austria).
The UV/vis analyses were performed in a Lambda 25, Perkin Elmer, with quartz cells with a 1 cm path length (PerkinElmer, Inc., Waltham, MA, USA).

Preparation of the Dispersed Catalyst
During the dry milling treatment, the [FeCl 2 {κ 3 -HC(pz) 3 }] (pz = pyrazol-1-yl) and the magnetic ferrite (ratio 1:1) were mixed mechanically, during 1 h, in a Retsch PM100/200 planetary ball mill, equipped with a 50 mL grinding bowl and 10 stainless steel balls of a 10 mm size.The rotational speed was 450 rpm, with rotational inversions every 5 min.All the mixtures were prepared in the absence of any added solvent (dry milling).After, the produced mixture was left under N 2 , at 200 • C, for 12 h, to eliminate the possible presence of CO 2 and water from the mixture.

Procedure for the Catalytic Oxidations
The microwave-assisted cyclohexane oxidation using CH 3 CN as a solvent (up to 5 mL total volume) was performed in a glass reactor (i.e., a Pyrex tube).The catalyst was added to the solvent as a solid in the reaction medium.Cyclohexane (5 mmol) was then introduced, and the reaction started when 70% TBHP aqueous solution as the oxidant (10 mmol) was added in one portion and the tube was sealed.The tube was placed in the microwave reactor and the system was left under stirring and under MW-irradiation (5 W), at 50 • C for 1-6 h.The reaction was stopped after the desired time and 5 mL of diethyl ether was added for the extraction of the organic products.
The reaction mixtures were analyzed by gas chromatography (GC) upon the addition of an excess of triphenylphosphine to reduce cyclohexyl hydroperoxide to cyclohexanol, following a method developed by Shul'pin [29].Nitromethane (0.05 mL) was used as an internal standard.Chromatographic measurements were performed in a Fisons Instruments GC 8000 series gas chromatograph with a BP20/SGE (30 m × 0.22 mm × 0.25 mm) capillary column (flame ionization detector (FID) detector) and Jasco-Borwin v.1.50software (Jasco, Tokyo, Japan), using helium as a carrier gas.All products formed were identified and their retention times were confirmed with those of commercially available samples.Helium was used as the carrier gas.Blank experiments in CH 3 CN were performed and it was confirmed that no cyclohexane oxidation products (or only traces, below 1%) were obtained in the absence of the iron catalyst.
The cyclohexane oxidation under an external magnetic field using CH 3 CN as the solvent (up to 5 mL total volume) was performed in a common beaker.The catalyst was added to the solvent as a solid in the reaction medium.Cyclohexane (5 mmol) was then introduced, and the reaction started when 70% TBHP aqueous solution as the oxidant (10 mmol) was added in one portion.The beaker was placed in the magnetic field (see Figure 2) and the system was left under mechanical stirring at

Figure 2 .
Figure 2. Cyclohexane oxidation reaction performed under different external NdFeB magnetic fields.(a) One magnetic block N40 grade; (b) Two magnetic blocks N42 grade; (c) Two magnetic blocks N42 grade, a magnetic cube N42 grade, and a magnetic cube N48 grade.

Figure 2 .
Figure 2. Cyclohexane oxidation reaction performed under different external NdFeB magnetic fields.(a) One magnetic block N40 grade; (b) Two magnetic blocks N42 grade; (c) Two magnetic blocks N42 grade, a magnetic cube N42 grade, and a magnetic cube N48 grade.