Mechanistic Insight into Catalyst-Transfer Polymerization of Unusual

Sep 20, 2012 - Frederic Monnaie , Ward Ceunen , Julien De Winter , Pascal Gerbaux , Valentina Cocchi , Elisabetta Salatelli , and Guy Koeckelberghs ...
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Mechanistic Insight into Catalyst-Transfer Polymerization of Unusual Anion-Radical Naphthalene Diimide Monomers: An Observation of Ni(0) Intermediates Volodymyr Senkovskyy,* Roman Tkachov, Hartmut Komber, Andreas John, Jens-Uwe Sommer, and Anton Kiriy* Leibniz-Institut für Polymerforschung Dresden e.V., Hohe Straße 6, 01069 Dresden, Germany S Supporting Information *

ABSTRACT: Ni-catalyzed polymerization of anion-radical complexes formed upon mixing of 2,6-bis(2-bromothien-5-yl)naphthalene-1,4,5,8-tetracarboxylic-N,N′-bis(2-octyldodecyl) diimide and activated Zn powder was investigated. We provide experimental evidence that the polymerization involves the chaingrowth mechanism and proceeds via Ni(0) complexes which were previously proposed to be key intermediates in other chain-growth catalyst-transfer polycondensations (CTPs), such as Kumada CTP of thiophenic monomers. DFT calculations predict a remarkable stability of the naphthalene diimide-based Ni(0) complexes. A plausible polymerization mechanism is proposed.

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INTRODUCTION Conjugated polymers (CPs) are an important class of materials for organic electronics.1 CPs are typically prepared via stepgrowth condensation polymerizations (mostly Stille and Suzuki couplings), which give little control over molecular weight (MW), polydispersity (PDI), or end-groups.2 This often results in batch-to-batch differences and altered material properties, which is undesirable for reproducibility. The development of synthetic routes to polymerize π-conjugated polymers using controlled polymerizations3 is of great importance as they can provide access to conjugated polymers with well-defined structures and all-conjugated block copolymers. Recently, a number of electron-rich polymers such as polythiophenes,3 polyfluorenes,4 polyphenylenes,5 and polypyrroles6 have been synthesized in a controlled manner using nickel-catalyzed Kumada chain-growth catalyst-transfer polymerizations (CTPs). The catalytic cycle of these catalyst-transfer polycondensations involves transmetalation (TM), reductive elimination (RE), and oxidative addition (OA) elementary steps. McCullough et al. suggested that the Ni(0) species formed at the reductive elimination step associates with the nearest thiophene ring, forming an associated pair (Ni(0) complex), then “moves” toward a growing polymer chain end, and finally inserts into the terminal C−Br bond.7,8 Thus, the unique propensity of Ni(0) species to transfer intramolecularly via a “ring walking” process9 is responsible for the observed chaingrowth nature of the polycondensation. Although it is now generally accepted that these Ni(0) complexes are important intermediates in CTPs,10 they were never observed experimentally. © 2012 American Chemical Society

Controlled polymerizations of a few electron-deficient monomers were also reported recently.11 Particularly, we reported a nickel-catalyzed chain-growth polymerization of a highly unusual anion-radical monomer, Br-Ar1-Br/Zn, prepared from 2,6-bis(2-bromothien-5-yl)naphthalene-1,4,5,8-tetracarboxylic-N,N′-bis(2-octyldodecyl) diimide (Br-Ar1-Br) and activated Zn powder.11a The polymerization led to the corresponding naphthalene diimide (NDI)-based conjugated polymer P(TNDIT) with controlled molecular weight (MW) and relatively low polydispersity index, PDI (Scheme 1).12 Preliminary studies suggested that the polymerization follows a chain-growth mechanism and allows a satisfactory control over MW in a broad range of monomer-to-catalyst ratio (feed ratio) (i.e., from 1/10 to 1/70) although the polymerization behavior is still far from being perfectly living. By analogy with other metal-catalyzed chain-growth polycondensations,3 the catalysttransfer mechanism was postulated for polycondensation of BrAr1-Br/Zn11a although mechanistic details remained unclear. In the present work we prove that the polymerization of Br-Ar1Br/Zn involves the intramolecular catalyst-transfer process, similar to one observed previously in the chain-growth polycondensations of electron-rich monomers.3 Furthermore, Ni(0) complexes, which were previously proposed to be key intermediates in chain-growth CTPs, were detected in this work for the first time. Received: August 27, 2012 Revised: September 5, 2012 Published: September 20, 2012 7770

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and it is convenient for NMR studies.3,10,11 Table S1 (Supporting Information) summarizes Mn and Mw (numberaverage and weight-average molecular weight, respectively) values evaluated against polystyrene (PS) standards (this procedure usually gives overestimated values for stiff polymers) as well as more accurate Mn and Mw values estimated against oligo-(TNDIT) standards. Because of relatively high MW of the Ar1 repeat unit (992 g/mol), oligo-(TNDIT)s up to decamers can be clearly resolved by GPC as individual compounds. To determine MWs of higher P(TNDIT) polymers, a “MW vs retention time” dependence obtained for oligo-(TNDIT) was extrapolated to higher MW values. It is noteworthy that relatively high MW products were formed early in the reaction when significant amount of the monomer was present in the polymerization mixture (Figure S1). For example, P(TNDIT) with degree of polymerization DP = 11 (having Mn = 11 800 g/mol against the oligo-(TNDIT) standards and Mn = 38 200 g/mol against the PS standards) was formed at 55% monomer conversion (Table S1). Such behavior is a clear sign of the chain-growth propagation mechanism. However, the formation of some amounts of oligomers during the polymerization (Figure 1) and relatively high polydispersities of the resulting polymers up to 1.6 indicate that it is far from being living. To further investigate the polymerization character, a chainextension experiment was attempted. To this end, the first portion of Br-Ar1-Br/Zn was polymerized in the presence of 0.06 mol % Ni(dppe)Br2. The monomer conversion of about 80% was reached in 4 h of the polymerization (GPC data, Figure S2a−c). Afterward, the second portion of the monomer equal to the first one was added. Several samples of the polymerization mixture were withdrawn at different polymerization time and investigated by GPC. Right after the addition of the second monomer portion, the intensity of the monomer peak was correspondingly increased (compare GPC curves in Figure S2a,b). During the polymerization, the polymer peak gradually increased at the expense of the monomer peak (Figure S2c and Table S2). Importantly, virtually no formation of oligomers was observed. At the same time, only a moderate increase of Mn of the resulting polymer (from 20 200 to 29 100 g/mol, as determined against oligo-(TNDIT) standards) and substantial broadening of PDI (from 1.5 to 2.3) were observed upon the polymerization of the second monomer portion. It is noteworthy that Mn of the resulting polymer can be controlled better in a “single-addition” experiment compared to the “chain-extension” experiment. Hence, the obtained data further demonstrate that the Ni-catalyzed polymerization of Br-Ar1-Br/ Zn involves the chain-growth mechanism; however, it is not living due to important chain-termination and reinitiation side reactions. For comparison, Yamamoto polymerization was also carried out by reacting Br-Ar1-Br with equimolar amounts of Ni(COD)2/bipyridine.16 As seen from GPC data given in Figure S3, the monomer was almost fully consumed already after 10 min of the polymerization; however, only low MW oligomers (predominantly dimers and trimers) were formed at this point. This is a typical step-growth polymerization behavior, and it differs significantly from one observed in the chain-growth polymerization of Br-Ar1-Br/Zn discussed above. As demonstrated previously, many metal-catalyzed polycondensations of AB-type monomers (i.e., having two dissimilar and self-complementary reactive groups) involve a unique intramolecular catalyst-transfer mechanism responsible for the

Scheme 1. Preparation of the Br-Ar1-Br/Zn Radical-Anion Monomer and Its Polymerization

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RESULTS AND DISCUSSION Monomer Structure. Our initial intention was to prepare an organo-zinc monomer Br-Ar1-ZnBr and polymerize it under usual Negishi/Rieke13,15 polymerization conditions.11a To this end, Br-Ar1-Br was mixed with equimolar amounts of activated Zn, which resulted in an immediate color change from redorange (inherent to Br-Ar1-Br) to deep-green and accompanied by dissolution of the solid phase. It is remarkable that the acidic work-up of the thus-prepared Br-Ar1-Br/Zn complex resulted in quantitative recovering of Br-Ar1-Br but not of Br-Ar1-H (Scheme 1). This indicates that both C−Br bonds of Br-Ar1-Br remained intact, and hence Br-Ar1-ZnBr was not formed under these conditions since otherwise hydrolysis of Br-Ar1-ZnBr should lead to Br-Ar1-H. Further investigations reveal a paramagnetic character of the complex, suggesting that electron transfer occurs from Zn to the electron-deficient Br-Ar1-Br which leads to a radical-anion-based complex, Br-Ar1-Br/Zn (Scheme 1).11a It was further found that Br-Ar1-Br/Zn monomer undergoes rapid polymerization at room temperature in the presence of Ni catalysts into the corresponding polymer, Br/H P(TNDIT), as shown in Scheme 1. Chain-Growth Character. GPC data (Figure 1) show consumption of the monomer and accumulation of the polymerization products during the polymerization of Br-Ar1Br/Zn in the presence of 0.04 mol % of Ni(dppe)Br2 (dppe = Ph2P(CH2)2PPh2). Ni(dppe)Br2 was chosen as it suitable catalyst for chain-growth polymerizations of various monomers,

Figure 1. GPC traces of the polymerization mixtures obtained at different reaction time: [Br-Ar1-Br/Zn]/[Ni(dppe)Br2] = 25. 7771

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chain-growth propagation.3 In this case, Ni(0) species formed in the RE step migrates to the chain end and oxidatively adds to the C−Br bond. It was proved for such polymerizations that Ni(0) does so even if much more reactive aryl halides are present in the reaction mixture. Alternatively, the chain-growth polymerization behavior may be achieved in the intermolecular catalyst transfer process, for which a much higher reactivity of the polymer chain end than of the monomer must be provided.14 To elucidate whether the transfer of Ni(dppe) species occurs intramolecularly or intermolecularly, polymerizations of 15 mol parts of Br-Ar1-Br/Zn initiated by 1 equiv of Ni(dppe)Br2 were attempted in the presence of different amounts (0, 2, and 15 equiv) of the monomer precursor Br-Ar1-Br. We assumed that if Ni(dppe) transfers intermolecularly during the polymerization, it should react with added Br-Ar1-Br initiating new chains. In this case, one should expect 2-fold decrease of the polymerization degree in the second experiment compared to the first (control) one. Furthermore, a very low polymerization degree is expected in the third experiment because initial concentrations of Br-Ar1-Br/Zn (monomer) and of Br-Ar1-Br (chain-transfer agent) are equal in that experiment. In reality, we did not find any substantial difference in the three polymerization runs (polymers with DP ∼ 13 were obtained in all cases) which shows that Ni(dppe) undergoes exclusively intramolecular OA. Transmetalation versus Reductive Coupling Steps. The polymerization studied herein has obvious similarities with Rieke13,15 polymerization because both of them utilize Ni complexes as catalysts, aryl dihalides as monomer precursors, and Zn dust as metal source. However, structures of monomers used in Rieke polymerization and in the polymerization studied in this work are different. Monomers in Rieke polymerization are conventional organozinc compounds, X-Th-ZnX (Th = thiophene), in which Zn is inserted between carbon and one of the halogen atoms (X). Coupling of monomer molecules with polymerizing chain end in Rieke polymerization occurs in the TM step as a nucleophilic attack of the monomer Th-ZnX onto electrophilic Ni of Th-Ni(dppe)-X concomitant with the ZnX2 elimination. However, there is no well-defined carbanionic center in Br-Ar1-Br/Zn monomer. Consequently, another process should be responsible for the coupling of Br-Ar1-Br/ Zn monomers with the growing chain. On the other hand, the polymerization studied in this work also shares common features with a kind of Yamamoto16 polymerization which utilizes aryl dihalides as monomers, catalytic amounts of Ni, and reagent amounts of reducing metals (Zn or Mg). An important contribution for understanding of the reductive coupling mechanism (which is the underlying process of the Yamamoto polymerization) was made by Amatore and Jutand.17a When performed under a constant electrochemical reductive driving force, the mechanism of coupling of aryl halides by nickel catalysts was found to proceed through a chain reaction involving Ni(0), Ni(I), Ni(II), and Ni(III) complexes: Ni(dppe) + Ph−Br → Ph−Ni II(dppe)−Br

(1)

Ph−Ni II(dppe)−Br + 1e− → Ph−Ni I(dppe) + Br −

(2)

Ph−Ni I(dppe) + Ar−Br → Ph 2Ni IIIBr(dppe)

(3)

Ph 2Ni IIIBr(dppe) → Ph−Ph + Ni IBr(dppe)

(4)

Ni IBr(dppe) + 1e− → Ni0(dppe) + Br −

(5)

According to Amatore and Jutand, propagation steps include the oxidative addition of PhBr to the electrogenerated unsaturated Ni(dppe) complex (1). The resulting Ph− NiII(dppe)−Br is reduced to Ph−NiI(dppe) via a single electron uptake (2). The latter species undergoes oxidative addition with PhBr to afford the trivalent nickel complex Ph2NiIIIBr(dppe) (3) which reductively eliminates Ph−Ph to yield NiIBr(dppe) (4). The cycle is closed by further single electron transfer to the monovalent nickel which restores the starting zerovalent nickel (5). The cycle is initiated by the twostep reduction of NiII(dppe)Cl2 into Ni(dppe), which readily reacts with bromobenzene. Although this mechanism was established for the electrochemical reductive coupling, it was proposed that chemical reductive coupling (i.e., when Mg or Zn is used as reducing agent) involves a similar mechanism.17b−f As such, we suggest that the Ni-catalyzed reductive coupling process can satisfactory explain the addition of Br-Ar1-Br/Zn monomers to the chain-propagating species Ar-Ni(dppe)-Br in the studied polymerization (Scheme 2). In this process, Zn Scheme 2. Mechanism of Catalyst-Transfer Polymerization of Br-Ar1-Br/Zn

atom associated with the monomer precursor molecule acts as a homogeneous reducing agent (see Scheme 2 of our preceding paper and Scheme S1; see Supporting Information, p S9).11a We further propose that the intramolecular catalyst-transfer process involving Ni(0) intermediates is responsible for the chain-growth propagation behavior observed in the Nicatalyzed polymerization of Br-Ar1-Br/Zn. NMR and Chemical Transformations Studies. In situ NMR studies were performed since such experiments were previously demonstrated to be a powerful tool in the investigation of KCTP.10,18 The aim of these studies was to identify structure of intermediates and their evolution during 7772

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Ni(dppe)-Br prepared by reaction of 1 equiv of Br-Ar1-Br/Zn and Ni(dppe)Br2. The identity of this complex was proved in our previous work by NMR spectroscopy.11a Furthermore, the 31 P NMR spectrum of the polymerization mixture at the end of the polymerization is also similar to the spectrum of another related Ni(II) complex, Ar1-Ni(dppe)-Br, formed upon reaction of Ni(dppe)Br2 with equimolar amounts of Ar1-Br/Zn (see below). We now focus our attention on identification of the second intermediate representing the catalyst-resting state and having in the 31P NMR spectrum signals between 49 and 47 ppm with JPP values in the 25−30 Hz range (Figure 2a). In general, these signals can be attributed either to bis-aryl complex (Ar1)nNi(dppe)-Ar1-Br formed before the RE step or to Ni(0) complex (Ar1)n+1-Br/Ni(dppe) formed right after it (Scheme 2). Complexes of such type were previously suggested to be key intermediates in related catalyst-transfer polycondensations of electron-rich monomers. Analysis of 31P NMR spectra alone, such as shown in Figure 2a, does not allow unambiguously discriminate between the two structures ((Ar1)n-Ni(dppe)-Ar1-Br vs (Ar1)n+1-Br/Ni(dppe)). Analysis of literature also shows that the spectra do not exclude any of the two possibilities.19 For example, as previously reported by McNeil et al.10 and Tkachov et al.,18 bisaryl complexes appear in 31P NMR spectra as two doublets (or multiplets for asymmetric complexes) at 45−50 ppm with coupling constants between ∼9 and 24 Hz for phenylene- and thiophene-based complexes, respectively. On the other hand, 31 P NMR spectra of (η2-naphthalene)(iPr2P(CH2)niPr2P)Ni0 (n = 2, 3) complexes were reported by Benn et al.20 Nonequivalence of the phosphorus atoms was observed in solution with a coupling constant of 36.6 Hz for the propane derivative (n = 3). Thus, the splitting constant of ∼27 Hz observed in our experiments does not contradict with the Ni(0) complex structure. Information useful for identification of the abovediscussed intermediate was obtained in model reactions (Scheme 3). A stable Ni(0) complex was prepared by reaction of Ni(dppe)Br2 with Ar1/Zn (Scheme 3A). In the absence of C−Br bonds, rearrangement via OA of the Ni(0) complex into (Ar1)n-Ni(dppe)-Br is not possible. A reaction was evident by change of the color of the reaction mixture from deep-green to dark-violet. A compound with two doublets at 49.3 and 47.8 ppm (JPP = 28.3 Hz, AB type quartet) was observed in the 31P NMR spectrum (Figure 2b) very similar to the spectrum of the catalyst-resting intermediate observed for polymerization of Br-Ar1-Br/Zn (Figure 2a) in both 31P chemical shifts and JPP with the difference that several overlapping doublets were observed for the polymerization. In all cases 1H signal integration proves an Ar1/dppe ratio of 1:1. The Ar1 unit is nonsymmetric resulting in two signal sets for the thiophene moieties. The COSY spectrum and 1H signal assignments for both thiophene moieties are given in the Supporting Information (Figure S4). The complex formed from Ar1/Zn and Ni(dppe)Br2 was stable in the absence of oxygen and moisture, however, reacted with Ar1-Br, resulting in Ar1Ni(dppe)Br and Ar1 (Scheme 3B). These data suggest that the product of the reaction between Ar1/Zn and Ni(II) complexes is the Ni(0) complex. It forms in a two-electron reduction process in which Ni(II) acts as the oxidizing and Ar1/Zn as the reducing agent and ZnBr2 is the byproduct (Scheme 3A). If Ar1-Br is added to the reaction mixture, Ni(dppe) transfers from Ar1 to Ar1-Br in a reversible process and then undergoes an irreversible intermolecular OA to C-Br, giving Ar1-

polymerization. Adding several equivalents of Br-Ar1-Br/Zn to Ni(dppe)Br2 resulted in the formation of closely located signals in the 31P NMR spectrum between 49 and 47 ppm (Figures 2a).11a The intensity of these signals decreased with polymer-

Figure 2. 31P NMR spectra (AB type quartets) of products obtained by reaction of Ni(dppe)Br2 with Br-Ar1-Br/Zn (a, polymerization), Ar1/Zn (b), NDI(OD)/Zn (c), and NDI(EH)/Zn (d) (solvent: THFd8, 273 K). Four overlapping AB type quartets for (a) are attributed to location of the Ni(0)dppe at different sites along the polymer chain, whereas three pairs of doublets observed for (d) are attributed to location of Ni(0)dppe between ethyl/ethyl, ethyl/butyl, and butyl/ butyl residues of the two chiral 2-ethylhexyl substituents. The diastereomeric effect disappears for (c) because both substituents of the methine carbon are long alkyl chains. (e) 31P NMR spectrum of (Ar1)n-Ni(dppe)-Ar1-Br (solvent: THF-d8, 303 K).

ization time at the expense of two new distal doublets at 61 and 46 ppm (Figure 2e). Since the latter doublets were the only signals after the full consumption of Br-Ar1-Br/Zn, they may be assigned to the propagating chain end (Ar1)n-Ni(dppe)-Br (Scheme 2).11a Such a picture is reminiscent of well-studied polymerizations of electron-rich monomers for which X-ArNi(dppe)-X were complexes onto which Ni catalysts stalled at the end of the polymerization.10 This assignment is in agreement with the structure of related complex Br-Ar17773

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Scheme 3. Model Reactions Highlighting the Formation of Ni0 Complexes and Their Oxidative Addition Reactions

Ni(dppe)Br (Scheme 3B). The same compound can be more straightforwardly obtained from Ar1-Br/Zn and Ni(dppe)Br2 (Scheme 3C). In this case, the reduction of Ni(dppe) by Ar1Br/Zn leads to the Ni(0) complex in which Ni(dppe) is associated with A1-Br; an intramolecular OA leads to Ar1Ni(dppe)Br.11a To allow a comparison with the 13C NMR data reported by Benn et al.20 for the Ni(0) naphthalene compounds, the Zn monomer was further reacted with naphthalene-1,4,5,8tetracarboxylic-N,N′-bis(alkyl) diimide, Ar2, having alkyl = 2ethylhexyl (EH) and alkyl = 2-octyldodecyl (OD) with the latter showing a better solubility in THF. As observed for Ar1, also these naphthalene diimide derivatives react smoothly with activated Zn powder, forming anion-radical complexes Ar2/Zn. The reaction of these complexes with Ni(dppe)Br2 again results in 31P NMR signals in the 48−52 ppm region (Figure 2c,d). The 1H NMR signals of the dppe and Ar2 moiety can be well distinguished, and the Ar2:dppe ratio determined from the 1H NMR spectra is again 1:1 (Figure SI5). Two proton signals for the NDI core at 303 K prove lowered symmetry (Figure 3a). The high-field shift observed for H2/H3 nearby at the η2complexed Ni(0) ligand is in accordance with data reported by Stanger for bis(tributylphosphione)(anthracene)nickel(0).19d The 13C spectrum of (η2-Ar2)(dppe)Ni0 (R = OD) shows broadened signals at 85.8 and 97.7 ppm, indicating remarkable chemical shift effects (Figure 3c). Benn et al.20 reported 84.7 and 91.1 ppm for exchange-averaged signals 1/4 and 2/3 for (η2-naphthalene)(iPr2P(CH2)niPr2P)Ni0 (n = 2, 3) complexes. Taking into account chemical shift effects of the two imide moieties and based on a HSQC spectrum (Figure S6), the NMR data given in Figure 3 confirm for the first time the structure of (η2-Ar2)(dppe)Ni0 (see also Table S3). The 13C signals are averaged signals because of fast exchange of the Ni(dppe) moiety between neighboring cisoid double bonds of the aromatic ring which results also in averaging of the proton

Figure 3. 1H NMR spectra at 303 K (a) and 213 K (b) and 13C NMR spectrum (c, 323 K) of (η2-NDI)(dppe)Ni0 (R = OD) measured in THF-d8. Only the region of aromatic protons and carbons is depicted. Signals of the dppe ligand are not labeled. Signals of naphthalene from monomer synthesis are crossed out.

signals of H2/H3 and H6/H7 (Figure 3a). This exchange is slowed down at lower temperatures, resulting in signal splitting at 213 K (Figure 3b and Figure S7). The 1H EXCY spectrum indicates that there is also Ni(dppe) migration between the two ring of the naphthalene core, but this process is slow on the NMR time scale (Figure S7). Also, temperature-dependent 31P NMR studies (Figures S8 and S9) confirm dynamic processes as reported by Benn et al.20 Figure S9 exemplifies the temperature effect on the 1H and 31P NMR spectra of (η2NDI)(dppe)Ni0 (R = EH). A detailed understanding of the involved processes is beyond the scope of this work. However, Stanger could show for a anthracene Ni(0) complex that the haptotropic rearrangement is intramolecular.19d In summary, our experiments reveal a remarkable stability of naphthalene diimide-based Ni(0) complexes in the absence of arylhalides and further suggest that these complexes could be important intermediates in polymerization of naphthalene diimide-based monomers. DFT Calculations of Ni(0) Complexes.21 DFT calculations22 were performed to theoretically verify formation of 7774

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Figure 3) is an exothermic process with ΔG298 ∼ −60 kJ/mol (or ΔG298 ∼ −150 kJ/mol relative to starting Ni(dppe) and BrAr1-Br compounds). Thus, our calculations confirmed a high stability of naphthalene-diimide-based Ni(0) complexes and feasibility of the ring-walking process of the Ni(dppe) species along πconjugated Br-Ar1-Br system until it undergoes the intramolecular OA reaction into C−Br. With these regards, it may look surprising why parent thiophene-based Ni(0) complexes, which were proposed to be important intermediates in chain-growth polycondensations, where previously not detected.13 We propose that the stability of the Ni(0) complexes studied herein is provided by an electron-deficient nature of the naphthalene diimide-based aromatics and that the corresponding thiophene-based complexes are much less stable in the absence of electronwithdrawing groups. Our calculations showed that indeed, the complex between Ni(dppe) and 2-bromothiophene [(η1-TBr)(dppe)Ni0, Figure S15] has much lower stability (ΔG298 = −36 kJ/mol) than the corresponding NDI-based complexes. Discussion of the Polymerization Mechanism. A plausible polymerization mechanism, consistent with the experimental data and theoretical calculations, is shown in Scheme 2. Chain Initiation. In the first step, the catalyst precursor Ni(dppe)Br2 is reduced by the monomer (Br-Ar1-Br/Zn) in a two-electron transfer process to give the coordination unsaturated Ni(0) complex, Ni(dppe). The latter complex associates with the monomer residue Br-Ar1-Br to give (η2-BrAr1-Br)(dppe)Ni0, which rearranges (via OA of Ni(dppe) to the C−Br bond) into Br-Ar1-Ni(dppe)-Br. The latter species acts as the initiator in the polymerization of Br-Ar1-Br/Zn. The reduction of Ni(dppe)Br2 and the formation of Ni(0) complexes were confirmed by model experiments discussed above. Particularly, near quantitative formation of the Ni(0) complex, (η2-Ar1)(dppe)Ni0, was observed upon reduction of Ni(dppe)Br2 by the monomer analogous, Ar1/Zn, containing no halogen atoms. We also demonstrated that (η2-Ar1)(dppe)Ni0 reacts with arylhalides, e.g., Br-Ar1-Br or Ar1-Br, giving the intermolecular OA products, Br-Ar1-Ni(dppe)-Br and Ar1Ni(dppe)-Br, respectively. Addition of these complexes to BrAr1-Br/Zn induces its polymerization. On the other hand, BrAr1-Ni(dppe)-Br and Ar1-Ni(dppe)-Br were near quantitatively obtained by reacting Ni(dppe)Br2 with stoichometric amounts of Br-Ar1-Br/Zn and Ar1-Br/Zn, respectively.11a These complexes induce polymerization of Br-Ar1-Br/Zn, leading to Br/H or H/H terminated polymer, respectively, confirming the initiation mechanism. Chain Propagation. The chain propagation is based on two major processes: (i) Ni-catalyzed reductive coupling process responsible for the addition of the monomer molecules to growing chains and (ii) intramolecular catalyst transfer which explains the chain-growth propagation behavior. The chain propagation starts with a single-electron redox process between the chain end, (Ar1)n-Ni(dppe)-Br (or the initiator Br-Ar1Ni(dppe)-Br), and the monomer molecule, Br0Ar1-Br/Zn (Scheme S2). Presumably, the reduction leads to Ni(I) species, (Ar1)n-Ni(dppe), which oxidatively adds to the C−Br bond of the anion-radical monomer followed by elimination of bromide anion, leading to (Ar1)n-Ni(dppe)-Ar1-Br. Such a mechanism is analogous to the mechanism of reductive coupling of aryl halides, investigated in by Amatore and Jutand.17a A detailed

naphthalene diimide-based Ni(0) complexes and to shed a light on their structure. To reduce calculation costs, we replaced long-chain substituents in the amide moieties of (η2-Br-Ar1Br)(dppe)Ni0 and (η2-Ar2)(dppe)Ni0 complexes by methyl groups, but we kept unchanged the structure of the phosphorus ligand as it was shown that the nature of ligand strongly affects the stability of Ni(0) complexes.23 We did not aim to explore comprehensively the whole potential energy surface of the Ni(0) complexes and to locate transition states between the complexes. Rather, we sampled several “typical” Ni(0) complexes to estimate their stability by positioning of the Ni center at different locations above the aromatic system (Figure 4 and Figures S10−S13).

Figure 4. Relative energies for selected complexes between Ni(dppe) and 2,6-bis(2-bromothien-5-yl)naphthalene-1,4,5,8-tetracarboxylicN,N′-bis(methyl) diimide (kJ/mol).

The formation of such complexes from Ni(dppe) and Br-Ar1Br (or Ar2) was found to be strongly favored, exothermic processes (with ΔG298 up to −150 kJ/mol). Because Ni(dppe) is the coordination-unsaturated species, more reliable data about stability of the complexes can be obtained if THF (solvent) molecules are included into calculations. Complex formation between Ni(dppe) and one THF molecule was calculated to be an exothermic process with ΔG298 = −28 kJ/ mol. Taking into account these values, more realistic complex formation energies for the (η2-Ar2)(dppe)Ni0 complexes lie in ΔG298 = −53 to −122 kJ/mol range. In all complexes, Ni atom is sandwiched between the aromatic core and the dppe ligand, the latter adopting a “propeller-like” conformation. This relatively strained conformation allows for phenyl rings to form a “crown” above the Ni atom rather than surround it from all sides. This enables a short contact between Ni and the aromatic system (∼1.9 Å) and contributes to the remarkable stability of the complexes. Complex formation energies were found to be dependent on the position of Ni(dppe) relative to the aromatic core. Complexes in which Ni(dppe) is located close to the amide moiety are relatively unstable. The most energetically favored isomer is such in which Ni(dppe) is associated with the naphthalene core (ΔG298 ∼ −122 kJ/mol; here and below ΔG298 values are given for the formation of complexes from Ni(dppe)/THF species). The complex with Ni(dppe) associated with the thiophene ring of Br-Ar1-Br is somewhat less stable (ΔG298 ∼ −90 kJ/ mol). However, the difference in energies between isomeric Ni(0) complexes makes it possible a thermal fluctuation of Ni(dppe) (catalyst ring-walking)24 along π-conjugated system of Br-Ar1-Br. Structure and energy of the oxidative addition product were also calculated (Figure S14).25 The formation of Br-Ar1-Ni(dppe)-Br from Ni(0) complex in which Ni(dppe) is associated with the thiophene ring next to the C−Br bond (see 7775

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diimide and activated Zn powder was investigated. We provide experimental evidence that the polymerization proceeds via Ni(0) complexes which were previously proposed to be key intermediates in chain-growth catalyst-transfer polycondensations. DFT calculations predict a remarkable stability of the naphthalene diimide-based Ni(0) complexes. A plausible polymerization mechanism involved reductive coupling and intramolecular catalyst-transfer steps is proposed.

investigation of this polymerization step is beyond the scope of the present work. Thus-formed (Ar1)n-Ni(dppe)-Ar1-Br is usual intermediates in catalyst-transfer polymerizations which rearrange via RE followed by intramolecular OA into (Ar1)n+1-Ni(dppe)-Br. It was previously proposed that the intramolecular catalyst transfer process involves Ni(0) intermediates, which was never confirmed experimentally. From the observation of the Ni(0) catalyst resting intermediates in 31P NMR spectra during the Ni-catalyzed polymerization of Br-Ar1-Br/Zn, one may tentatively suggest that OA is a relatively slow, rate limiting step. At first sight, such a result is rather unusual because OA reactions in many cases are very fast processes. However, we suggest that the high stability of the Ni(0) complexes, as follows from DFT calculations, accounts for their reduced reactivity and hence explains a relatively slow OA process. Furthermore, a catalyst walking process would be an additional factor contributing to the lowered OA rate. Indeed, for the OA process to occur, the Ni(dppe) species should “walk” over a relatively large distance (∼1.5 nm) from the point where it was located right after RE (i.e., nearby the internal thiophene ring of the last monomer unit) and up to the terminal thiophene ring. In contrast, in the polymerization of smaller thiophene monomers, the catalyst-transfer path is much shorter (∼0.4 nm) which greatly increases probability for Ni(dppe) to be located nearby the C−Br bond, thus facilitating OA. In addition, it is likely that the terminal C−Br bond in thiophenic monomers induces rather strong “orientation” (polarization) effect (which is a driving force for Ni(dppe) to “walk” unidirectionally toward more electron-deficient chain end to add to the C−Br bond). Indeed, as it was found in our previous work,24a the contribution of the random catalyst walking process (which is unfavored for polymerization process) was minor in this polymerization. The situation is different in the polymerization of Br-Ar1-Br/Zn monomers: in addition to much larger monomer size, highly electron-deficient naphthalene diimide group positioned between the thiophene rings screens the polarization effect induced by the terminal C−Br bond. As a result, one can expect much higher contribution of the random catalyst walking process in such polymerization. We believe that 31P NMR spectra of the polymeric and smallmolecule Ni(0) complexes shown in Figures 2a and 2b, respectively, illustrates this idea: a more complicated spectrum in Figure 2a is due to a superposition of signals originated from a number of different Ni(0) complexes forming as a result of the ring-walking of Ni(dppe) along the polymer chain. Our DFT calculations also corroborate with this assumption showing that the complex with Ni(dppe) associated with naphthalene group is 32 kJ/mol more stable than the corresponding complex with Ni(dppe) associated with the terminal thiophene ring. As such, we suggest that in the Nicatalyzed polymerization of Br-Ar1-Br/Zn, the “true” ratelimiting step is not the OA process itself, but rather the preceding ring-walking process. Such a “delocalization” of the Ni(dppe) species among extended π-conjugated system decreases the population of the Ni(0) intermediate with Ni(dppe) located next to the terminal C−Br bond (which acts as the reactant in the OA process, see Figure S15).

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ASSOCIATED CONTENT

S Supporting Information *

Description of instrumentation and materials used, GPC and NMR spectroscopic data, DFT-derived structures of Ni(0) complexes. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (V.S.); kiriy@ ipfdd.de (A.K.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the DFG for financial support (SPP 1355 “Elementary Processes of Organic Photovoltaics”, grant KI1094/4).

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REFERENCES

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CONCLUSION Ni-catalyzed chain-growth polymerization of anion-radical complexes formed upon mixing of 2,6-bis(2-bromothien-5yl)naphthalene-1,4,5,8-tetracarboxylic-N,N′-bis(2-octyldodecyl) 7776

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Macromolecules

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