Examining the Spin State and Redox Chemistry of Ni(Diimine) Catalysts during the Synthesis of π‐Conjugated Polymers

Abstract

Polymeric semiconductors are lightweight organic materials, unlike their lattice-based inorganic counterparts such as silicon. This confers advantages in terms of flexibility and processability, making them of particular interest for application in electronic devices with specialized form-factors. [4-6] The most common approach to synthesize π-conjugated polymers is by transition-metal catalyzed cross-coupling reactions between metalated and halogenated monomers. A variety of reactions have been utilized to polymerize π-conjugated monomers, including Stille, [7] Suzuki, [8] Murahashi, [9] Negishi, [10] and Kumada [11,12] couplings. Direct (hetero)arylation has also emerged as an alternative crosscoupling strategy to polymerize unfunctionalized monomers directly via CH bonds. [13] Most cross-coupling polymerizations follow a step-growth mechanism, however some metal catalysts can undergo rapid intramolecular transfer along polymer chains after each successive CC linkage is formed between adjacent monomers. If the catalyst undergoes intramolecular oxidative addition into the terminal CX bond of a polymer chain, quasi-controlled chain-growth character can be achieved. [14] The earliest and most prominent example of this is the synthesis of poly(3-hexylthiophene) utilizing Kumada coupling, initiated with a Ni(bisphosphine)X 2 precatalyst such as Ni(dppp) Cl 2 (dppp = bis(diphenylphosphino)propane) or Ni(dppe)Cl 2 (dppe = bis(diphenylphosphino)ethane). This polymerization, termed Kumada catalyst transfer polymerization (KCTP), was discovered to follow a quasi-controlled chain-growth mechanism by the McCullough and Yokozawa groups while working in independent laboratories. [15,16] Despite being the first reported example of a quasi-controlled π-conjugated polymerization mechanism, and in spite of significant efforts to expand the catalyst and monomer scope of KCTP, this system remains the most well-controlled π-conjugated polymerization to date allowing for the formation of well-defined block copolymers, [17] polymers with complex architectures, [14] and even discrete high oligomers [18] and isolated living polymer chains. [19] Kumada catalyst transfer polymerization (KCTP) is currently an unparalleled catalytic method for preparing π-conjugated materials with well-defined end-groups, targeted molecular weights, and narrow dispersity. Polymerization control is both catalyst and monomer dependent. Polymerizations using bidentate Ni(phosphine) catalysts and 3-alkylthiophene (or related) monomers lead to the highest degrees of polymerization control; however the scope of monomers that Ni(phosphines) can polymerize is quite limited. Here, two possible mechanistic limitations of Ni(diimine) catalysts are evaluated: 1) the role of spin states, and 2) the redox non-innocence of diimine ligands. Density functional theory (DFT) calculations are utilized to evaluate singlet and triplet spin states of Ni(diimine) species throughout the KCTP catalytic cycle. It is found that in the energetically favored triplet state, Ni(diimin...

Unprecedented Square-Planar α-Diimine Dibromonickel Complexes and Their Ethylene Polymerizations Modulated by Ni–Phenyl Interactions

“…High-spin tetrahedral LNiBr 2 complexes often show paramagnetism and are hardly characterized by NMR analysis. , Indeed, Ni-3 was not characterized by NMR analysis according to previous report . On the contrary, low-spin square-planar Ni-1 and Ni-2 complexes were herein characterized by NMR spectroscopies.…”

“…Only an atypical crystallization of the sandwich-like α-diimine nickel complex involves square-planar and tetrahedral geometries within a same single crystal because of weak metal-to-ligand coordination . As a matter of fact, the relative distortion of the square-planar and tetrahedral geometries of LNiX 2 complexes has been used to quantify the ligand effects of the steric and electronic environments around the nickel metal, which is closely related to catalytic performance. , …”

Precision Synthesis of Conjugated Polymers Using the Kumada Methodology

“…Optimization of electronic and steric properties of diimine ligands has resulted in improved BTz polymerization; however, they are outperformed by phosphine ligands overall. 58 For example, Ni(II)diimine catalysts yield PBTz and P3HT with multiple end-group distributions and moderate dispersities (Đ ≈ 1.7), 10 whereas Ni(II)phosphine catalysts can synthesize poly(3hexylthiophene) with uniform end groups and narrow dispersities (Đ ≈ 1.1). Beyond evaluating the strength of the catalyst−polymer association complex, we disclosed the role of spin-state and redox chemistry on Ni(II)diimine catalyst performance.…”

A Chain‐Growth Mechanism for Conjugated Polymer Synthesis Facilitated by Dinuclear Complexes with Redox‐Active Ligands

“…To bridge the gap between traditional metal catalyzed cross‐couplings and metal‐free radical pathways, we set out to explore conjugated polymer synthesis mediated by first‐row transition metal dinuclear complexes with redox‐active ligands, which can accommodate single‐electron processes. Indeed, redox‐active ligands had been previously shown to support open shell intermediates in a conjugated polymer synthesis [24] . Meanwhile, our focus on dinuclear complexes stems from the observation that they often provide access to mechanistic manifolds that complement their mononuclear counterparts.…”

“…Indeed, redox-active ligands had been previously shown to support open shell intermediates in a conjugated polymer synthesis. [24] Meanwhile, our focus on dinuclear complexes stems from the observation that they often provide access to mechanistic manifolds that complement their mononuclear counterparts. For example, Hoffmann and Trinquier predicted that concerted reductive eliminations at dinuclear complexes would generally be symmetry forbidden.…”