Mechanistic Origin of Ligand Effects on Exhaustive Functionalization During Pd-Catalyzed Cross-Coupling of Dihaloarenes

We describe a detailed investigation into why bulky ligands-those that enable catalysis at "12e -" Pd0-tend to promote overfunctionalization during Pd-catalyzed cross-couplings of dihalogenated substrates. After one cross-coupling event takes place, PdL initially remains coordinated to the π system of the nascent product. Selectivity for mono- vs. difunctionalization arises from the relative rates of π-decomplexation versus a second oxidative addition. Under the Suzuki coupling conditions in this work, direct dissociation of 12e - PdL from the π-complex cannot outcompete oxidative addition. Instead, Pd must be displaced from the π-complex as 14e - PdL(L') by a second incoming ligand L'. The incoming ligand is another molecule of dichloroarene if the reaction conditions do not include π-coordinating solvents or additives. More overfunctionalization tends to result when increased ligand or substrate sterics raises the energy of the bimolecular transition state for separating 14e - PdL(L') from the mono-cross-coupled product. This work has practical implications for optimizing selectivity in cross-couplings involving multiple halogens. For example, we demonstrate that small coordinating additives like DMSO can largely suppress overfunctionalization and that precatalyst structure can also impact selectivity.

Tandem Kumada-Tamao catalyst-transfer condensation polymerization and Suzuki-Miyaura coupling for the synthesis of end-functionalized poly(3-hexylthiophene)

Successive Kumada-Tamao catalyst-transfer condensation polymerization of 2-bromo-5-chloromagnesio-3-hexylthiophene and Suzuki-Miyaura end-functionalization with pinacol arylboronate in one pot afforded poly(3-hexylthiophene) (P3HT) with a base-sensitive functional group at both ends. The use of poly(methyl methacrylate) (PMMA) bearing a boronic acid ester moiety at one end enabled one-pot synthesis of PMMA-b-P3HT-b-PMMA triblock copolymer.

[IPr#-PEPPSI]: A Well-Defined, Highly Hindered and Broadly Applicable Pd(II)-NHC (NHC = N-Heterocyclic Carbene) Precatalyst for Cross-Coupling Reactions

In this Special Issue, "Featured Papers in Organometallic Chemistry", we report on the synthesis and characterization of [IPr#-PEPPSI], a new, well-defined, highly hindered Pd(II)-NHC precatalyst for cross-coupling reactions. This catalyst was commercialized in collaboration with MilliporeSigma, Burlington, ON, Canada (no. 925489) to provide academic and industrial researchers with broad access to reaction screening and optimization. The broad activity of [IPr#-PEPPSI] in cross-coupling reactions in a range of bond activations with C-N, C-O, C-Cl, C-Br, C-S and C-H cleavage is presented. A comprehensive evaluation of the steric and electronic properties is provided. Easy access to the [IPr#-PEPPSI] class of precatalysts based on modular pyridine ligands, together with the steric impact of the IPr# peralkylation framework, will facilitate the implementation of well-defined, air- and moisture-stable Pd(II)-NHC precatalysts in chemistry research.

Statistical Copolymers of Thiophene-3-Carboxylates and Selenophene-3-Carboxylates; 77Se NMR as a Tool to Examine Copolymer Sequence in Selenophene-Based Conjugated Polymers

Herein, we demonstrate that homopolymerization and statistical copolymerization of 2-ethylhexyl thiophene-3-carboxylate and 2-ethylhexyl selenophene-3-carboxylate monomers is possible via Suzuki-Miyaura cross-coupling. A commercially available palladium catalyst ([1,3-bis(2,6-di-3-pentylphenyl)imidazol-2-ylidene](3-chloropyridyl)dichloropalladium(II) or PEPPSI-IPent) was employed...

Precision synthesis of a fluorene-thiophene alternating copolymer by means of the Suzuki–Miyaura catalyst-transfer condensation polymerization: the importance of the position of an alkyl substituent on thiophene of the biaryl monomer to suppress disproportionation

The Suzuki–Miyaura coupling polymerization of PinB-F8T(3)-Br was accompanied by disproportionation, whereas that of PinB-F8T(4)-Br proceeded in a chain-growth polymerization manner to afford a well-defined fluorene-thiophene alternating copolymer.

Controlled Pd(0)/Ad3P-Catalyzed Suzuki Cross-Coupling Polymerization of AB-Type Monomers with Ad3P-Coordinated Acetanilide-Based Palladacycle Complex as Initiator

Controlled Pd(0)-catalyzed Suzuki cross-coupling polymerizations of AB-type monomers with tris(1-adamantyl)phosphine (Ad₃P) as the ligand was described. Ad₃P-coordinated acetanilide-based palladacycle complex (1) was demonstrated to be an efficient initiator for controlled Suzuki cross-coupling polymerization, affording polymers with narrow Đs and well-controlled end groups. Our study provided an efficient ligand and an efficient initiator for controlled Suzuki cross-coupling polymerizations.

Catalyst-dependent intrinsic ring-walking behavior on π-face of conjugated polymers

The balance between the ring-walking process and the oxidative-addition state are key determinants of catalyst mobility in catalyst-transfer condensation polymerization.

Synthesis of conjugated copolymers by combining different coupling reactions

Different coupling reactions are combined in the same copolymerization to tune the structure of the resulting conjugated copolymer.

Matchmaking in Catalyst-Transfer Polycondensation: Optimizing Catalysts based on Mechanistic Insight

Catalyst-transfer polycondensation (CTP) has emerged as a useful living, chain-growth polymerization method for synthesizing conjugated (hetero)arene-based polymers with targetable molecular weights, narrow dispersities, and controllable copolymer sequences-all properties that significantly influence their performance in devices. Over the past decade, several phosphine- and carbene-ligated Ni- and Pd-based precatalysts have been shown to be effective in CTP. One current limitation is that these traditional CTP catalysts lead to nonliving, non-chain-growth behavior when complex monomer scaffolds are utilized. Because these monomers are often found in the highest-performing materials, there is a significant need to identify alternative CTP catalysts. Recent mechanistic insight into CTP has laid the foundation for designing new catalysts to expand the CTP monomer scope. Building off this insight, we have designed and implemented model systems to identify effective catalysts by understanding their underlying mechanistic behaviors and systematically modifying catalyst structures to improve their chain-growth behavior. In this Account, we describe how each catalyst parameter-the ancillary ligand(s), reactive ligand(s), and transition metal-influences CTP. As an example, ancillary ligands often dictate the turnover-limiting step of the catalytic cycle, and perhaps more importantly, they can be used to promote the formation of the key intermediate (a metal-arene associative complex) and its subsequent reactivity. The fidelity of this intermediate is central to the mechanism for the living, chain-growth polymerization. Reactive ligands, on the other hand, can be used to improve catalyst solubility and accelerate initiation. Additional advantages of the reactive ligand include providing access points for postpolymerization modification and synthesizing polymers directly off surfaces. While the most frequently used CTP catalysts contain nickel, palladium-based catalysts exhibit a higher functional group tolerance and broader substrate scope (e.g., monomers with boron, magnesium, tin, and gold transmetalating agents). Overall, we anticipate that applying the tools and lessons detailed in this Account to other monomers should facilitate a better "matchmaking" process that will lead to new catalyst-transfer polycondensations.

Preparation of an Aurylated Alkylthiophene Monomer via C-H Activation for Use in Pd-PEPPSI-iPr Catalyzed-Controlled Chain Growth Polymerization

In the search for new synthetic routes toward greener and more facile syntheses of conjugated polymers, C-H functionalization provides a promising solution by minimizing the production and processing of aryl halide monomer precursors used in traditional organometallic coupling reactions. In this paper, we investigate the use of Au(I) and its ability to directly C-H activate 2-bromo-3-hexylthiophene to form a reactive monomer species, bypassing the typical Grignard monomer formation from a dihalogenated thiophene. Addition of Pd-PEPPSI-iPr as a palladium catalyst source in the presence of the resultant aurylated thiophene monomer yielded poly(3-hexylthiophene) as observed by both NMR and GPC. Studies on the growth of these polymers show linear dependence between M_n and monomer conversion, low dispersities, as well as M_n predicted by catalyst loading, which is supportive of a living-type chain growth mechanism. This Au-Pd system represents a novel methodology for incorporating C-H activation into the synthesis of P3HT with control over M_n.

Limitations of Using Small Molecules to Identify Catalyst-Transfer Polycondensation Reactions

Catalyst-transfer polycondensation (CTP) is a relatively new method for synthesizing conjugated polymers in a chain-growth fashion using transition metal catalysis. Recent research has focused on screening catalysts to broaden the monomer scope. In this effort, small molecule reactions have played an important role. Specifically, when selective difunctionalization occurs, even with limiting quantities of reaction partner, it suggests an associative intermediate similar to CTP. Several new chain-growth polymerizations have been discovered using this approach. We report herein an attempt to use this method to develop chain-growth conditions for synthesizing poly(2,5-bis(hexyloxy)phenylene ethynylene) via Sonogashira cross-coupling. Hundreds of small molecule experiments were performed and selective difunctionalization was observed with a Buchwald-type precatalyst. Unexpectedly, these same reaction conditions led to a step-growth polymerization. Further investigation revealed that the product ratios in the small molecule reactions were dictated by reactivity differences rather than an associative intermediate. The lessons learned from these studies have broad implications on other small molecule reactions being used to identify new catalysts for CTP.

Effect of Solvent on Surface Ordering of Poly(3-hexylthiophene) Thin Films

Enhancement of charge transport in organic polymer semiconductors is a crucial step in developing optimized devices. A variety of sample preparation conditions, such as film fabrication method, solvent species, and annealing, were found to influence the hole mobility of organic polymers. Despite the fact that many factors can influence their performance, it is believed that polymer surface ordering plays a key role in determining organic polymer function. Here, sum frequency generation (SFG) vibrational spectroscopy was used to nondestructively map the surface/interfacial ordering of poly(3-hexylthiophene) (P3HT) films prepared using different solvents; we believe that solvent interactions determine the degree of surface/interfacial ordering. Both X-ray diffraction (XRD) spectroscopy and scanning electron microscopy (SEM) were used to supplement SFG to systematically study bulk crystallinity and surface morphology. We conclude that SFG is a powerful tool to elucidate the surface/interfacial structural information on polymer semiconducting films. We demonstrate that the solvent composition used to prepare P3HT thin films influences the resulting film surface morphology, surface/interfacial ordering, and bulk crystallinity.

Controlled synthesis of high molecular weight poly(3-hexylthiophene)s via Kumada catalyst transfer polycondensation with Ni(IPr)(acac)2 as the catalyst

Ni(IPr)(acac)₂ is a great catalyst for the controlled synthesis of poly(3-hexylthiophene)s with M_ns of 9.90–350 kg mol⁻¹via Kumada catalyst transfer polycondensation.

Kumada catalyst transfer polycondensation for controlled synthesis of polyfluorenes using 1,3-bis(diarylphosphino)propanes as ligands

The moderately hindered catalyst Ni(acac)₂/L2 outperformed other catalysts, affording PF8s with M_n up to 91.1 in a controlled manner.

Intramolecular catalyst transfer polymerisation of conjugated monomers: from lessons learned to future challenges

Eleven years after the first reports on intramolecular catalyst transfer polycondensations, this review aims to critically recap on the fundamental “lessons” that can be learned from the historic literature as well as from the fervid activity that has emerged in the last three years.

Direct visualization of microphase separation in block copoly(3-alkylthiophene)s

A copoly(3-alkylthiophene) block copolymer was synthesized in a one-pot block copolymerization reaction, starting from a functional o-tolyl initiator in order to maximize A–B diblock copolymer formation. The microphase separation behaviour was directly visualized using STM.

Selective and general exhaustive cross-coupling of di-chloroarenes with a deficit of nucleophiles mediated by a Pd–NHC complex

We report the first example of a general, exhaustive Pd-mediated cross-coupling of polychloroarenes in the presence of a deficit of nucleophiles, mediated by the highly active PEPPSI-IPent catalyst.

Controlled Synthesis of Fully π-Conjugated Donor-Acceptor Block Copolymers Using a Ni(II) Diimine Catalyst

We use a Ni(II) diimine catalyst to prepare the first examples of the controlled synthesis of electron-rich/electron-deficient all-conjugated diblock copolymers. These catalysts are able to control polymerizations of both electron-rich and electron-deficient monomers, which we attribute to strong association to both monomer types. Block copolymers are prepared by controlled chain extension, and their structure is verified by gel permeation chromatography, ¹H NMR, electrochemistry, calorimetry, and atomic force microscopy.

Co-crystallization phase transformations in all π-conjugated block copolymers with different main-chain moieties

Driven by molecular affinity and balance in the crystallization kinetics, the ability to co-crystallize dissimilar yet self-crystallizable blocks of a block copolymer (BCP) into a uniform domain may strongly affect its phase diagram. In this study, we synthesize a new series of crystalline and monodisperse all-π-conjugated poly(2,5-dihexyloxy-p-phenylene)-b-poly(3-(2-ethylhexyl)thiophene) (PPP-P3EHT) BCPs and investigate this multi-crystallization effect. Despite vastly different side-chain and main-chain structures, PPP and P3EHT blocks are able to co-crystallize into a single uniform domain comprising PPP and P3EHT main-chains with mutually interdigitated side-chains spaced in-between. With increasing P3EHT fraction, PPP-P3EHTs undergo sequential phase transitions and form hierarchical superstructures including predominately PPP nanofibrils, co-crystalline nanofibrils, a bilayer co-crystalline/pure P3EHT lamellar structure, a microphase-separated bilayer PPP-P3EHT lamellar structure, and finally P3EHT nanofibrils. In particular, the presence of the new co-crystalline lamellar structure is the manifestation of the interaction balance between self-crystallization and co-crystallization of the dissimilar polymers on the resulting nanostructure of the BCP. The current study demonstrates the co-crystallization nature of all-conjugated BCPs with different main-chain moieties and may provide new guidelines for the organization of π-conjugated BCPs for future optoelectronic applications.

Bandgap engineering through controlled oxidation of polythiophenes

The use of Rozen's reagent (HOF⋅CH3 CN) to convert polythiophenes to polymers containing thiophene-1,1-dioxide (TDO) is described. The oxidation of polythiophenes can be controlled with this potent, yet orthogonal reagent under mild conditions. The oxidation of poly(3-alkylthiophenes) proceeds at room temperature in a matter of minutes, introducing up to 60 % TDO moieties in the polymer backbone. The resulting polymers have a markedly low-lying lowest unoccupied molecular orbital (LUMO), consequently exhibiting a small bandgap. This approach demonstrates that modulating the backbone electronic structure of well-defined polymers, rather than varying the monomers, is an efficient means of tuning the electronic properties of conjugated polymers.

Polythiophene synthesis via halogen dance

Polymerization of thiophene with a Ni(ii) catalyst through halogen dance leads to a new class of polythiophenes.