Iron amino-bis(phenolate) complexes for the formation of organic carbonates from CO2 and oxiranes

Table S1. Crystallographic data and structure refinement for 1 and 2. S3 Table S2. Selected Bond lengths (Å) and angles (°) for 1and 2 S4 Figure S1. Molecular structure (ORTEP) and partial numbering scheme for 2. S5 Ellipsoids are shown at the 50% probability level (H-atoms omitted for clarity). Figure S2. Electronic absorption spectrum of 2 in dichloromethane. S6 Figure S3. Electronic absorption spectrum of 3 in dichloromethane. S6 Figure S4. Electronic absorption spectrum of 4 in dichloromethane. S7 Figure S5. Electronic absorption spectrum of 5 in dichloromethane. S7 Figure S6. 1 H NMR spectrum (300 MHz, 298 K, CDCl3) of H L3. S8 Figure S7. 13 C NMR spectrum (300 MHz, 298 K, CDCl3) of H L3. S9 Figure S8. MALDI-TOF mass spectrum of H2L3. S10


Introduction
Utilization of carbon dioxide (CO2) in the preparation of chemicals with commercial value has become important as it is a green, cheap, non-toxic and abundant feedstock. [1][2][3][4][5][6][7][8] Highly reactive substrates such as epoxides allows for the thermodynamic stability of CO2 to be overcome. 1, 2,9 The interest in cyclic carbonates as CO2-derived molecules is driven by their wide applications as aprotic solvents (including their use to prepare electrolyte solutions in lithium ion batteries) and as starting materials for polycarbonates. 10 Industrially, the production of cyclic carbonates requires demanding reaction conditions such as elevated CO 2 pressures and high temperatures. Therefore, numerous efforts have been devoted to the design of efficient catalysts for this transformation under mild reaction conditions, 10 including catalysts of aluminum, [11][12][13][14] chromium, [15][16][17][18] cobalt, [17][18][19][20][21][22] zinc, 23, 24 manganese 25 and magnesium. 26 In order to address the potential toxicity associated with some of these metals, iron complexes have been used as a promising class of catalyst. Moreover, compared with some catalysts, because of iron's high natural abundance, they are often cheap and some recent examples have shown exceptional catalytic activity in the conversion of CO 2 and epoxides to carbonates. [27][28][29] To date, several iron-based catalysts have shown excellent activity in the production of both cyclic carbonates and polycarbonates. [26][27][28][29][30][31][32][33] A series of dinuclear iron catalysts based on macrocyclic ligands was reported by the Williams group. 27 Their systems were able to catalyze the reaction of CO 2 and epoxide to produce cyclic carbonates or polycarbonates at only 1 atm pressure of CO 2 in the presence of PPNCl as a co-catalyst.
In 2011, a mononuclear iron(II)-system based on a tetraamine ligand was able to produce propylene carbonate without the addition of a co-catalyst. 28 Related cycloaddition reactions catalyzed by monometallic and dimetallic iron(III) complexes chelated with aminotriphenolate ligands have been investigated by Kleij and co-workers ( Figure 1). 29 Later, the same iron complexes were shown to have high activity and selectivity in supercritical carbon dioxide (sc-CO 2) for the production of both cyclic carbonate and polycarbonate at 80 °C with a strong dependence on co-catalyst loading. 30 In 2013, a new family of ionic monometallic iron(II) and (III) complexes containing N2O2 ligands demonstrated high activity for the conversion of CO 2 and epoxide to cyclic carbonates with 99% yield and TON up to 500 without the addition of a co-catalyst. 31 Wang and co-workers have reported very active iron(II) complexes which could catalyze the cycloaddition of CO 2 and epoxides to generate cyclic carbonates with nearly 100% yield and TON of 1000 in 6 h ( Figure 2). 32 Recently, bimetallic iron(III) thioether-triphenolate complexes have shown high activity towards the production of propylene carbonate from the coupling of CO 2 and propylene oxide under solvent-free conditions with the highest reported TON to date, 3480, in only 6 h ( Figure 3). 33 Pescarmona and co-workers reported an effective bifunctional iron(III) pyridylamino-bis(phenolate) complexes FeX[O2NN'], (X=Cl, Br) to produce either cyclic carbonates (CHC) or polycyclohexene (PCHC) carbonates under solvent free conditions at 60 °C and scCO2 medium within 18 h. 34 In the present study, we report the synthesis of new iron(III) complexes supported by tetradentate amino-bis(phenolate) ligands and their catalytic activity for the coupling reaction of CO2 and various epoxides. Details of the spectroscopic and magnetic properties of these complexes have been included.

Synthesis and characterization of iron complexes
A series of tetradentate amino-bis(phenol) compounds H2[O2N2] RR'Pip ( Figure 4) were synthesized using a method similar to literature procedures reported by Kerton and co-workers. 35 As shown in Scheme 1, the desired iron(III) complexes were obtained via a method reported by Kozak and co-workers, 36 which employs dropwise addition of a methanol solution of anhydrous FeX 3 (X = Cl or Br) to a methanolic slurry of the ligand at room temperature. The resulting solution was neutralized using NEt 3 and evaporated to dryness. Extraction into an appropriate solvent, such as acetone, followed by filtration and removal of the solvent afforded analytically pure paramagnetic complexes with the formulation Fe[L]X. [37][38][39] The complexes were characterized using MALDI-TOF mass spectrometry, elemental analysis, X-ray diffraction and UV-vis spectroscopy. Scheme 1. Synthesis of iron(III) complexes.

Crystal Structure Determination
Single crystals of 1 and 2 suitable for X-ray diffraction analysis were obtained by slow evaporation and cooling of a saturated methanol or acetone solution at -20 °C. The ORTEP diagrams of the structures are shown in Figure 5 and S1 and the crystallographic data are collected in Table S1. Both complexes exhibit monometallic structures with the iron centres bonded to the two phenolate oxygen atoms and two amine nitrogen atoms of the ligand, which define the basal plane of the square pyramid. The apical sites are occupied by chloride ions and the coordination geometry around each iron atom can be described as a distorted square pyramid for both complexes. Selected bond lengths (Å) and angles (°) for compounds 1 and 2 are given in Table S2. Since both of these complexes are structurally

UV-visible spectroscopic and magnetic data
Based on previous work with iron(III) compounds supported by tetradentate aminobis(phenolate) ligands, similar electronic absorption spectra were obtained for all of the present complexes. 36,38,50 Since all of these complexes showed similar absorption bands, we can assume that the compounds contain Fe in similar geometries.

Cyclization of Propylene Oxide with Carbon Dioxide
Inspired by the promising results reported by the Wang, 32 . 3, respectively).
The cycloaddition reaction was also tested for complexes 2-5, in order to identify the most active catalyst and any ligand effects (entries [15][16][17][18]. As previously reported by others, introducing electron-withdrawing substituents in the ortho and para-positions of the phenolate ring generates more reactive complexes for the use in the coupling reaction of CO2 and epoxides. 34,56,57 Our results are in good agreement with this observation as 4 displays the highest catalytic activity with a TOF of 173 h -1 (entry 17). A possible explanation is that a decrease in the donor ability of the ligand leads to increased Lewis acidity of the metal centre and enhances the ability of the metal to bind to the epoxide. 1, 2 The substitution of the axial ligand by a bromide led to a drop of the catalytic activity with conversions achieving only 34% (entry 18 cf. entry 3 for the corresponding chloride complex, conversion 74%). A similar trend was also observed by Pescarmona and coworkers. 34 and they attributed the low activity to the larger radius of bromide which causes steric repulsion for the incoming epoxide substrate when approaching the metal centre.
Other reasons for the decrease in activity may be a difference in lability or nucleophilicity of the halide anion. Therefore, overall in the present work activity decreased in the order 4>1>3≥5>2. In contrast to the work reported by Pescarmona's group, only a small amount of polypropylene carbonate (4%) could be produced at higher temperature and pressure conditions (70 °C and 70 bar of CO2). 34 To expand the scope of the catalytic system, several commercially available epoxides with different electronic and steric properties were examined as substrates using 1 ( Table 2). The reaction conditions were chosen according to the conditions presented in Table 1. 1 was able to produce cyclic carbonate from various terminal epoxides containing functional groups. It has recently been noted that the presence of such groups can have a significant effect on the underlying mechanism of the reaction and functional groups such as -OH in glycidol can serve a role in activating carbon dioxide. 58 In our study, epichlorohydrin and glycidol reached conversions higher than those observed for PO (  29,32,33,59 Reducing the electron-withdrawing nature of the substituents on the oxirane ring resulted in the production of cyclic carbonate in smaller amounts and low catalytic activity (Table 2, entries 4 and 5). Styrene oxide (SO) exhibited lower reactivity with conversion reaching only 31% (entry 6). This might be due to electronic effects that have been studied computationally, which show that the alkoxide formed from ringopening of SO is less nucleophilic and therefore less reactive towards carbon dioxide. 60 Further to this, switching the substrate to cyclohexene oxide led to very low conversions compared to all other epoxides used and no polymer was formed (

Kinetic Measurements
At elevated temperatures, it is known that propylene carbonate is produced as the dominant product in the coupling reaction of PO and CO2. 19 The production of cyclic carbonates is proposed to occur via a backbiting mechanism from either a carbonate or an alkoxide chain end during the coupling process. 19,27 In an effort to better understand the mechanistic aspects of the propylene oxide/carbon dioxide coupling process, a kinetic study for the formation of cyclic propylene carbonate catalyzed by 1 and TBAB was undertaken. Figure 7 shows the reaction profile obtained using in situ infrared spectroscopy. During the course of the reaction, a strong absorption band at 1806 cm -1 was seen to increase in intensity and can be assigned to the cyclic carbonate carbonyl group. Furthermore, as discussed above, temperature has a clear influence on the reaction; therefore, the formation of cyclic carbonate was monitored with respect to increases in temperature ( Figure 8). During the course of the reaction, the temperature was gradually increased and maintained for approximately 25 minutes at each temperature. No cyclic carbonate was observed at room temperature and a small amount formed at 30 and 40 °C.
As expected, increasing the temperature further resulted in significant increases in the rate of cyclic carbonate formation. In addition, as shown in the Arrhenius plot (Figure 9), the activation energy for the formation of the cyclic carbonate could be calculated from the kinetic data. The activation barrier using the 1/TBAB catalytic system was determined to be 98.4 kJ mol -1 , which is in good agreement with the values reported by the groups of Rieger (93.8 kJ mol -1 ) and Darensbourg (100 kJ mol -1 ), 28,61 for the cycloaddition of PO with CO2 using an iron(II) complex containing a tetradentate bis(amino)-bis(pyridyl) ligand and chromium(III) salen complex, respectively. Thus implying that the reaction pathways followed by these catalytic systems are likely very similar.    4 ]. In addition to reactions described below that use a 100 mL pressure vessel equipped for IR-monitoring, cycloaddition reactions were also carried out in a 300 mL stainless steel Parr® 5500 autoclave reactor with a Parr® 4836 controller.

In situ monitoring of the cycloaddition reaction by IR spectroscopy
In situ monitoring was carried out using a modified 100 mL stainless steel reactor