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New Trendy Magnetic C-Scorpionate Iron Catalyst and Its Performance towards Cyclohexane Oxidation Ana P. C. Ribeiro 1, * ID , Inês A. S. Matias 1 ID , Elisabete C. B. A. Alegria 1,2 ID , Ana M. Ferraria 3 , Ana M. Botelho do Rego 3 , Armando J. L. Pombeiro 1 and Luísa M. D. R. S. Martins 1, * ID 1

2 3

*

Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001 Lisboa, Portugal; [email protected] (I.A.S.M.); [email protected] (E.C.B.A.A.); [email protected] (A.J.L.P.) Departamento de Engenharia Química, Instituto Superior de Engenharia de Lisboa, Instituto Politécnico de Lisboa, R. Conselheiro Emídio Navarro, 1959-007 Lisboa, Portugal Centro de Química-Física Molecular and Institute of Nanoscience and Nanotechnology, DEQ, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001 Lisboa, Portugal; [email protected] (A.M.F.); [email protected] (A.M.B.d.R.) Correspondence: [email protected] (A.P.C.R.); [email protected] (L.M.D.R.S.M.); Tel.: +351-218419264 (L.M.D.R.S.M.)

Received: 23 January 2018; Accepted: 4 February 2018; Published: 8 February 2018

Abstract: For the first time, a magnetic C-scorpionate catalyst was prepared from the iron(II) complex [FeCl2 {κ3 -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. Keywords: magnetic; C-scorpionate; catalyst; oxidation; selectivity; mechanochemical; cyclohexane; iron

1. 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 [1–6]. 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 [7–9]. 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 [10–12]. Within the known homogeneous catalysts for alkanes oxidation, metallic complexes with C-scorpionate tris(pyrazol-1-yl)methane ligands have gained significant importance in the last years [13–17]. The bio-inspired tris(pyrazol-1-yl)methane iron(II) complex [FeCl2 {κ3 -HC(pz)3 }], which was previously proved to act as an efficient catalyst in C–H activation reactions [13–20], was also chosen due to its easy (one-step) synthesis in water at room temperature [18]. 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) Catalysts 2018, 8, 69; doi:10.3390/catal8020069

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Catalysts 2018, 8, 69 2 of 11 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  catalyst (e.g., its recyclability and reuse by easy magnetic separation) prepared by an eco-friendly route. catalytic performance of our new magnetic material.  Moreover, we also aimed at an evaluation of the effect of an external magnetic field on the catalytic To our knowledge, this is the first time that the successful preparation of a magnetic scorpionate  performance of our new magnetic material. catalyst  and  the  use  of this an  is external  for  tuning  the  oxidation  of  alkanes  has  been  To our knowledge, the firstmagnetic  time that field  the successful preparation of a magnetic scorpionate reported.  catalyst and the use of an external magnetic field for tuning the oxidation of alkanes has been reported.

2. Results and Discussion  2. Results and Discussion The iron(II) complex [FeCl The iron(II) complex [FeCl22{κ33‐HC(pz) -HC(pz)33}] (1, pz = pyrazol‐1‐yl) was synthesized according to a  }] (1, pz = pyrazol-1-yl) was synthesized according to a reported procedure [18] and characterized by spectroscopic and analytic techniques.  reported procedure [18] and characterized by spectroscopic and analytic techniques. The new magnetic iron catalyst (2) was prepared by dry milling treatment of [FeCl {κ33-HC(pz) ‐HC(pz)33}]  The new magnetic iron catalyst (2) was prepared by dry milling treatment of [FeCl22{κ }] and ferrite (1:1 mass ratio) at room temperature (see Section 3.1).  and ferrite (1:1 mass ratio) at room temperature (see Section 3.1). Three samples were analysed by XPS: The C‐scorpionate iron complex ([FeCl Three samples were analysed by XPS: The C-scorpionate iron complex ([FeCl22{κ33‐HC(pz) -HC(pz)33}], 1);  }], 1); 3 3 the ferrite sample (Fe O44); and the mixture [FeCl {κ -HC(pz) ‐HC(pz)33}] }] + Fe the ferrite sample (Fe33O ); and the mixture [FeCl22{κ + Fe33O4 4(1:1, wt. %, 2). Figure 1 shows  (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 photoelectron C 1s and Fe 2p XPS regions and the valence band of the mixture compared with the {κ3‐HC(pz) 3}] and Fe 4 valence bands.  the sum of [FeCl sum of [FeCl2 {κ32-HC(pz) Fe3 O43Ovalence bands. 3 }] and

C 1s

Fe 2p

10400

5400

Fe3+ + Fe2+

Fe3O4

8400

-C=

3400

O-C=O 2400

Complex

Intensity (c.p.s.)

C-N

4400

Intensity (c.p.s.)

9400

Fe3O4

7400

Fe2+

6400 5400 4400

Complex Mixture

3400

1400

Mixture

2400

Fe 2p1/2 Fe 2p3/2

1400

400

292

289 286 283 Binding Energy (eV)

280

(a) 

734 724 714 Binding Energy (eV) (b)

Figure 1. Cont.

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240

O 2s

Valence Band

C 2s

Cl 3p, Fe 3d and O 2p

CH

190

Intensity (c.p.s.)

N 2s 140

C-N

90

40

Mixture Sum(Complex+Fe3O4)

-10 40

36

32

28

24

20

16

12

Binding Energy (eV)

8

4

0

-4

 

(c) Figure 1. XPS regions (a) C 1s and (b) Fe 2p of (from top to bottom): Fe O4, [FeCl 2{κ3‐HC(pz) 3}] (1) and  3 Figure 1. XPS regions (a) C 1s and (b) Fe 2p of (from top to bottom): 3Fe 3 O4 , [FeCl2 {κ -HC(pz)3 }] (1) their mixture (2). (c) Comparison of the mixture valence band with the sum of [FeCl 2{κ3‐HC(pz) 3}] and  3 and their mixture (2). (c) Comparison of the mixture valence band with the sum of [FeCl2 {κ -HC(pz) 3 }] Fe 3O4 valence bands.  and Fe O valence bands. 3

4

2  The iron(II) complex C 1s region (Figure 1a) was fitted with two main peaks, assigned to sp The iron(II) complex C 1s region (Figure 1a) was fitted with two main peaks, assigned to sp2 carbon carbon  atoms  (–C=)  and  carbon  singly  bound  to  nitrogen  (C–N),  plus  two  extra  peaks  from  some  atoms (–C=) and carbon singly bound to nitrogen (C–N), plus two extra peaks from some aliphatic aliphatic  carbon  (grey  curve)  and  one  peak  centered  at  higher  binding  energies,  characteristic  of  carbon (grey curve) and one peak centered at higher binding energies, characteristic of carbon atoms carbon  atoms  in  very  electronegative  neighborhoods  such  as  carboxyl  and/or  carboxylate  groups.  in very electronegative neighborhoods such as carboxyl and/or carboxylate groups. After mixing with After mixing with Fe3O4, C 1s fitting is qualitatively very similar to that of the complex [FeCl 2{κ3‐ Fe3 O4 , C 1s fitting is qualitatively very similar to that of the complex [FeCl2 {κ3 -HC(pz)3 }], although HC(pz)3}],  although  the  relative  amount  of  aliphatic  carbon  has  increased.  Such  carbonaceous  the relative amount of aliphatic carbon has increased. Such carbonaceous contamination mainly contamination mainly results from Fe3O4, as attested by the carbon detected in the ferrite sample. Fe  results from Fe3 O4 , as attested by the carbon detected in the ferrite sample. Fe 2p regions (Figure 1b) 2p  regions  (Figure  1b)  were  fitted  with  several  doublets,  but  only  the  most  intense  are  shown.  were fitted with several doublets, but only the most intense are shown. Centered at a lower binding Centered at a lower binding energy, the complex 1 has a component, Fe 2p3/2, centered at 709.4 ± 0.2  energy, the complex 1 has a component, Fe 2p3/2 , centered at 709.4 ± 0.2 eV, which is typical of Fe2+ . eV, which is typical of Fe2+. As expected, this component binding energy increases in the presence of  As expected, this component binding energy increases in the presence of Fe3 O4 to 710.7 ± 0.2 eV, Fe3O4  to  710.7  ± 0.2  eV, attesting 2+that  Fe  2p  is  a  sum  of  Fe2+ and Fe3+  ions  [21].  Fe  2p3/2 in  Fe3O4  is  attesting that Fe 2p is a sum of Fe and Fe3+ ions [21]. Fe 2p3/2 in Fe3 O4 is centered at 711.5 ± 0.2 eV. centered at 711.5 ± 0.2 eV. Roughly between 715 and 723 eV and above 729 eV, multiplet structures  Roughly between 715 and 723 eV and above 729 eV, multiplet structures arising from spin-spin arising from spin‐spin coupling (between unpaired core electron and the unpaired electrons in the  coupling (between unpaired core electron and the unpaired electrons in the outer shell) are detected. outer shell) are detected.  XPS analysis shows that the as-synthesised iron(II) complex [FeCl {κ3 -HC(pz) }] has the expected XPS analysis shows that the as‐synthesised iron(II) complex [FeCl22{κ3‐HC(pz)33}] has the expected  Fe/N ratio (0.17, Table 1); however, the atomic ratio N/Cl = 4.0 (against 3.0 for the anticipated 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. 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 22/C-N atomic ratio In fact, from the quantitative analysis of C 1s fitted peaks, the experimental C sp /C‐N atomic ratio  (computed from the areas of the main peaks identified in Figure 1a) is close to the one predicted for (computed from the areas of the main peaks identified in Figure 1a) is close to the one predicted for  the as-synthesized [FeCl {κ3 -HC(pz) }] (~1.2), which means that the detected O–C=O groups must be the as‐synthesized [FeCl22{κ3‐HC(pz)33}] (~1.2), which means that the detected O–C=O groups must be  from carboxylated iron, which could promote some dimerization. from carboxylated iron, which could promote some dimerization.  When mixed with Fe O4 , complex 1 seems to lose nitrogen atoms, as shown by the ratio N/Cl When mixed with Fe33O4, complex 1 seems to lose nitrogen atoms, as shown by the ratio N/Cl (N  (N and Cl only exist in the organometallic compound)