DOI: 10.1002/chem.201304593

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& Iron Catalysts

Well-Defined Iron Complexes as Efficient Catalysts for “Green” Atom-Transfer Radical Polymerization of Styrene, Methyl Methacrylate, and Butyl Acrylate with Low Catalyst Loadings and Catalyst Recycling So-ichiro Nakanishi,[b] Mitsunobu Kawamura,[a] Hidetomo Kai,[c] Ren-Hua Jin,[d] Yusuke Sunada,[a] and Hideo Nagashima*[a, b]

Abstract: Environmentally friendly iron(II) catalysts for atomtransfer radical polymerization (ATRP) were synthesized by careful selection of the nitrogen substituents of N,N,N-trialkylated-1,4,9-triazacyclononane (R3TACN) ligands. Two types of structures were confirmed by crystallography: “[(R3TACN)FeX2]” complexes with relatively small R groups have ionic and dinuclear structures including a [(R3TACN)Fe(m-X)3Fe(R3TACN)] + moiety, whereas those with more bulky R groups are neutral and mononuclear. The twelve [(R3TACN)FeX2]n complexes that were synthesized were subjected to bulk ATRP of styrene, methyl methacrylate (MMA), and butyl acrylate (BA). Among the iron complexes examined, [{(cyclopentyl)3TACN}FeBr2] (4 b) was the best catalyst

Introduction The discovery of transition-metal-catalyzed atom-transfer radical polymerization (ATRP) has provided one of the most reliable methods for controlled radical polymerization.[1–3] ATRP is performed by activation of an alkyl halide initiator by a transition-metal compound in the presence of vinyl monomers. ATRP demonstrates significant advantages for the preparation of various polymer architectures in terms of chain topology, composition, and functionality, which has led to the development of a range of advanced materials.[4] It is crucial for ATRP [a] Dr. M. Kawamura, Dr. Y. Sunada, Prof. Dr. H. Nagashima Institute for Materials Chemistry and Engineering Kyushu University, 6-1 Kasugakoen, Kasuga, Fukuoka 816-8580 (Japan) Fax: (+ 81)92-583-7819 E-mail: [email protected] [b] S.-i. Nakanishi, Prof. Dr. H. Nagashima Graduate School of Engineering Sciences Kyushu University, 6-1 Kasugakoen, Kasuga, Fukuoka 816-8580 (Japan) [c] H. Kai Central Research Laboratories, DIC Corporation Sakura, Chiba 285-8668 (Japan) [d] Prof. Dr. R.-H. Jin Department of Material and Life Chemistry Kanagawa University, 3-2-7 Rokkakubashi, Yokohama 221-8686 (Japan) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201304593. Chem. Eur. J. 2014, 20, 5802 – 5814

for the well-controlled ATRP of all three monomers. This species allowed easy catalyst separation and recycling, a lowering of the catalyst concentration needed for the reaction, and the absence of additional reducing reagents. The lowest catalyst loading was accomplished in the ATRP of MMA with 4 b (59 ppm of Fe based on the charged monomer). Catalyst recycling in ATRP with low catalyst loadings was also successful. The ATRP of styrene with 4 b (117 ppm Fe atom) was followed by precipitation from methanol to give polystyrene that contained residual iron below the calculated detection limit (0.28 ppm). Mechanisms that involve equilibria between the multinuclear and mononuclear species were also examined.

studies that efficient catalysts are developed. New catalysts should enable a precise control over the polymer molecular weight, a narrow molecular-weight distribution, and the introduction of useful end groups onto the polymer that provide a vehicle for further post-polymerization options. A number of papers have been published on ATRP since its initial conception by Matyjaszewski and Sawamoto et al.[2] The research into ATRP mediated by copper–amine catalysts has focused on many facets of the reactions, and problems found in the early stages have been solved over the last ten years.[3d–i] For example, one problem was that a large quantity of copper catalyst was required to enable a fine control over the molecular weight. Catalyst immobilization on either solid or liquid phases was investigated as one of the solutions.[5] Efforts to lower the catalyst concentration have been made independently by studies on catalyst recycling.[3d, 6] The addition of reducing reagents to the reaction medium produces activators that are regenerated by electron-transfer atom-transfer radical polymerization (ARGET). Furthermore, initiators for continuous activator regeneration (ICAR) and related processes that achieve polymerization with a parts-per-million order of the metal for the charged monomer have been devised.[6] The present ATRP process, however, still has many issues that must be addressed,[7] and continuous efforts are being made to develop superior catalytic systems.[3e, 8, 9] A promising strategy to access the ideal catalyst systems should be a rational catalyst design

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Full Paper based on the profound insights into the catalytically active species.[8–10] Among the ATRP catalysts investigated so far, iron-based catalysts have received considerable attention owing to the environmentally benign nature of iron as an element.[11] Several iron(II) compounds have been reported as catalysts,[12] such as FeII–phosphine complexes,[12a] iron–carbonyl complexes,[12b] and onium salts.[12c,d,e] Several well-defined iron–amine catalysts have also been investigated.[13, 14] In particular, Gibson and coworkers reported a series of studies that utilized four-coordinate diamine–FeCl2 complexes, the structures of which were thoroughly characterized by X-ray crystallography. These complexes generated stimulating discussions on the relationship between the polymerization behavior and the structural features of the catalyst.[13i, j] Nevertheless, the iron catalyst systems reported to date have not yet achieved a sufficiently high catalyst performance relative to the copper catalyst systems. For example, it is widely known that a single catalyst system that achieves well-controlled ATRP of styrenes, methacrylates, and acrylates is rare,[3i] although these are typical vinyl monomers for free-radical polymerizations. In particular, the well-controlled ATRP of acrylates was reported in copper-catalyzed ATRP;[15] however, to the best of our knowledge, there have been no reports on iron catalysts that promote the ATRP of acrylates with complete monomer conversion to give polyacrylates with a narrow molecular-weight distribution.[16–18] With regard to “green” ATRP, iron-based ARGET and ICAR have been investigated.[6c] Iron onium salts washable by water[19a] and heterogeneous catalyst systems composed of iron oxides[19b] and Bu4NBr are unique green ATRP procedures that have recently been highlighted. However, complicated multicomponent systems are often used, and the monomers are limited to methyl methacrylate (MMA) and styrene. In this manuscript, we report our investigations into the development of new iron catalysts that solve the problems outlined in the previous discussion of iron-mediated ATRP. The following parameters should be satisfied for any new catalyst: Firstly, the well-controlled ATRP of styrenes, methacrylate, and acrylate monomers by a single catalyst must be developed. Secondly green ATRP must be developed that satisfies the following conditions: 1) polymerization without solvents (bulk polymerization) should be achieved, because minimum amounts are preferable from an environmental viewpoint; 2) the catalytic activity should be high enough to lower the catalyst loadings without additives, as highly active singlecomponent catalysts are desirable for the synthesis of polymers with a high purity; and 3) the catalyst should be water soluble and easily separated from the reaction mixture after polymerization. It would be particularly advantageous if the catalyst were recoverable and reusable. In addition to these three requirements, a challenging goal of this research is the well-controlled ATRP with a low catalyst concentration and catalyst recycling. Our strategy involves the rational design of well-defined iron(II) complexes that lead to high-performance ATRP. In our preliminary communications, we reported two types of iron(II) complexes that bear N,N,N-substituted-1,4,9-triazanonane (TACN) ligands for ATRP catalysis, which were well Chem. Eur. J. 2014, 20, 5802 – 5814

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characterized by crystallography.[14] The ionic complex [(Me3TACN)Fe(m2-Cl)3Fe(Me3TACN)] + [(Me3TACN)FeCl3] (2 a) achieved facile catalyst recovery by extraction of the crude polymer with methanol, and the recovered catalyst was also reusable.[14a] However, the complex was only useful for the well-controlled ATRP of styrene. In contrast, a mononuclear complex, [(iPr3TACN)FeBr2] (7 b), displayed a higher activity than 1 a, and promoted the polymerization of styrene, MMA, and butyl acrylate (BA) with lower catalyst loadings.[14b] Unfortunately, 7 b was not soluble in methanol; therefore, separation of the catalyst from the polymer was not easy and recycling was not possible. We were interested in the development of new iron catalysts that exhibit the merits of both 2 a and 7 b. We carried out preparation and characterization of a series of “[(R3TACN)FeX2]” complexes by systematically changing the R group on the TACN ligand, and then we studied the ATRP initiated by these catalysts. As a result, the iron complex [{(cyclopentyl)3TACN}FeBr2]n (4 b) was found to be the best catalyst for the well-controlled ATRP of styrene, MMA, and BA, and also satisfied the three previously described parameters for green ATRP. This has never been achieved by iron catalysts for ATRP. Furthermore, the excellent catalytic performance of 4 b enabled catalyst recycling in the ATRP of styrene and MMA even with low catalyst concentration. The iron catalysts were efficiently removed from the crude polymer by precipitation into methanol. In an extreme case, the residual iron in the formed polymer was below the detection limit of inductively coupled plasma mass spectrometry (ICPMS) analysis (0.28 ppm). Block copolymerization to form polystyrene-b-polyMMA (pSt-bpMMA) was also achieved by using low catalyst concentrations. Mechanisms that involve the equilibrium between the highly active mononuclear and stable ionic dinuclear species are discussed in regard to the structurally well-defined iron complexes.

Results and Discussion Preparation and characterization of (TACN)FeII complexes Preparation Ligation of N,N,N-trialkyl-1,4,7-triazanonane (R3TACN; 1 a–f) to iron(II) salts is expected to afford coordinatively unsaturated 16-electron iron(II) species, [(R3TACN)FeX2]. The first complex of [(R3TACN)FeX2] was reported by Rauchfuss and co-workers.[20] Treatment of N,N,N-trimethyl-1,4,7-triazanonane (Me3TACN) with FeCl2 in CH3CN gave 2 a with an ionic structure shown as A in Figure 1. The second example of [(R3TACN)FeX2] was mononuclear, shown as B in Figure 1. [(iPr3TACN)FeX2] (X = Cl (7 a), Br (7 b)) was prepared by our group from N,N,N-isopropyl1,4,7-triazanonane (iPr3TACN) and either FeCl2 or FeBr2 in CH2Cl2.[14b] By using six different TACN ligands (R = Me, pMeOC6H4CH2, cyclopentyl, cyclohepyl, sBu, and iPr) and two halogen ligands (X = Cl, Br) as shown in Figure 1, a series of iron complexes, [(R3TACN)FeX2], was synthesized by slightly modified procedures of those reported in the two papers above.[14b, 20] Details are described in the Supporting Information. All of these complexes are paramagnetic, and recrystalli-

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Full Paper iron. Each iron center in the cationic moiety adopts the octahedral coordination geometry with three nitrogen atoms derived from the facially coordinated TACN ligand and three bridging Cl atoms. The Fe (m-Cl) bond lengths of 4 a are in the range of 2.480(2)–2.5137(13) , whereas those of Fe N of 4 a-ionic are 2.254(4)–2.281(4) . The molecular structures of 4 a-neutral and 4 b-neutral as determined by Xray crystallography are similar. The coordination geometry around the iron center of 4 b was found to be a trigonal bipyrFigure 1. A series of [(R3TACN)FeX2] in which nuclearity is dependent on the steric bulkiness of the R group and X. amid with one Br atom (Br2) and two N atoms (N1 and N2) on the equatorial plane. The Fe N3 (axial) bond length was longer zation of the crude product from a mixture of CH2Cl2 and than the other two Fe N bonds (2.347(9)  versus 2.213(8) and hexane gave samples that provided satisfactory data for ele2.225(7) ). Similarly, the Fe Br1 (axial) bond was elongated mental analyses, which confirmed them to be [(R3TACN)FeX2]. considerably relative to the Fe Br2 (equatorial) bond (2.608(2) versus 2.453(2) ). These are characteristic of mononuclear Structure in the solid state [(R3TACN)FeX2] complexes with trigonal-bipyramidal coordinaX-ray-quality crystals were grown from the layer interface betion geometries.[14b] Similar structural features can be observed tween CH2Cl2 and hexane. The molecular structures of a series in other ionic dinuclear and mononuclear [(R3TACN)FeX2] comof the (R3TACN)FeII complexes determined by crystallography plexes, and details are described in the Supporting Information. have either a structure that includes an ionic diiron moiety A The molecular structures of a series of [(R3TACN)FeX2] comor that of a neutral mononuclear species B. Complex 2 a is composed of a cationic dinuclear moiety, [(Me3TACN)Fe(m2plexes clearly demonstrate that sterically less bulky R groups on the TACN ligand tend to afford ionic dinuclear structures Cl)3Fe(Me3TACN)] + , and a mononuclear anion, because the reaction of R3TACN with FeX2 initially affords coor[(Me3TACN)FeCl3] .[20] Interestingly, the bromo analogue 2 b has a similar bromo-bridged dinuclear moiety, [(Me3TACN)Fe(m2dinatively unsaturated [(R3TACN)FeX2] species with 16-electron Br)3Fe(Me3TACN)] + , as a cation, but the counteranion is Br . configurations. In the cases in which R = Me, MeOC6H4CH2, and cyclopentyl (X = Cl), [(R3TACN)FeX2] species dimerize to proComplexes 3 a and 3 b, in which N-methoxybenzyl groups are on the TACN ligand, are dinuclear, similar to 2 b. Mononuclear duce the [(R3TACN)Fe(m2-X)3Fe(R3TACN)] + moiety, in which each structures similar to those of 7 a and 7 b[14b] were confirmed by iron center is coordinatively saturated. The initial counteranion of the dinuclear cationic species is X , but in some cases furcrystallography for the complexes with N,N,N-(cyclohepther reaction of X takes place. The counteranion of 2 a is tyl)3TACN and N,N,N-sBu3TACN ligands, namely, 5 a, 5 b, 6 a, and 6 b. The N-cyclopentyl derivatives of [(R3TACN)FeX2] were differ[(Me3TACN)FeCl3] , which was formed by the reaction of Cl with 16-electron [(R3TACN)FeX2] species. The crystal structure ent from these. From the same mother liquor of 4 a, both of 4 a-ionic contains two [{(cyclopentyl)3TACN}Fe(m2single crystals that contained a dinuclear structure and those with a mononuclear structure were obtained by recrystallizaCl)3Fe{(cyclopentyl)3TACN}] + units and Fe2Cl62 . In sharp contion to give the bromo-homologue 4 b. The three molecular structures of 4 a and 4 b are depicted in Figure 2 as examples of dinuclear and mononuclear structures, respectively. The molecular structure of 4 a-ionic was found to be constructed from a cationic dinuclear component, [(R3TACN)Fe(m2-Cl)3Fe(R3TACN)] + , and a 1=2 (Fe2Cl6)2 anion outside Figure 2. ORTEP drawings for 4 a-ionic (left), 4 a-neutral (center), and 4 b-neutral (right). Hydrogen atoms and the of the coordination sphere of counteranion of 4 a-ionic (Fe2Cl62 ) were omitted for clarity. Chem. Eur. J. 2014, 20, 5802 – 5814

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Full Paper trast, coordinatively unsaturated complexes are generally unfrom d = + 39 to + 121 ppm, and it was impossible to assign stable but isolable when bulky ligands sterically protect the the environments accurately. metal center.[21] When the R groups are a secondary alkyl In contrast, ESI mass spectra[24] of the compounds clearly moiety (R = iPr, sBu, cycloheptyl, cyclopentyl), three of the R demonstrated the difference in nuclearity for possible detecgroups protect the 16-electron iron center to make the comtion of the signals. The ionic compounds 2 a, 2 b, 3 a, and 3 b plexes 4–7 stable. presented a parent signal owing to [(R3TACN)Fe(m2-X)3FeFacile dimerization of 16-electron L3FeX3 (L = amine ligands) (R3TACN)] + . A typical example is seen in the spectrum of 2 b, in species by halogen bridges is a special feature of which a single signal at m/z 693.12 for C18H42N6Fe2Br3 and its [(R3TACN)FeX2]. Six [NNN] pincer complexes were examined as well-resolved characteristic isotope patterns were clearly visible and correspond to [(Me3TACN)Fe(m2-Br)3Fe(Me3TACN)] + under the five-coordinated iron(II) catalyst for ATRP in the literatur[13f] e. the standard conditions of cone voltage = 20 V and [2 b] = Among them, [(PMDETA)FeCl2] (PMDETA = N,N,N-penta0.02 mm in CH2Cl2 (Figure 3). In contrast, neutral mononuclear methyldiethylenetriamine) is the closest analogue to 2 a as a five-coordinated triamine complex.[22] Although this is a 16-electron complex with a vacant site for coordination of a chlorine atom, [(PMDETA)FeCl2] exists as a neutral mononuclear complex, and no evidence for the formation of the chloride-bridged dinuclear complex has been reported. When the ionic dinuclear structures are examined in detail, it can be observed that the bulkiness of R groups on the TACN ligand does not affect the structure of the Fe(m2-Cl)3Fe core, but increases the distance between Fe and the TACN ligands. For instance, [(R3TACN)FeCl2] complexes with the methyl, MeOC6H4CH2, and cyclopentyl groups (2 a, 3 a, and 4 a) contain [(R3TACN)Fe(m2-Cl)3Fe(R3TACN)] + in the solid state, in which Fe Cl lengths are similar (2.47–2.52 ), but the bond lengths of Fe N in 4 a (2.254(4)–2.281(4) ) are significantly elongated relative to those of 2 a and 3 a (2.193(3)– 2.212(3)  for 2 a, 2.196(4)–2.218(4)  for 3 a). Since cyclopentyl groups in 4 a are more sterically hindered than Me or MeOC6H4CH2 moieties in 2 a or 3 a, interli- Figure 3. ESI-MS spectrum of 2 b showing a signal due to [(Me3TACN)Fe(m2-Br)3Fe+ gand steric repulsion among the cyclopentyl groups (Me3TACN)] . on the TACN ligand of each iron center forces the Fe N bonds to lengthen. It is important to note that complexes 5–7 showed no signals under the same condithe larger interligand steric repulsion in [(R3TACN)Fe(m2-X)3Fetions.[25] Of particular interest are the ESI mass spectra of cyclo(R3TACN)] + X makes the dinuclear structure less stable. Thus, the complexes with R groups that are more bulky than cyclopentyl derivatives 4 a and 4 b. Interestingly, ESI mass spectra of pentyl favor the neutral and mononuclear structures. 4 a and 4 b under the standard conditions are similar in that they give signals that correspond to [{(cyclopentyl)3TACN}Fe(m2-X)3Fe{(cyclopentyl)3TACN}] + , although both ionic dinuclear Species in the solution state and neutral mononuclear structures were found for 4 a, whereas only the mononuclear structure was confirmed for 4 b by The nuclearity of a series of [(R3TACN)FeX2] complexes determeans of crystallography. Since ESI-MS measurement was carmined by crystallography may be maintained in the solution ried out using the solution from which single crystals of 4 b state. Although NMR spectroscopy is generally a powerful tool were grown, the result suggests the possible existence of both for the analysis of chemical species in solution, the paramagthe mononuclear and dinuclear structures in the solution state netism of [(R3TACN)FeX2] prevented the acquisition of well-refor 4 b. solved 1H resonances for determination of iron species in the solution state.[23] In fact, The tri-iron complex 2 a in CD3CN showed five broad signals at d = 39.3, 46.0, 52.5, 101.9, Cyclic voltammetry 120 ppm, which have been assigned in the literature as Me Since ATRP is promoted by the redox reaction of transitionand CH2 signals.[20] In contrast, the bromo-analogue 2 b exhibitmetal species, correlation of the reaction rate with the redox ed three broad signals at d = 41.4, 72.1, and 112.1 ppm in an potential of the catalyst and the initiator has been discussed in integral ratio of 2:3:2, which are assignable to three of the six the literature.[3] The cyclic voltammogram of [(PMDETA)FeCl2], CH2 groups, Me protons, and another three of six CH2 groups 1 of the Me3TACN ligand, respectively. H NMR spectra of other which is an analogue of mononuclear [(R3TACN)FeCl2] comcomplexes in CD3CN or CD2Cl2 gave ill-resolved broad signals plexes in terms of its pentacoordinated molecular structure Chem. Eur. J. 2014, 20, 5802 – 5814

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Full Paper given by the tridentate aliphatic amine ligand,[13e] gives a quasi-reversible wave, with Eox, E1/2, and DEp of 370, 280, and 170 mV, respectively, in MeCN. The Eox value of the other five [NNN] pincer complexes with aminopyridine or iminopyridine ligands varied from 240 to + 260 mV. The Eox, E1/2, and DEp values determined by cyclic voltammetry of eight representative [(R3TACN)FeX2] complexes are shown in the Supporting Information. Since [(R3TACN)FeBr2] complexes are not very soluble in MeCN, MeCN was used for measurements of four chloride complexes for comparison with [(PMDETA)FeCl2], whereas those of all eight complexes were performed in CH2Cl2. In MeCN, cyclic voltammograms of the chloride complexes 3 a, 4 a, and 7 a provided Eox values of 540– 574 mV under conditions almost identical to those of [(PMDETA)FeCl2] reported in the literature,[13e] thus indicating that they were oxidized more easily than [(PMDETA)FeCl2] by approximately 200 mV. Cyclic voltammograms of all eight complexes in CH2Cl2 revealed the trend of Eox : the trinuclear chloride 2 a ( 451 mV) > di- and mononuclear chloride compounds 3 a, 4 a, and 7 a ( 326 to 362 mV) > bromide complexes 2 b, 3 b, 4 b, and 7 b ( 146 to 205 mV). It is reasonable that Eox values for other complexes are dependent on X, but not on the R groups of the TACN ligand. The actual voltammograms are shown in the Supporting Information. ATRP catalyzed by [(R3TACN)FeX2]n ATRP of three monomers by [(R3TACN)FeX2] with different R and X groups

Table 1. Polymerization of three monomers by [(TACN)FeX2].[a] Entry

Catalyst

1 2 3 4 5 6 7 8 9 10 11 12

2a 2b 3a 3b 4a 4b 5a 5b 6a 6b 7a 7b

t [h] 20 48 4 4 4 4 4 4 4 4 4 4

Conv. [%] Mn (exptl) Polymerization of styrene > 99 76 84 88 94 88 96 93 96 96 > 99 > 99

24 600 18 800 18 300 26 800 24 900 23 200 24 000 22 200 25 000 26 300 27 200 28 300

Mn (calcd)

Mw/Mn

25 000 19 000 21 000 22 000 23 500 22 000 24 000 23 250 24 000 24 000 25 000 23 750

1.23 1.21 1.14 1.32 1.17 1.09 1.26 1.28 1.28 1.28 1.26 1.23

Polymerization of MMA 13 14 15 16 17 18 19 20 21 22 23 24

2a 2b 3a 3b 4a 4b 5a 5b 6a 6b 7a 7b

48 48 1 1 1 1 1 1 1 1 1 1

> 99 86 81 96 93 93 93 93 97 94 > 99 95

30 000 25 000 18 500 25 000 24 900 26 200 24 800 24 200 23 200 25 500 29 900 24 900

25 000 21 500 20 250 24 000 23 250 23 250 23 250 23 250 24 250 23 500 25 000 22 500

1.94 1.97 1.22 1.33 1.28 1.24 1.28 1.24 1.24 1.24 1.23 1.27

25 000 25 000 25 000 21 750 22 500 22 500 25 000 22 250 22 250 22 500 22 500 22 500

2.01 1.97 3.89 2.05 4.24 1.24 4.77 1.25 4.37 1.24 4.63 1.21

Polymerization of BA 25 26 27 28 29 30 31 32 33 34 35 36

2a 2b 3a 3b 4a 4b 5a 5b 6a 6b 7a 7b

20 20 20 20 20 20 20 20 20 20 20 20

> 99 > 99 > 99 87 90 90 > 99 89 89 90 90 90

45 000 42 000 48 100 59 600 34 000 24 000 35 400 25 470 31 600 24 900 27 200 24 900

The results of bulk polymerization of three monomers by a series of [(R3TACN)FeX2] prepared as described above are summarized in Table 1. Iron complexes carefully purified by recrystallization were used in these studies to avoid the effect of iron im[a] Standard reaction conditions are reported in the text. purities. Polymerizations were performed in the presence a small amount of internal standard (benzyl methyl ether) to determine the conversion by 1 H NMR spectroscopy. In all cases, the molar ratio of the monomerization by 2 a required 20 h to complete the reaction. In mer, the initiator, and the catalyst was 250:1:1. The ATRP of stycontrast, the reaction with 2 b was slower than that of 2 a; the rene was performed at 120 8C, and (1-chloroethyl)benzene and conversion was 76 % after 48 h. For comparison of the catalytic (1-bromoethyl)benzene were used as the initiators for activity of the other ten complexes, the reaction time was set [(R3TACN)FeCl2] and [(R3TACN)FeBr2], respectively. For MMA and to 4 h in the experiments shown in Table 1, entries 3–12. In all cases, prolonged heating resulted in achieving complete conBA polymerizations (100 8C), the initiator for [(R3TACN)FeCl2] version of the monomer. The conversion of styrene under was methyl trichloroacetate, whereas that for [(R3TACN)FeBr2] these conditions was around 85 % when 3 a, 3 b, or 4 b was was methyl 2-bromoisobutylate. Polymerizations with 2 a and used as the catalyst, whereas it reached over 93 % with the 7 b, which were reported previously, were also performed other five catalysts. The narrow molecular-weight distribution under the same conditions for comparison. suggests the living nature of the polymerization, which was inIn the ATRP of styrene, all of the catalysts examined providdicated by the linear relationship seen in the plot of convered polystyrene with narrow molecular-weight distribution. The sion of the monomer versus Mn. For representative examples, Mn determined by size-exclusion chromatography (SEC) was consistent with that calculated from conversion of the monothe graphs for 4 a and 4 b in ATRP of styrene are shown in mer and the ratio of charged polystyrene with the initiator. Of Figure 4 with the plot of monomer conversion versus molecuinterest is the significant difference in the reaction rate. Polylar-weight distribution (see other examples in the Supporting Chem. Eur. J. 2014, 20, 5802 – 5814

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Full Paper when the reaction was complete after 1 h. Except for the polymerization using 3 a as the catalyst, the monomer conversion was higher than 93 % after 1 h. As described in the introduction, the well-controlled ATRP of acrylates is difficult to achieve with iron catalysts. In the ATRP of BA at 100 8C shown in Table 1, entries 25–36, the key to accomplishing precise control of the ATRP was the use of mononuclear iron bromide complexes, [(R3TACN)FeBr2], 4 b, 5 b, 6 b, and 7 b. Detailed studies on the poorly controlled ATRP of MMA and BA suggest the involvement of a free radical chain extension at the initial stage of the reaction.[26] The reaction profiles of Table 1, entries 13 and 14 (MMA), and 25–36 (BA) showed that rapid monomer conversion at the initial stage (typically up to 10–20 % conversion for MMA and  70 % for BA) was followed by a slow reaction. The initial rapid reaction was mediated by free radicals, and Mn increased independently of the monomer conversion. In the later slow reaction, a linear correlation of Mn to monomer conversion was observed (ATRP). As a consequence, the molecular weight of the polymer was always larger than the theoretical values. These results are in sharp contrast to the well-controlled ATRP experiments of MMA BA by 4 b, in which Mn increased linearly with monomer conversion.

Figure 4. A typical plot of conversion versus Mn : ATRP of styrene by 4 a and 4 b.

Removal of the catalyst from the polymer and catalyst recycling

Figure 5. Plots of time versus ln([M]/[M]0)

Information). For analysis of the reaction rate in detail, the plots of the reaction time versus ln([M]/[M]0) for 2 a, 2 b, 4 a, 4 b, 7 a, and 7 b are illustrated in Figure 5. The reaction rates decreased in the order 7 a = 7 b > 4 a > 4 b @ 2 a @ 2 b. In the reactions that used [(R3TACN)FeX2], in which R = Me (2 a, 2 b) and cyclopentyl (4 a, 4 b), the Cl complexes catalyzed the reaction faster than the corresponding Br complexes. In contrast, there was no substantial difference in the reaction rates between [(iPr3TACN)FeCl2] and [(iPr3TACN)FeBr2]. It is noteworthy that induction period was clearly visible in the reactions with 2 a and 2 b. The polymerization behavior of MMA was somewhat different from that of styrene. The reactions with 2 a and 2 b were slow and gave polyMMA with broader molecular-weight distributions (Table 1, entries 13 and 14). In sharp contrast, the reactions with the other ten catalysts were much faster, and the molecular-weight distribution was narrow in all cases (Mw/Mn < 1.33). The results shown in Table 1, entries 15–24 are the data Chem. Eur. J. 2014, 20, 5802 – 5814

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In the ATRP of styrene catalyzed by 2 a with an ionic dinuclear structure, the iron residue was easily separable from the crude polymer upon precipitation from methanol, and the recovered catalyst was reusable.[14a] Similar facile catalyst removal from the polymer by methanol was seen in the polymerizations with 2 b, 3 a, and 3 b with ionic dinuclear structures. In sharp contrast, a mononuclear complex 7 b efficiently catalyzed the ATRP of styrene, MMA, and BA, but precipitation from methanol was not effective for removal of the iron residue.[14b] As was determined for 7 b, mononuclear complexes 5 a, 5 b, 6 a, 6 b, and 7 a are also good catalysts for ATRP, but removal of the iron residue from the product was difficult. Of interest are 4 a and 4 b, which both have neutral mononuclear and ionic dinuclear structures in the solution state. Although the polymerization behavior of 4 a and 4 b was similar to mononuclear complexes 7 a and 7 b, the separation of iron residue was as easy as for 2 a and 2 b. Table 2 summarizes the typical results of ICPMS analysis for the polymer after precipitation in ATRP of styrene catalyzed by 2 a, 2 b, 4 a, 4 b, 7 a, and 7 b. In ATRP catalyzed by 2 a and 2 b, the iron contents in the purified polystyrene were below 30 ppm, which means that over 99 % of charged iron was removed by precipitation. In contrast, more than half of the charged iron species remained in the purified polystyrene in ATRP catalyzed by 7 a and 7 b by using the same precipitation procedure as that used for ATRP by 2 a and 2 b. Separation of iron residue from the crude polymers obtained by catalysis of 4 a or 4 b was easily accomplished by precipitation into methanol. ICPMS data (Table 2, entries 3 and 4) showed residual iron contents of 13 and 17 ppm that corresponded to less than 1 % of the charged iron species, and are as good as those by 2 a or 2 b. The catalyst recycling experi-

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Full Paper Table 2. Removal of the iron species by precipitation from methanol. Entry Catalyst X Charged Fe [ppm]

Fe content after workup [ppm][a]

1 2 3 4 5 6

14 22 13 17 1226 1260

2a 2b 4a 4b 7a 7b

Cl Br Cl Br Cl Br

2150 2150 2150 2150 2150 2150

Removal efficiency [%][b] 99.3 99.0 99.4 99.2 43.0 41.4

[a] Polymerization was carried out with styrene (10 mmol), (1-chloroethyl)benzene (0.04 mmol), and the catalyst (0.04 mmol) at 120 8C: 2 a (20 h), 2 b (48 h), 4 a (4 h), 4 b (6 h), 7 a (4 h), and 7 b (4 h). After the reaction, the product was dissolved in THF (5 mL) and the solution was poured into methanol (30 mL) to precipitate the polymer. The crude polymer was washed with methanol three times (total methanol volume was 5 mL). The purified polymer was subjected to ICPMS analysis to determine the content of Fe. [b] Calculated by {1-([Fe content after workup [ppm]]/ [charged Fe [ppm]])}  100.

ments were carried out for ATRP of styrene, MMA, and BA by using 4 b as representative examples (Table 3, entries 4–12). For comparison, those of the ATRP of styrene catalyzed by 2 a was carried out, and the results are shown in Table 3, entries 1–3. The results clearly demonstrate that 4 b was reusable three times for the ATRP of all three monomers. No change was apparent among the first, second, and third runs on the reaction rate, the monomer conversion, Mn, or Mw/Mn. ATRP with low catalyst concentration We next carried out ATRP of three monomers by 4 b with lower catalyst loadings as summarized in Table 4. Experiments that used 7 b under similar conditions are also listed for comparison. Since the molecular weight of the formed polymer was determined by the molar ratio of the monomer to the initiator, we estimated the catalytic activity by the ratio of the initiator to the catalyst (I/C), whereas the ratio of the monomer to the initiator was fixed at 250:1. Well-controlled ATRP of styrene initiated by (1-bromoethyl)benzene and that of MMA using methyl 2bromoisobutylate as the initiator was achieved with I/C = 4 and 20 by using 4 b or 7 b as the catalyst. Although polymerization with lower catalyst loadings required longer reaction times for complete consumption of the monomer, molecular weight and molecular-weight distribution of the formed polymers were identical to those obtained with the same catalyst under the standard conditions (I/C = 1). With the lower catalyst loading (I/C = 40), well-controlled ATRP was achieved with MMA (Table 4, entries 5 and 13), but some increase in the Mw/Mn was seen in the ATRP of styrene (Table 4, entry 10). The ATRP of BA was rather difficult to accomplish with lower catalyst concentrations of 4 b or 7 b. As shown in Table 4, enChem. Eur. J. 2014, 20, 5802 – 5814

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tries 6 and 7, BA polymerizations by 4 b with I/C = 4 and 8 were complete in 20 h to give poly(BA) with a narrow molecular-weight distribution. The same experiment with I/C = 20 for 4 b was uncontrollable. Well-controlled ATRP with lower catalyst concentration was also possible by using other methanol-soluble dinuclear complexes: 3 a, 3 b, and 4 a. The ATRP of styrene (I/C = 4) by using (1-chloroethyl)benzene as the initiator was carried out at 120 8C. The catalyst 3 a gave polystyrene of Mn = 26 200 with Mw/Mn = 1.19 after 20 h (conversion > 95 %). In the case of 3 b, (1-bromoethyl)benzene was used as the initiator. After 20 h, conversion reached 91 %, and polystyrene of Mn = 23 200 with Mw/Mn = 1.25 was obtained. ATRP with 4 a as the catalyst proceeded faster than that with 3 a under similar conditions to give polystyrene of Mn = 28 300 with Mw/Mn = 1.15 after 4 h (conversion > 95 %).]

Table 3. Recycling of the catalyst in ATRP of three monomers by complex 4 b. Entry [a]

1 2[a] 3[a] 4[b] 5[b] 6[b] 7[c] 8[c] 9[c] 10[c] 11[c] 12[c]

Catalyst Monomer Recycle t [h] Conv. [%] Mn (exptl) Mn (calcd) Mw/Mn 2a 2a 2a 4b 4b 4b 4b 4b 4b 4b 4b 4b

styrene

styrene

MMA

BA

first second third first second third first second third first second third

20 20 20 4 4 4 1 1 1 20 20 20

95 93 92 > 95 90 92 > 95 > 95 > 95 > 95 > 95 > 95

32 300 29 000 31 000 24 200 27 400 26 900 24 000 29 400 29 400 24 400 28 900 29 000

23 800 23 300 23 000 25 000 22 000 23 000 25 000 22 000 23 000 25 000 22 000 23 000

1.31 1.33 1.39 1.24 1.25 1.24 1.24 1.22 1.28 1.21 1.25 1.30

[a] Data reported in ref. [14a]. Initiator = (1-chloroethyl)benzene, [catalyst]/[initiator]/[ monomer] = 1:1:250. [b] Initiator = (1-bromoethyl)benzene, [catalyst]/[initiator]/[ monomer] = 1:1:250. [c] Initiator = methyl 2-bromoisobutylate, [catalyst]/[initiator]/[ monomer] = 1:1:250.

Table 4. ATRP by 4 b and 7 b with lower catalyst loadings.[a] Entry Catalyst Monomer Catalyst: initiator/ monomer

I/ C

t Conv. [h] [%]

Mn Mw/ Mn (exptl) (calcd) Mn

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

4 20 4 20 40 4 8 4 20 40 4 20 40 4 8

6 24 1 4 4 20 20 6 24 48 2 4 4 20 20

24 300 23 200 24 000 27 800 28 000 16 400 16 800 25 400 27 800 35 600 24 500 27 200 26 100 27 300 27 800

4b

styrene MMA

BA 7b

styrene

MMA

BA

250:1:0.25 250:1:0.05 250:1:0.25 250:1:0.05 250:1:0.025 250:1:0.25 250:1:0.125 250:1:0.25 250:1:0.05 250:1:0.025 250:1:0.25 250:1:0.05 250:1:0.025 250:1:0.25 250:1:0.125

96 92 96 95 90 95 70 96 92 89 96 90 92 95 95

24 000 23 000 25 000 25 000 22 000 17 000 22 000 24 000 23 000 25 000 24 000 22 000 22 000 27 350 25 000

1.24 1.19 1.24 1.21 1.28 1.24 1.25 1.24 1.26 1.50 1.24 1.25 1.24 1.25 1.32

[a] Polymerization of styrene was performed by using (1-bromoethyl)benzene as the initiator at 120 8C. Polymerization of MMA or BA was carried out with methyl-2-bromoisobutylate as the initiator at 100 8C.

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Full Paper Catalyst recycling for ATRP with low catalyst concentration and post-polymerization Good catalytic performance for well-controlled ATRP of all three monomers, catalyst recycling, and polymerizations with lower catalyst loadings generated an intriguing idea that catalyst recycling in the reaction with low catalyst concentration might be possible using 4 b as the catalyst. The results are summarized in Table 5. The monomer/initiator/catalyst ratio

Table 5. Catalyst recycling experiments by 4 b at low catalyst loadings.[a] Entry Monomer Recycle t [h] Conv. [%] Mn (exptl) Mn (calcd) Mw/Mn 1 2 3 4 5 6 7 8 9

styrene

MMA

BA

first 6 second 6 third 6 first 1 second 1 third 1 first 20 second 20 third 20

96 > 95 > 95 > 95 90 92 75 > 95 > 95

25 000 27 000 24 000 24 000 27 000 27 000 24 000 27 000 24 000

24 000 23 000 25 000 25 000 22 000 23 000 25 000 23 000 25 000

1.25 1.26 1.42 1.24 1.25 1.24 1.21 3.01 3.12

[a] Ratio of [monomer]/[initiator]/[catalyst] = 250:1:0.25; I/C = 4. Polymerization of styrene was performed by using (1-bromoethyl)benzene as the initiator at 120 8C. Polymerization of MMA or BA was carried out with methyl-2-bromoisobutylate as the initiator at 100 8C.

tection limit of the instrument, which corresponded to 0.28 ppm. This clearly suggests that the high catalytic activity of 4 b associated with the washable property by methanol resulted in substantial removal of the iron residues. Post-polymerization reactions are important for providing evidence that a halogen atom is bonded at the polymer end. In typical examples, we carried out two experiments for the preparation of pSt-b-pMMA by using 4 b as the catalyst according to the following procedure. The pre-polymerization was performed with styrene using (1-bromoethyl)benzene as the initiator at 120 8C for 4 h (styrene/(1-bromoethyl)benzene/4 b ratio of 100:1:1; I/C = 1). After the reaction was over, the polystyrene obtained was dissolved in a small amount of toluene. Post-polymerization of MMA (the ratio of polystyrene formed/ MMA = 1:150) was performed at 100 8C for 4 h. We also carried out block copolymerization with a lower catalyst concentration (I/C = 4). Both of the experiments gave satisfactory results. Conversion of the monomer or the pre-polymer reached > 95 %, and the molecular-weight distribution of the obtained block copolymer was 1.20–1.25. After the reaction, the catalyst was easily removed from the crude copolymer by precipitation into MeOH. Details are described in the Supporting Information.

Mechanistic considerations A general mechanistic scheme for ATRP is shown in Scheme 1. was fixed to 250:1:0.25 (I/C = 4). Since the catalyst showed One-electron redox of transition metals is involved in the catasome sensitivity towards oxygen, careful manipulation was lytic cycle, and chain extension is promoted by radical reaction. necessary, particularly at the catalyst recovery stage. NevertheATRP is a type of reversible deactivation radical polymerization less, the catalyst was recycled three times with no significant in which the metal species behaves as a halogen atom carrier, change in the rate or the molecular-weight distribution of the and reversible activation of the carbon halogen bond at the polymer in ATRP of styrene and MMA. Since ATRP of BA is polymer end is one of the keys to achieving controlled polymore difficult to control than styrene and MMA, well-controlled ATRP of BA was not achieved in the second and third recycling experiments as shown in Table 5, entries 8 and 9. As reported previously,[14b] removal of the iron residue from the crude polymer by precipitation from methanol is easier in ATRP with lower catalyst concentration than in that using one molar amount of the catalyst to the initiator. We examined the polymerization of styrene with the monomer/initiator/catalyst (4 b) ratio of 250:1:0.05. The initial content of iron was 117 ppm with respect to the charged styrene monomer. The crude polymer was precipitated from methanol and the resulting polystyrene was analyzed by ICPMS. The iron content was below the de- Scheme 1. General scheme for the mechanism of ATRP and the catalytically active [(R3TACN)FeX2]. Chem. Eur. J. 2014, 20, 5802 – 5814

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Full Paper merization. The equilibrium (kdeact versus kact) controls the concentration of active radical species at the polymer end to a certain level, such that the molecular weight of the polymer increases linearly with monomer conversion. In the ATRP by [(R3TACN)FeX2], catalytically active iron(II) species require a vacant site for coordination of X of the C X bond in the initiator or at the polymer end. The iron(II) species responsible for catalysis should be coordinatively unsaturated. Mononuclear 16-electron [(R3TACN)FeX2] compounds 5–7 are the catalytically active iron(II) species. This is supported Scheme 2. Possible equilibrium between washable dinuclear and catalytically active by rapid and well-controlled ATRP of styrene and mononuclear species. MMA by these catalysts (Table 1, entries 7–12 and 19–24) and the reaction profile of ATRP of styrene catalyzed by 7 a or 7 b with no induction period (Figure 5). tion (Scheme 2). In other words, the equilibrium provides a sufSince the redox potential of the mononuclear complexes is in ficient amount of catalytically active mononuclear species to the range in which efficient activation of a C X bond occurs,[3] the reaction medium, whereas it gives ionic dinuclear species they are active enough to carry out the reaction with low catathat are favorable for the catalyst separation during the lyst loadings. In contrast, in complexes with ionic dinuclear workup process. As described in the section on the molecular moieties, typically 2 and 3, the iron centers are coordinatively structures, the steric bulkiness of the R group dictates that the saturated. It is likely that a mononuclear 16-electron structure of [(R3TACN)FeX2] be either ionic dinuclear or neutral mononuclear. The medium-sized cyclopentyl group is small [(R3TACN)FeX2] species is generated from the di- or trinuclear enough to adopt the dinuclear structure; however, significant species by cleavage of the Fe (m2-X) linkage in [(R3TACN)Fe(m2steric repulsion between the two (cyclopentyl)TACN ligands reX)3Fe(R3TACN)] + in the reaction medium, and these species catalyze the reaction. Since smaller R groups tend to form sults in easy cleavage of the Fe (m2-X) Fe bonds to form catamore stable [(R3TACN)Fe(m2-X)3Fe(R3TACN)] + moieties, formalytically active species. As shown in Figure 5, the rate was decreased in the order 7 a = 7 b > 4 a > 4 b @ 2 a @ 2 b. One extion of the [(R3TACN)FeX2] species from 2 does not occur planation for this trend is the amount of active species in solusmoothly.[27] The induction period seen in ATRP of styrene by 2 tion, which is equal to the charged amount of the catalyst in shown in Figure 5 indicates the slow generation of the catalytithe rapid reactions of 7 a and 7 b, whereas relatively small cally active species, which suggests that insufficient amounts amounts of active species are formed by decomposition of of active species are supplied in the initial stage of the reaction multinuclear species in the slow reactions with 2 a and 2 b. The medium. This explains the slower ATRP of styrene and MMA moderate rate of 4 a and 4 b might be due to the existence of (Table 1, entries 1, 2, 13, and 14) and the poorly controlled an equilibrium that generates relatively large amounts of polymerization of MMA (Table 1, entries 13 and 14), because an active species. The iron catalysts reported in the literature so insufficient amount of the catalyst species slows down the refar did not achieve well-controlled ATRP of acrylates.[16] In this action and cannot suppress the free radical chain extension that occurs concomitantly. Easy separation of the iron species paper, we found that a combination of [(R3TACN)FeBr2] and from the polymer by precipitation into methanol can be exbromo initiators was a good catalyst system for ATRP of styplained if we assume that the ionic dinuclear precatalyst is rerene, MMA, and BA. It is clear that the FeBr2 catalyst and the formed upon workup.[27] This is reasonable considering the bromo initiator provides significantly larger kdeact than kact in mechanism of formation of ionic complexes with a cationic dithe equilibrium shown in Scheme 1. Further investigations are iron moiety from FeX2 and R3TACN; the initial species formed required for an explanation of the special role of bromine in iron-catalyzed ATRP. should be mononuclear [(R3TACN)FeX2]. Supporting evidence for reformation of the ionic di-iron species from the mononuclear active species was obtained from 1H NMR spectroscopy. Conclusion After ATRP of styrene with 2 a and subsequent workup with methanol, the iron species was recovered from the methanol As described in the Introduction, the goals of ATRP studies are solution. The 1H NMR spectrum of the recovered iron species to develop highly active catalysts that realize the well-controlled polymerization of various monomers. It is also imporwas in agreement with that of 2 a.[14a] tant that the product is obtained without contamination by Interestingly, the cyclopentyl–TACN complex 4 b achieved the metal residue in the product, because purification of polyboth a high catalytic activity and easy separation. As described mers is more difficult than that of organic molecules with low above, X-ray crystallography and ESI-MS suggest that both molecular weights. As described above, we successfully solved mono- and dinuclear structures of 4 b exist in the solution these issues by rational catalyst design of [(R3TACN)FeX2] comstate. If we assume that 4 b exists as an equilibrium mixture of mononuclear and dinuclear species in solution,[28, 29] the situaplexes, in which the appropriate steric bulkiness of the R group controls the reversibility between an ionic dinuclear and tion is favorable for both catalytic activity and catalyst separaChem. Eur. J. 2014, 20, 5802 – 5814

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Full Paper a neutral mononuclear species. A series of [(R3TACN)FeX2] complexes was prepared and characterized by X-ray crystallography, ESI-MS, and cyclic voltammetry. The ionic dinuclear species contributes to the easy separation of the catalyst from the polymer, whereas the mononuclear species is active enough to allow the catalyst concentration to be decreased. As a result of polymerization studies, we found that [{(cyclopentyl)3TACN}FeBr2] (4 b) is the best catalyst for the ATRP of styrene, MMA, and BA. By using 4 b carefully purified by recrystallization, ATRP with low catalyst loadings was realized as well as easy removal of the iron residue by precipitation into methanol, thereby leading to the production of polymers that contained only a negligible amount of iron (< 0.28 ppm). There is no precedence for which high catalytic activity and solubility control of the metal species are achieved simply by using the reversibility between the ionic dinuclear and the neutral mononuclear structure of the catalyst. We achieved this by reversible cleavage of the Fe (m2-X) Fe linkage, which is a new concept for catalyst design. It should also be pointed out that paramagnetism of iron compounds prevents their effective characterization by NMR spectroscopy.[23] Preparation of a series of [(R3TACN)FeX2] complexes followed by characterization with a combination of crystallography and spectroscopy provided one successful example of the catalyst design of paramagnetic compounds, thereby providing a solid methodology for the research of iron-catalyzed reactions. We consider the present report to be the first step toward catalyst evolution of [(R3TACN)FeX2], and further studies are in progress.

X-ray data collection and reduction X-ray crystallography was performed using a Rigaku Saturn CCD area detector with graphite-monochromated MoKa radiation (l = 0.71070 A). The detailed crystallographic data are summarized in the Supporting Information. CCDC-967810 (2 b), -967811 (3 a), -967812 (3 b), -967814 (4 a-neutral), -967813 (4 a-ionic), -967815 (4 b), -967816 (5 a), -967817 (5 b), -967818 (6 a), -967809 (6 b), -720058 (7 a), and -720057 (7 b) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

Synthesis of the TACN ligands The TACN ligands presented in this paper were prepared according to the following procedure. In a 30 mL round-bottomed flask, triazacyclononane (230 mg, 1.8 mmol), alkyl bromide (5.4 mmol), and K2CO3 (830 mg, 6.0 mmol) were suspended in acetonitrile (5 mL), and the mixture was heated under reflux conditions for 16 h. The resulting mixture was then filtered, and the filtrate was concentrated under vacuum. The obtained product was dissolved in Et2O (10 mL) and washed with H2O (3  5 mL). The organic layer was collected and dried over MgSO4. After removing the solvent under vacuum, the desired product was obtained as a slightly yellow oil. Three TACN compounds—1,4,7-trimethyl-1,4,7-triazacyclononane (Me3TACN) (1 a),[30] 1,4,7-tri(4-methoxybenzyl)-1,4,7-triazacyclononane [(MeOC6H4CH2)3TACN] (1 b),[31] and 1,4,7-triisopropyl-1,4,7-triazacyclononane (iPr3TACN) (1 f)[32]—were identified by the spectroscopic data reported in the literature. The representative spectroscopic data of 1,4,7-tricyclopentyl-1,4,7-triazacyclononane (1 c) is described below.

1,4,7-Tricyclopentyl-1,4,7-triazacyclononane (1 c)

Experimental Section General Commercially available compounds of butyl acrylate (BA), methyl methacrylate (MMA), styrene, (1-chloroethyl)benzene, (1-bromoethyl)benzene, and methyl 2-bromoisobutylate were dried over CaH2 and distilled under reduced pressure. Anhydrous solvents were purchased from Kanto Chemical and used without further purification. All of these chemicals were degassed just prior to use. 1H and 13C NMR spectra were recorded using a JEOL Lambda 600 or a Lambda 400 spectrometer at ambient temperature unless otherwise noted. 1H and 13C NMR spectroscopic chemical shifts (d = values) were given in ppm relative to the solvent signal (for 1H: CDCl3, d = 7.26 ppm; CD3CN, d = 1.94 ppm; and for 13C: CDCl3, d = 77.16 ppm; CD3CN, d = 118.20 ppm). Elemental analysis was obtained using a Perkin–Elmer EA2400. ESI-MS spectra were collected using a JEOL JMS-T100CS. Actual charts are shown in Figures S4MS–S15-MS of the Supporting Information. Melting points were measured using a Yanaco SMP3 micro melting-point apparatus. Electrochemical experiments were performed using a CV-50 W voltammetric analyzer. Cyclic voltammograms of each compound are shown in Figures S4-CV–S9-CV and S14-CV–S15-CV of the Supporting Information, and the summary is shown in Figure S16 of the Supporting Information. The molecular weight and the molecularweight distribution of the obtained polymer were determined by SEC analysis using a Shodex KF-804L and KF805L apparatus (elution: THF) at 45 8C. The ICPMS measurement was performed using a Shimadzu ICPM 8500 spectrometer for which the detection limit of the Fe was estimated to be below 0.28 ppm. Chem. Eur. J. 2014, 20, 5802 – 5814

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Yield: 40 %; 1H NMR (400 MHz, [D]CHCl3, RT): d = 2.98 (quint, 3 J(H,H) = 8.24 Hz, 3 H; NCH), 2.76 (s, 12 H; NCH2), 1.77–1.68, 1.64– 1.54, 1.51–1.41, and 1.37–1.27 ppm (m, 6 H each; CH2 of the cyclopentyl ring); 13C NMR (125 MHz, [D]CHCl3, RT): d = 66.7 (NCH), 53.4 (NCH2), 29.7, 24.0 ppm (CH2 of the cyclopentyl ring); HRMS (EI, 70 eV): m/z calcd for C21H39N3 : 333.3144; found: 333.3142.

General procedure for the preparation of [(TACN)FeX2] complexes Solutions of TACN derivatives (1.88 mmol) in THF (1 mL) were added to a suspension of FeCl2 or FeBr2 (1.81 mmol) in acetonitrile or CH2Cl2 (19 mL), then the reaction mixture was stirred for 4 h at room temperature. The resulting mixture was filtered through a pad of Celite, and the filtrate was concentrated under vacuum to give a white powder. The crude product was dissolved in CH2Cl2 (10 mL) and layered with hexane (30 mL), from which crystals suitable for X-ray crystallographic analysis were obtained. The obtained crystals were washed three times with diethyl ether (3 mL) and dried under vacuum. The representative spectroscopic data of 4 b are described below.

[{(Cyclopentyl)3TACN}FeBr2] (4 b) Yield: 703 mg, 71 %. X-ray structural determination revealed a coordinatively unsaturated mononuclear structure, [{(cyclopentyl)3TACN}FeBr2] (4 b-neutral). In contrast, the ESI-MS spectrum showed a parent peak that corresponded to [{(cyclopentyl)3TACN}Fe(m-Br)3Fe{(cyclopentyl)3TACN}] + (4 b-ionic): m.p.

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Full Paper 391.6 8C (decomp); ESI-MS (CH2Cl2, RT, under standard conditions): m/z: 1013.59, 1014.59, 1015.57, 1016.58, 1017.56, 1018.57, 1019.57, 1020.57, 1021.57, 1022.58, 1023.57 (C42H78N6Fe2Br3 ; a parent peak due to 2 a; 100 %), 466.31, 467.30, 468.30, 469.31, 470.30, 471.31, 472.29 ([C21H39N3FeBr]; a fragment peak due to {(cyclopentyl)3TACNFeBr} + ); elemental analysis calcd (%) for C21H39N3Br2Fe1: C 45.93, H 7.16, N 7.65; found: C 45.69, H 7.42, N 7.69.

Polymerization mediated by [(TACN)FeX2] complexes A general procedure for the polymerization is as follows. In an N2filled glovebox, the catalyst was placed in a glass tube fitted with a glass stopper, to which were added a monomer (10 mmol), benzyl methyl ether (1.0 mmol; internal standard to determine monomer conversion), and an initiator (0.04 mmol). The tube was sealed by the stopper and heated with stirring. Conversion of the monomer was determined by 1H NMR spectroscopy. After the reaction was over, the sample was purified by precipitation from methanol, and the polymer obtained was subjected to SEC analyses for the determination of Mn and Mw/Mn. Under the standard conditions of polymerization reactions, the ratio of [monomer]/[Fe]/[initiator] was determined to be 250:1:1. For polymerization of styrene catalyzed by [(TACN)FeCl2] derivatives, reactions were performed at 120 8C by using (1-chloroethyl)benzene as the initiator. For MMA and BA polymerizations mediated by [(TACN)FeCl2] derivatives, methyl trichloroacetate was used as the initiator, and reactions were performed at 100 8C. For polymerization of styrene catalyzed by [(TACN)FeBr2] derivatives, reactions were performed at 100 8C by using (1-bromoethyl)benzene as the initiator. For MMA and BA polymerizations mediated by [(TACN)FeBr2] derivatives, methyl 2bromoisobutylate was used as the initiator, and reactions were performed at 100 8C.

Catalyst recycling experiments by 4 b with low catalyst loadings In a glovebox, catalyst 4 b (20.8 mg, 0.038 mmol) was dissolved in CH2Cl2 (1 mL). The catalyst solution (0.25 mL) was transferred to a 20 mL Schlenk tube and concentrated. A monomer (10 mmol) and an initiator (0.04 mmol) were added, and the reaction mixture was heated to 120 (ATRP of styrene) or 100 8C (ATRP of MMA and BA). After the reaction was over, THF (5 mL) was added to dissolve the polymer. The resulting THF solution was poured into methanol (30 mL) under an inert gas atmosphere, and the polystyrene precipitated was separated by filtration. The resulting solution that contained 4 b was transferred into the new reaction vessel, and the solvent was removed under vacuum. The monomer (10 mmol) and the initiator (0.04 mmol) were added, and the second run of polymerization was performed as described above. The sequence of polymerization followed by the catalyst recycling described above was repeated three times.

Acknowledgements This work was supported by the Core Research Evolutional Science and Technology (CREST) program of Japan Science and Technology Agency (JST), and this work was performed under the Cooperative Research Program of “Network Joint Research Center for Materials and Devices”. Support was also provided by a Grant-in-Aid for Scientific Research on Innovative Areas “Stimuli-Responsive Chemical Species for the Creation of Functional Molecules” (no. 25109534) and for Young Scientist (A) (no. 24685011) from the Ministry of Education, Culture, Sports, Science and Technology, Japan. Keywords: catalyst recycling · iron · polymerization · radical reactions · redox chemistry

Block copolymerization by 4 b A mixture of the catalyst 4 b (55 mg, 0.1 mmol), styrene (1.0 mL, 10 mmol), (1-bromoethyl)benzene (18.5 mg, 13.8 mL, 0.1 mmol), and benzyl methyl ether (126 mL, 1 mmol) was heated at 100 8C. After 4 h, the reaction vessel was cooled to room temperature. The resulting polymer was dissolved in toluene (1 mL) in a glovebox. A small portion of this mixture was separated and subjected to 1 H NMR spectroscopy and SEC analyses. MMA (1.5 g, 1.6 mL, 15 mmol) was then added to the toluene solution of the polystyrene, and the glass tube was sealed tightly. The reaction vessel was again heated at 100 8C for 4 h. The reaction mixture was then cooled to room temperature, dissolved with THF (5 mL), and subjected to 1H NMR spectroscopy and SEC analyses.

Polymerization of MMA, styrene, and BA with low catalyst loadings In general, the catalyst (0.038 mmol) was dissolved in CH2Cl2 (1 mL) in the N2-filled glovebox. A portion of the resulting catalyst solution (0.25 mL in the case of I/C = 4) was added to a 20 mL Schlenk tube. The solvent was removed under vacuum. A monomer (10 mmol), an initiator (0.04 mmol), and benzyl methyl ether (126 mL, 1 mmol) were added to the reaction vessel that contained the catalyst. The obtained reaction mixture was heated at 100 8C. The resulting polymer was dissolved in THF (5 mL) and subjected to 1H NMR spectroscopy and SEC analysis. Chem. Eur. J. 2014, 20, 5802 – 5814

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[1] For representative reviews on controlled radical polymerization, see: a) Frontiers in Polymer Chemistry (Ed.: V. Percec), Chem. Rev. 2001, 101, issue 12; b) V. Sciannamea, R. Jrçme, C. Detrembleur, Chem. Rev. 2008, 108, 1104 – 1126; c) P. B. Zetterlund, Y. Kagawa, M. Okubo, Chem. Rev. 2008, 108, 3747 – 3794; d) B. M. Rosen, V. Percec, Chem. Rev. 2009, 109, 5069 – 5119. [2] For representative original reports and accounts on ATRP, see: a) M. Kato, M. Kamigaito, M. Sawamoto, T. Higashimura, Macromolecules 1995, 28, 1721 – 1723; b) J. S. Wang, K. Matyjaszewski, J. Am. Chem. Soc. 1995, 117, 5614 – 5615; c) V. Percec, B. Barboiu, Macromolecules 1995, 28, 7970 – 7972; d) C. Granel, P. Dubois, R. Jrme, P. Teyssi, Macromolecules 1996, 29, 8576 – 8582; e) D. M. Haddleton, C. B. Jasieczek, M. J. Hannon, A. J. Shooter, Macromolecules 1997, 30, 2190 – 2193; f) T. Patten, K. Matyjaszewski, Acc. Chem. Res. 1999, 32, 895 – 903; g) M. Sawamoto, T. Ando, M. Kamigaito, Chem. Rec. 2004, 4, 159 – 175; h) M. Ouchi, T. Terashima, M. Sawamoto, Acc. Chem. Res. 2008, 41, 1120 – 1132. [3] For representative general reviews on ATRP, see: a) K. Matyjaszewski, J. Xia, Chem. Rev. 2001, 101, 2921 – 2990; b) M. Kamigaito, T. Ando, M. Sawamoto, Chem. Rev. 2001, 101, 3689 – 3745; c) V. Coessens, T. Pintauer, K. Matyjaszewski, Prog. Polym. Sci. 2001, 26, 337 – 377; d) N. V. Tsarevsky, K. Matyjaszewski, Chem. Rev. 2007, 107, 2270 – 2299; e) W. A. Braunecker, K. Matyjaszewski, Prog. Polym. Sci. 2007, 32, 93 – 146; f) M. Ouchi, T. Terashima, M. Sawamoto, Chem. Rev. 2009, 109, 4963 – 5050; g) F. di Lena, K. Matyjaszewski, Prog. Polym. Sci. 2010, 35, 959 – 1021; h) M. Kamigaito, Polym. J. 2011, 43, 105 – 120; i) K. Matyjaszewski, Macromolecules 2012, 45, 4015 – 4039; j) For large scale reactions see: N. Chan, J. Meuldijk, M. F. Cunningham, R. A. Hutchinson, Ind. Eng. Chem. Res. 2013, 52, 11931 – 11942.  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper [4] Reviews of ATRP for materials science: R. Barbey, L. Lavanant, D. Paripovic, N. Schwer, C. Sugnaux, S. Tugulu, H.-A. Klok, Chem. Rev. 2009, 109, 5437 – 5527. [5] Reviews for green ATRP (catalyst immobilization): a) Y. Shen, H. Tang, S. Ding, Prog. Polym. Sci. 2004, 29, 1053 – 1078; b) S. Faucher, S. Zhu, J. Polym. Sci. Part A 2007, 45, 553 – 565. [6] Reviews of green ATRP including lowering the catalyst loadings: a) T. Pintauer, K. Matyjaszewski, Chem. Soc. Rev. 2008, 37, 1087 – 1097; b) Q. Lou, D. A. Shipp, ChemPhysChem 2012, 13, 3257 – 3261; c) L. Bai. L. Zhang, Z. Chen, X. Zhu, Polym. Chem. 2012, 3, 2685 – 2697. [7] For example, levels of copper below 10 ppm affected the stability of certain polymers, see: A. Nese, S. S. Sheiko, K. Matyjaszewski, Euro. Poly. J. 2011, 47, 1198 – 1202. [8] Highly active copper catalysts: a) V. Percec, T. Guliashvili, J. S. Ladislaw, A. Wistrand, A. Stjerndahl, M. J. Sienkowska, M. J. Monteiro, S. Sahoo, J. Am. Chem. Soc. 2006, 128, 14156 – 14165; b) H. Tang, N. Arulsamy, M. Radosz, Y. Shen, N. V. Tsarevsky, W. A. Braunecker, W. Tang, K. Matyjaszewski, J. Am. Chem. Soc. 2006, 128, 16277 – 16285. [9] Recent progress in active catalysts for ARGET: a) L. Zhang, H. Tang, J. Tang, Y. Shen, L. Meng, M. Radosz, N. Arulsamy, Macromolecules 2009, 42, 4531 – 4538; b) K. Schrçder, R. T. Mathers, J. Buback, D. Konkolewicz, A. J. D. Magenau, K. Matyjaszewski, ACS Macro Lett. 2012, 1, 1037 – 1040; c) A. J. D. Magenau, Y. Kwak, K. Schrçder, K. Matyjaszewski, ACS Macro Lett. 2012, 1, 508 – 512. [10] Reviews for mechanistic aspects for copper-catalyzed ATRP from coordination chemistry: a) T. Pintauer, K. Matyjaszewski, Coord. Chem. Rev. 2005, 249, 1155 – 1184; b) W. A. Braunecker, K. Matyjaszewski, J. Mol. Catal. A 2006, 254, 155 – 164; see also the following papers and references cited therein: c) S. Perrier, D. Berthier, I. Willoughby, D. Batt-Coutrot, D. M. Haddleton, Macromolecules 2002, 35, 2941 – 2948; d) J. M. MuÇozMolina, T. R. Belderran, P. J. Prez, Macromolecules 2010, 43, 3221 – 3227; e) S. A. Turner, Z. D. Remillard, D. T. Gijima, E. Gao, R. D. Pike, C. Goh, Inorg. Chem. 2012, 51, 10762 – 10763. [11] Iron-catalyzed reactions in organic synthesis: a) C. Bolm, J. Legros, J. Le Paih, L. Zani, Chem. Rev. 2004, 104, 6217; b) B. Plietker, Iron Catalysis in Organic Chemistry, Wiley-VCH, Wienheim, Germany, 2008; c) Iron Catalysis: Fundamentals and Applications (Ed.: B. Plietker), Springer, Berlin, 2010; d) A. Thayer, Chem. Eng. News 2005, 83, 40 – 58; see also the Committee for Medicinal Products for Human Use (CHMP), European Medicines Agency, Guideline on the Specification Limits for Residues of Metal, EMEA/CHMP/SWP/4446/2000, http://www.ema.europa.eu/ema/ 2008. [12] For representative iron catalysts, also see the references cited in these articles and recent reviews (refs. [3d] and [6c]): a) T. Ando, M. Kamigaito, M. Sawamoto, Macromolecules 1997, 30, 4507 – 4510; b) Y. Kotani, M. Kamigaito, M. Sawamoto, Macromolecules 1999, 32, 6877 – 6880; c) M. Teodorescu, S. G. Gaynor, K. Matyjaszewski, Macromolecules 2000, 33, 2335 – 2339; d) B. Gçbelt, K. Matyjaszewski, Macromol. Chem. Phys. 2000, 201, 1619 – 1624; e) Y. Wang, K. Matyjaszewski, Macromolecules 2010, 43, 4003 – 4005; f) Y. Wang, K. Matyjaszewski, Macromolecules 2011, 44, 1226 – 1228; g) Y. Wang, Y. Kwak, K. Matyjaszewski, Macromolecules 2012, 45, 5911 – 5915; h) K. Nishizawa, M. Ouchi, M. Sawamoto, Macromolecules 2013, 46, 3342 – 3349; i) M. Y. Khan, X. Chen, S. W. Lee, S. K. Noh, Macromol. Rapid Commun. 2013, 34, 1225 – 1230. [13] Well-defined iron catalysts that bear nitrogen-containing ligands: a) V. C. Gibson, R. K. O’Reilly, W. Reed, D. F. Wass, A. J. P. White, D. J. Williams, Chem. Commun. 2002, 1850 – 1851; b) V. C. Gibson, R. K. O’Reilly, D. F. Wass, A. J. P. White, D. J. William, Macromolecules 2003, 36, 2591 – 2593; c) R. K. O’Reilly, V. C. Gibson, A. J. P. White, D. J. Williams, J. Am. Chem. Soc. 2003, 125, 8450 – 8451; d) V. C. Gibson, R. K. O’Reilly, D. F. Wass, A. J. P. White, D. J. Williams, Dalton Trans. 2003, 2824 – 2830; e) R. K. O’Reilly, V. C. Gibson, A. J. P. White, D. J. Williams, Polyhedron 2004, 23, 2921 – 2928; f) M. P. Shaver, L. E. N. Allan, H. S. Rzepa, V. C. Gibson, Angew. Chem. 2006, 118, 1263 – 1266; Angew. Chem. Int. Ed. 2006, 45, 1241 – 1244; g) R. K. O’Reilly, M. P. Shaver, V. C. Gibson, A. J. P. White, Macromolecules 2007, 40, 7441 – 7452; h) L. E. N. Allan, M. P. Shaver, A. J. P. White, V. C. Gibson, Inorg. Chem. 2007, 46, 8963 – 8970; i) M. P. Shaver, L. E. N. Allan, V. C. Gibson, Organometallics 2007, 26, 4725 – 4730; j) R. Ferro, S. Milione, V. Bertolasi, C. Capacchione, A. Grassi, Macromolecules 2007, 40, 8544 – 8546; k) L. E. N. Allan, J. P. MacDonald, Chem. Eur. J. 2014, 20, 5802 – 5814

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A. M. Reckling, C. M. Kozak, M. P. Shaver, Macromol. Rapid Commun. 2012, 33, 414 – 418. a) S. Niibayashi, H. Hayakawa, R.-J. Jin, H. Nagashima, Chem. Commun. 2007, 1855 – 1857; b) M. Kawamura, Y. Sunada, H. Kai, N. Koike, A. Hamada, H. Hayakawa, R.-H. Jin, H. Nagashima, Adv. Synth. Catal. 2009, 351, 2086 – 2090. a) J.-S. Wang, K. Matyjaszewski, Macromolecules 1995, 28, 7901 – 7910; b) For recent examples, see the following papers and reviews (ref. [3]): ref. [12f]. a) ATRP of methyl acrylate catalyzed by iron–diimine complexes was slow, and the molecular-weight distributions were 1.5–1.6; b) ATRP of decyl acrylate with CBr4/FeCl3/2,2’-bipyridine/azobisisobutyronitrile (AIBN) or benzoyl peroxide (BPO) in DMF was reported, in which the molecular weight of the formed polymer was not in accordance with the theoretical value calculated from the ratio of the monomer to the initiator: P. J. Saikia, A. K. Hazarika, S. D. Baruah, Polym. Bull. 2013, 70, 1483 – 1498. Iron-based ARGET and ICAR: a) R. Luo, A. Sen, Macromolecules 2008, 41, 4514 – 4518; b) L. Zhang, Z. Cheng, S. Shi, Q. H. Li, X. Zhu, Polymer 2008, 49, 3054 – 3059; c) L. Zhang, Z. Cheng, Y. L, X. Zhu, Macromol. Rapid Commun. 2009, 30, 543 – 547; d) L. Bai, L. Zhang, Z. Zhang, Y. Tu, N. Zhou, Z. Cheng, X. Zhu, Macromolecules 2010, 43, 9283 – 9290; e) L. Zhang, J. Miao, Z. Cheng, X. Zhu, Macromol. Rapid Commun. 2010, 31, 275 – 280; f) Y. Wang, Y. Zhang, B. Parker, K. Matyjaszewski, Macromolecules 2011, 44, 4022 – 4025; g) H. Jiang, L. Zhang, J. Pan, X. Jiang, Z. Cheng, X. Zhu, Polym. Chem. 2012, 50, 2244 – 2253. a) G. Zhu, L. Zhang, Z. Zhang, J. Zhu, Y. Tu, Z. Cheng, X. Zhu, Macromolecules 2011, 44, 3233 – 3239; b) see also ref. [17f]; c) K. Mukumoto, Y. Wang, K. Matyjaszewski, ACS Macro Lett. 2012, 1, 599 – 602. a) M. Ishio, M. Katsube, M. Ouchi, M. Sawamoto, Y. Inoue, Macromolecules 2009, 42, 188 – 193; b) A. Kanazawa, K. Satoh, M. Kamigaito, Macromolecules 2011, 44, 1927 – 1933. A. C. Moreland, T. B. Rauchfuss, Inorg. Chem. 2000, 39, 3029 – 3036. M. Jimnez-Tenorio, M. C. Puerta, P. Valerga, Eur. J. Inorg. Chem. 2004, 17 – 32. F. Calderazzo, U. Englert, G. Pampaloni, E. Vanni, C. R. Acad. Sci. Paris, t.2, Srie II c 1999, 311 – 319. B. E. Mann, Prop. Inorg. Organomet. Compd. 1997, 30, 1 – 209. a) R. Colton, A. D’Agostino, J. C. Traeger, Mass Spectrom. Rev. 1995, 14, 79 – 106; b) W. Henderson, B. K. Nicholson, L. J. McCaffrey, Polyhedron 1998, 17, 4291 – 4313; c) J. C. Traeger, Int. J. Mass Spectrom. 2000, 200, 387 – 401. The fragmentation signals are sensitive to the cone voltage. As was described in the text, neutral mononuclear complexes [(R3TACN)FeX2] gave no visible peaks under the standard conditions When a higher cone voltage was applied, abstraction of X from [(R3TACN)FeX2] took place in the mass chamber to afford signals that arose from [(R3TACN)FeCl] + with well-resolved characteristic isotope patterns. See details in the Supporting Information. It is noteworthy that a C X bond exists at the polymer end even in the case, in which controllability of ATRP was not good. As was reported previously, 2 a catalyzed the controlled ATRP of styrene, whereas the controllability of MMA and BA polymerization was poor. We carried out block copolymerization catalyzed by 2 a, in which the pre-polymerization of monomer A was followed by post-polymerization with monomer B. Six combinations of A and B out of three monomers—styrene, MMA, and BA—were prepared and subjected to copolymerization. In all cases, conversion of monomer A in the pre-polymerization and that of pre-polymer poly-A in the post-polymerization reached > 90 % and the corresponding block copolymers, (polymer of A)-b-(polymer of B), were formed (details are given in the Supporting Information). The bromine bridge in 2 b was actually cleaved by the solvent. The mononuclear complex stabilized by acetonitrile, [(Me3TACN)FeBr2(NCMe)], was isolated from a solution of 2 b in MeCN and characterized. M. Kawamura, S. Nakanishi, Y. Sunada, H. Nagashima, unpublished. In research on diamagnetic complexes, NMR spectroscopy is useful for obtaining direct evidence for equilibrium. In the cases when two species equilibrate in solution, both species could be detected by 1H NMR spectroscopy. Only a limited number of reactions that involve equilibria between two paramagnetic species have been analyzed by NMR spectroscopy in the cases when all of the signals were visible over a broad  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper region and the equilibrium caused substantial change in the chemical shifts (see also ref. [29]). In the case of [(TACN)FeX2], attempts to detect two species, cationic dinuclear and neutral mononuclear, by NMR spectroscopy have been unsuccessful so far. Attempts to detect the two species by UV-visible spectroscopy were also unsuccessful. [29] M. Grau, J. England, R. T. M. de Rosales, H. S. Rzepa, A. J. P. White, G. J. P. Britovsek, Inorg. Chem. 2013, 52, 11867 – 11874. [30] L. Wang, C. Wang, R. Bau, T. C. Flood, Organometallics 1996, 15, 491 – 498.

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[31] S. Mahapatra, J. A. Halfen, W. B. Tolman, J. Am. Chem. Soc. 1996, 118, 11575 – 11586. [32] J. L. Sessler, J. W. Sibert, V. Lynch, Inorg. Chim. Acta 1994, 216, 89 – 95.

Received: November 23, 2013 Revised: February 4, 2014 Published online on March 24, 2014

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