Original Article Published: 03 September 2019 A robust nickel catalyst with an unsymmetrical propyl-bridged diphosphine ligand for catalyst-transfer polymerization Matthew A. Baker1, Josué

Ayuso-Carrillo1, Martin R. M. Koos  ORCID: orcid.org/0000-0002-7829-47291, Samantha N. MacMillan  ORCID: orcid.org/0000-0001-6516-18232, Anthony J. Varni1, Roberto R. Gil  ORCID:

orcid.org/0000-0002-8810-50471 & …Kevin J. T. Noonan  ORCID: orcid.org/0000-0003-4061-75931 Show authors Polymer Journal volume 52, pages 83–92 (2020)Cite this article

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A new nickel diphosphine catalyst has been synthesized in which the bidentate ligand has two different phosphine donors, a typical PPh2 group and a stronger σ-donating PEt2 group. The

catalyst was highly effective for the chain-growth polymerization of a 3-alkylthiophene monomer using a Suzuki–Miyaura cross-coupling. The catalyst is particularly effective for this

polymerization in the presence of excess free ligand. The unsymmetrical diphosphine nickel complex reported here represents a new approach to tuning metal-ligand reactivity in the

chain-growth polymerization of aromatic monomers. In addition, this new nickel catalyst exhibited increased hydrolytic resistance in the polymerization as compared to commercially available

1,3-bis(diphenylphosphino)propane nickel dichloride.

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02 February 2023 Identification of a potent palladium-aryldiphosphine catalytic system for high-performance carbonylation of alkenes Article Open access 05 March 2024 1-octene polymerization

catalyzed by titanium and zirconium complexes supported by [PN] or [NPN] ligands Article Open access 18 February 2025 Introduction

Metal-catalyzed cross-couplings continue to be among the most powerful strategies for forming C–C bonds, and they are commonly employed to synthesize π-conjugated polymers. These reactions

typically proceed by a step-growth mechanism, but an interaction between the metal catalyst and the π-system [1, 2] of the growing macromolecule can result in a chain-growth process (known

as catalyst-transfer polymerization or CTP). This affords well-defined polymers with controllable molecular weights and narrow molecular weight distributions [3,4,5,6,7,8,9,10,11]. The

ancillary ligand bound to the metal is critical to the chain-growth process because it governs both the metal-polymer π-interactions and the cross-coupling efficiency [3,4,5,6,7,8,9,10,11].

Of the cross-coupling reactions used for CTP, the Suzuki–Miyaura reaction is advantageous due to the functional group compatibility of organoboron moieties and the relatively mild reaction

conditions needed to promote C–C bond formation. While palladium complexes are often used to catalyze this transformation [12,13,14,15,16,17,18,19,20,21,22,23,24], nickel catalysts such as

Ni(dppp)Cl2 and Ni(IPr)(PPh3)Cl2 can also be employed to polymerize X-Ar-B(OR)2 monomers [25, 26]. Herein, we prepared a nickel diphosphine precatalyst with two unique phosphine donors

(Scheme 1) for CTP.

Synthesis of the Ni(sepp)Cl2 precatalyst for CTP

The desymmetrization of the diphosphine ligand enables specific modifications of the steric and electronic environments on each donor atom bound to the metal. In this report, the bidentate

ligand is composed of diaryl (PPh2) and dialkyl (PEt2) phosphines with a bridging propyl linker. This diphosphine was synthesized according to a published procedure [27,28,29], and it is

abbreviated “split” diethyl diphenyl phosphine or sepp for simplicity. The ligand strongly binds nickel and offers improved hydrolytic resistance under basic conditions compared to

commercially available 1,3-bis(diphenylphosphino)propane nickel dichloride (Ni(dppp)Cl2). Most importantly, Ni(sepp)Cl2 can catalyze the controlled polymerization of a 3-hexylthiophene

monomer using Suzuki-Miyaura coupling.

The polymerization catalyst is particularly effective when the reaction is conducted in the presence of excess free ligand. Remarkably, although the catalyst is unsymmetric, a single

catalyst resting state was identified using 31P NMR spectroscopy, indicating that oxidative addition to the Ni0 occurs preferentially with the growing chain trans to the PEt2 group. An

externally initiated variant of this catalyst was also synthesized and shown to be highly effective for controlling the polymer end groups. Altogether, the data suggest that individual

modification of the donor ligands in bidentate structures may be a valuable ligand design strategy for chain-growth polycondensation and, more broadly, for cross-coupling

reactions.

All synthetic manipulations of air-sensitive compounds were carried out under an N2 atmosphere using standard Schlenk techniques or in an N2-filled glovebox. All glassware was dried

overnight in an oven and heated under vacuum prior to use. Solvents were dried and degassed prior to use. Deionized water for the polymerizations was thoroughly degassed by a continuous flow

of N2 for at least 30 min. The sepp ligand was prepared according to a literature procedure [27] though a different reducing agent was used for the final step [30]. The 1H and 31P NMR

spectra of sepp and the ligand precursors are provided in the Supporting Information (Figs. S1–S3). Monomer 1 (2-(5-bromo-4-hexylthiophen-2-yl)−4,4,5,5-tetramethyl-1,3,2-dioxaborolane) was

synthesized from literature procedures with minor modifications [23, 31]. All other compounds were purchased from commercial vendors and used as received.

All NMR spectra were recorded on a 500 Bruker Avance III or a 500 Bruker Avance Neo (1H, 500 MHz; 13C, 125.8 MHz; 31P, 202.5 MHz) spectrometer. NMR signals were referenced to residual

solvent for 1H and deuterated solvents for 13C{1H}. The proton-decoupled 31P{1H} NMR spectra were electronically referenced using internal Bruker software according to a universal scale

determined from the precise ratio, Ξ , of the resonance frequency of the 31P nuclide to the 1H resonance of TMS in a dilute solution (φ < 1%) [32]. All polymer samples subjected to 31P{1H}

NMR analysis were prepared by removing 0.5 mL of the polymer solution and transferring it into an NMR tube housed in a Schlenk tube under N2. Aliquots were removed from the polymerization at

specific time points and analyzed.

Gel-permeation chromatography (GPC) measurements were performed on a Waters Instrument equipped with a 717 plus autosampler, a Waters 2414 refractive index (RI) detector, and two SDV columns

(Porosity 1000 and 100,000 Å; Polymer Standard Services) with THF as the eluent (~1 mg/mL, flow rate 1 mL/min, 40 °C). A 10-point calibration based on polystyrene standards (Polystyrene,

ReadyCal Kit, Polymer Standard Services) was used to determine the molecular weights.

MALDI-TOF analyses were performed using an Applied Biosystems Voyager DE-STR mass spectrometer. One microliter of a solution of the matrix

(trans-2-[3-(4-t-butyl-phenyl)-2-methyl-2-propenylidene]malonitrile, DCTB) in THF (10 mg/mL) was spotted onto a well of the MALDI plate, and the solvent was allowed to evaporate. Polymer

sample solutions (2 mg/mL in THF) were prepared, and 1 μL of this solution was spotted onto the well by a layering method. The solvent was evaporated prior to analysis. Data were collected

in positive polarity mode in either linear or reflection mode.

Low-temperature X-ray diffraction data for (sepp)Ni(o-C6H4CO2Me)Br and Ni(sepp)Cl2 were collected on a Rigaku XtaLAB Synergy diffractometer coupled to a Rigaku Hypix detector with Cu Kα

radiation (λ = 1.54184 Å) from a PhotonJet microfocus X-ray source at 100 K. The diffraction images were processed and scaled using CrysAlisPro software [33]. The structures were solved

through intrinsic phasing using SHELXT [34] and refined against F2 on all data by full-matrix least squares with SHELXL [35] following established refinement strategies [36]. All nonhydrogen

atoms were refined anisotropically. All hydrogen atoms bound to carbon were included in the model at geometrically calculated positions and refined using a riding model. The isotropic

displacement parameters of all hydrogen atoms were fixed to 1.2 times the Ueq value of the atoms they are linked to (1.5 times for methyl groups). Details of the data quality and a summary

of the residual values of the refinements are listed in the Supporting Information (Tables S1 and S2). The crystallographic data for (sepp)Ni(o-C6H4CO2Me)Br and Ni(sepp)Cl2 have been

deposited with the Cambridge Crystallographic Data Center (CCDC) under the reference numbers CCDC 1911056 and 1911057, respectively. These data can be obtained free of charge via

www.ccdc.cam.ac.uk/data_request/cif.

The degree of polymerization (DP) for each poly(3-hexylthiophene) (P3HT) sample was estimated from the alkyl region of the 1H NMR spectrum [37,38,39]. For polymers prepared using

Ni(sepp)Cl2, the α-CH2 signal of the hexyl tail on the H-terminated end group (triplet at 2.62 ppm) [37, 39] was compared to the α-CH2 signal of the polymer (triplet centered at 2.81 ppm,

integrated from 2.90 to 2.65 ppm). The H-terminated end group signal was set to 2, and the DP was calculated as follows: DP = (integration of polymer CH2 signal/2) + 1 for the Br-terminated

end group + 1 for the H-terminated group. For polymers prepared using (sepp)Ni(o-C6H4CO2Me)Br, the CH3 signal of the o-C6H4CO2Me end group (singlet at 3.80 ppm) [38] was compared to the

α-CH2 signal of the polymer (triplet centered at 2.81 ppm, integration from 2.90 to 2.65 ppm). The end group signal was set to 3, and the DP was calculated as follows: DP = (integration of

polymer CH2 signal/2) + 1 for the H-terminated end group.

Density functional theory calculations were performed for all compounds using a mixed basis set with B3LYP 6–31 G(d) for all atoms in the ligand and SDD for nickel in the Gaussian 09 suite

[40]. The Cartesian coordinates of the optimized geometries are available as xyz files, and the total energies of the two isomers are included in Table S3 (ESI).

proceduresNi(sepp)Cl2

To an oven-dried 100 mL Schlenk flask were added NiCl2 (0.118 g, 0.91 mmol), sepp (0.300 g, 0.95 mmol) and degassed ethanol (15 mL) under an inert atmosphere. The suspension was stirred at

90 °C for 1 h and then cooled to 8 °C to facilitate precipitation of the target compound. The nickel dihalide complex was isolated as an air-stable, red-orange solid by vacuum filtration and

washed with ethanol and diethyl ether (0.140 g, 35% yield). Crystals were grown from a concentrated solution in CH2Cl2 (~20 mg/mL) layered with n-hexane or diethyl ether.

31P{1H} NMR (202 MHz, CD2Cl2) δ 8 (br s, 2P). 1H NMR (500 MHz, CD2Cl2) δ 7.97 (d, J = 7.4 Hz, 4H), 7.56 (t, J = 7.5 Hz, 4H), 7.50 (t, J = 7.4 Hz, 2H), 3.07 (br s, 2H), 2.48 (br s, 2H), 2.07

(br s, 2H), 1.91 (s, 2H), 1.70 (p, J = 5.8 Hz, 2H), 1.43 (t, J = 7.4 Hz, 6H). 13C{1H} NMR (126 MHz, CD2Cl2): δ 135.4, 134.1 (br), 131.8, 129.2, 26.1, 23.6 (br), 20.2, 19.1, 10.5. HRMS

(DART-MS) (m/z): [2M−Cl]+ calculated for C38H52Cl3Ni2P4, 853.0792; found, 853.0789.

In a glovebox, a scintillation vial equipped with a magnetic stir bar was charged with bis(1,5-cyclooctadiene)nickel (0.165 g, 0.6 mmol), triphenylphosphine (0.315 g, 1.20 mmol), and toluene

(6 mL). After complete dissolution of the reagents in toluene, methyl-2-bromobenzoate (0.142 g, 0.66 mmol) was added into the reaction mixture, and the solution was vigorously stirred at 23

 °C for 18 h, during which time a solid precipitated from the reaction mixture. The suspended solids were precipitated by hexanes (~60 mL), isolated by filtration and washed with hexanes

(~200 mL) and methanol (~200 mL). The resulting bright-yellow solid was dried under reduced pressure at 50 °C, affording 0.420 g (88% yield) of the nickel complex.

31P{1H} NMR (202 MHz, CD2Cl2): δ 20.8 (s). 1H NMR (500 MHz, CD2Cl2): δ 7.75–7.53 (br s, 12H), 7.50 (d, J = 7.5 Hz, 1H), 7.41–7.10 (br m, 18H), 6.68 (d, J = 7.9 Hz, 1H), 6.48 (t, J = 7.6 Hz,

1H), 6.32 (t, J = 7.4 Hz, 1H), 3.74 (s, 3H). 13C{1H} NMR (126 MHz, CD2Cl2): δ 169.5, 166.2, 136.8, 136.6, 135.1, 132.5, 131.8, 129.9, 128.9, 128.0, 121.5, 51.9. HRMS (DART-MS) (m/z): [M + 

H]+ calculated for C44H38BrNiO2P2, 797.0884; found, 797.6169.

In a glovebox, a scintillation vial equipped with a stir bar was charged with (PPh3)2Ni(o-C6H4CO2Me)Br (0.237 g, 0.30 mmol), sepp (0.103 g, 0.33 mmol), and THF (4.8 mL). The solution was

vigorously stirred at 23 °C for 18 h, during which time a precipitate formed in the reaction mixture. The suspended solids were precipitated by hexanes (~50 mL), isolated by filtration and

washed with hexanes (~150 mL). The resulting bright-yellow solid was dried under reduced pressure at 23 °C, affording 0.170 g (97% yield) of the nickel complex. Crystals were grown from a

concentrated solution in CH2Cl2 (~20 mg/mL) layered with n-hexane. 31P{1H} (202 MHz, THF-d8) δ 18.0 (d, JPP = 70.3 Hz, 1P), −5.7 (d, JPP = 70.2 Hz, 1P). 1H NMR (500 MHz, THF-d8) δ 8.55 (br

s, 2H), 7.59 (br s, 3H), 7.49–7.41 (m, 1H), 6.98 (dt, J = 7.7, 1.8 Hz, 2H), 6.80 (t, J = 7.7 Hz, 2H), 6.62 (tt, J = 7.4, 1.6 Hz, 1H), 6.55 (t, J = 8.9 Hz, 2H), 6.32 (t, J = 7.4 Hz, 1H), 3.81

(s, 3H), 2.53–2.31 (m, 2H; note: this signal overlaps with H2O), 2.28–2.15 (m, 1H), 2.11–1.99 (m, 2H), 1.89–1.76 (m, 1H), 1.71–1.66 (m, 2H; note: this signal overlaps with the solvent

signal), 1.66–1.55 (m, 1H), 1.40 (dt, J = 15.8, 7.6 Hz, 3H), 1.30–1.19 (m, 1H), 1.12 (dt, J = 14.6, 7.6 Hz, 3H). 13C{1H} NMR (126 MHz, THF-d8) δ 174.6 (dd, JPC = 79.7, 35.7 Hz), 172.3 (d,

JPC = 5.8 Hz), 139.2 (d, JPC = 2.6 Hz), 136.8 (d, JPC = 11.9 Hz), 135.7, 135.5 (dd, JPC = 47.2, 4.6 Hz), 132.7 (d, JPC = 44.9 Hz), 132.2, 131.6 (d, JPC = 7.5 Hz), 130.0 (dd, JPC = 4.5, 2.5 

Hz), 129.7 (d, JPC = 5.5 Hz), 129.5 (d, JPC = 10.0 Hz), 128.8 (d, JPC = 2.2 Hz), 127.7 (d, JPC = 9.5 Hz), 121.1, 51.6, 28.7 (dd, JPC = 21.7, 7.9 Hz), 20.6 (dd, JPC = 17.2, 3.5 Hz), 19.5 (d,

JPC = 3.5 Hz), 19.3 (d, JPC = 24.2 Hz), 18.8 (d, JPC = 28.1 Hz), 8.6, 8.2 (d, JPC = 3.8 Hz). HRMS (DART-MS) (m/z): [M−Br]+ calculated for C27H33NiO2P2, 509.1309; found,

509.1307.

In a glovebox, a scintillation vial equipped with a stir bar and an open-top Teflon screw cap was charged with a calculated amount of catalyst and K3PO4‧H2O (0.070 g, 0.30 mmol). In several

cases (entries 3–4 and 7–8 in Table 1), a calculated amount of sepp ligand (1.0 equiv relative to catalyst) was also added to the reaction. Monomer 1 (0.056 g, 0.15 mmol) and then THF (6 mL)

were added to the vial, which was then sealed and removed from the glovebox. The reaction was heated to 50 °C, and water (0.09 mL) was injected. Two hours after the addition of water, the

polymerization was quenched by the addition of 6 N methanolic HCl (50 mL) with stirring for 30 min. The resulting precipitate was isolated by filtration, washed with methanol (100 mL), water

(50 mL), and then methanol (200 mL) before drying. The experiment to obtain Mn versus conversion and the semilogarithmic plot was conducted according to a previous report [25].

Preparation of P3HT using a Suzuki–Miyaura cross-couplingFull size tableResults and discussion

The combination of the previously reported diphosphine [27,28,29] with NiCl2 in refluxing ethanol (Scheme 1) afforded Ni(sepp)Cl2 in a low yield (35%). The NMR signals (1H and 13C{1H}) of

the precatalyst were broad due to the very short T2 relaxation times induced by the quadrupolar chlorides [41]. An extremely broad, nearly imperceptible signal was present in the 31P{1H} NMR

spectrum, presumably due to this quadrupolar relaxation. Single crystal X-ray diffraction (Fig. 1) was then employed to unambiguously assign the structure.

Solid-state molecular structure of Ni(sepp)Cl2 (thermal ellipsoids at 50% probability) with H atoms omitted. Only one of the two independent molecules is shown

There are two independent molecules in the unit cell of Ni(sepp)Cl2. The average nickel–phosphine bond lengths for the two different phosphines are nearly identical (2.1609(4) Å for the

Ni−PPh2 moiety and 2.1712(4) Å for the Ni−PEt2 unit). Interestingly, the bite angles in the two independent precatalyst molecules differ by over 2° and average to 95°. This suggests a

slightly larger steric constraint imposed by the two unique phosphines in Ni(sepp)Cl2 compared to that of Ni(dppp)Cl2, in which the bite angle of the diphosphine is 91.8° [42].

Upon complete characterization of the Ni(sepp)Cl2 complex, its performance in CTP was evaluated. This precatalyst was used to initiate the polymerization of a 3-alkylthiophene monomer

bearing a bromine at the 2-position and a pinacol boronate at the 5 position (1) in THF with K3PO4·H2O as the base and minimal water at 50 °C. The concentrations of base and water used in

these experiments were identical to our optimized conditions for Ni(dppp)Cl2. Using 4 mol% Ni(sepp)Cl2, P3HT with a DP of 27, an Mn of 7.9 kg/mol, and Ð of 1.07 (Table 1, Entry 1) was

obtained.

The molecular weight and distribution of the P3HT sample suggested that Ni(sepp)Cl2 was a promising catalyst for CTP, but it also raised some questions. Prior reports on the mechanism of

Suzuki–Miyaura cross-coupling have indicated that ligand substitution reactions of metal-halogen bonds to form metal-hydroxo species are critical for the transmetalation with the aryl

boronate [43,44,45,46,47,48,49]. Considering this, activation of the nickel dihalide precatalyst likely requires exchange of the −Cl ligands for −OH ligands prior to transmetalation and

reductive elimination (depicted in Scheme 2). Consequently, this hydroxide exchange can potentially impact the efficiency of the precatalyst reduction. In our prior work on the

polymerization of 1 with Ni(dppp)Cl2 [25], NMR spectroscopy revealed a significant portion of free dppp in the reaction mixture. The presence of dppp likely stemmed from metal-hydroxide

formation involving the precatalyst and subsequent dissociation of the diphosphine ligand to afford the catalytically inactive nickel hydroxide (proposed pathway in Scheme 2). Miyaura and

coworkers have documented sensitivity of dihalide precatalysts for Suzuki-Miyaura cross-coupling in their previous work [50,51,52]. Although a portion of the Ni(dppp)Cl2 precatalyst was lost

to hydrolysis, this event was critical to the overall chain-growth polymerization process since the combination of the remaining active catalyst and free ligand were necessary to achieve

the controlled polymerization, as evidenced by control experiments [25]. The hydrolysis of Ni(dppp)Cl2 led us to consider whether Ni(sepp)Cl2 with the stronger PEt2 donor would show

different precatalyst initiation behavior.

Proposed pathway of precatalyst reduction and hydrolysis

Our interrogation of the polymerization reaction using 31P{1H} NMR spectroscopy revealed that Ni(sepp)Cl2 was significantly more resistant to precatalyst hydrolysis when compared to

Ni(dppp)Cl2 (Fig. 2). For comparison, free dppp and its oxide account for nearly 60% of the integration intensity in the spectrum of an aliquot of the Ni(dppp)Cl2-initiated polymerization,

while for Ni(sepp)Cl2, the free ligand and its oxide account for only 10% of the mixture. In addition to hydrolytic resistance, only one pair of doublets was observed from the major product

in the Ni(sepp)Cl2-initiated polymerization (Fig. 2). The chemical shifts are close to those of the pair of doublets observed when polymerizing with Ni(dppp)Cl2.

Crude 31P{1H} NMR spectra recorded in THF of the polymerization reaction conducted with Ni(dppp)Cl2 (top) and Ni(sepp)Cl2 (bottom). Aliquots were removed 60 min after initiation from 25 mM

polymerizations of monomer 1 with 10 mol % catalyst, 2 equiv of K3PO4·H2O, and 33.3 equiv of degassed water. * indicates unidentified side-product

Transmetalation has been identified as the rate-limiting step when using Ni(dppp)Cl2 as the catalyst in both Kumada–Corriu- and Suzuki–Miyaura-initiated CTPs [25, 53, 54]. The resting state

of the catalyst, (dppp)Ni(P3HT)Br, can be observed using 31P{1H} NMR spectroscopy [53, 54]. The two doublets in the top of Fig. 2 correspond to this Ni2+ species that has undergone oxidative

addition. The downfield signal near 20 ppm (2JPP = 64.4 Hz) corresponds to the PPh2 group trans to the halogen, and the upfield signal near −3 ppm corresponds to the PPh2 group trans to the

growing chain (2JPP = 65.8 Hz) [55]. A similar pattern is observed for the polymerization with Ni(sepp)Cl2. One doublet appeared at 19.1 ppm (2JPP = 71.0 Hz), and the other appeared further

upfield at −4.5 ppm (2JPP = 70.9 Hz).

The similarity to the Ni(dppp)Cl2 polymerization suggested that the oxidative addition product is also the resting state of the catalyst in this case. Moreover, the upfield shift of the

second signal was indicative that the more electron-rich PEt2 is trans to the growing chain. If the oxidative addition was not selective, then one would expect to see two pairs of doublets

for the two possible stereoisomers. Some minor doublets are indeed observed in the spectrum, so it is likely that the other stereoisomer forms (along with several other minor species), but

the major signals, are the result of the selective arrangement of the P3HT and Br around the unsymmetrical nickel catalyst.

To confirm the positions of the phosphino groups relative to the corresponding aryl bromide, a model compound was synthesized. Luscombe and co-workers [38, 56] previously synthesized an

externally initiated (dppp)Ni(o-tolyl)Br catalyst for cross-coupling polymerizations. Unfortunately, the isolation and characterization of the analogous (sepp)Ni(o-tolyl)Br was unsuccessful.

Methylbenzoate was considered as a replacement for the o-tolyl ligand since the methyl ester should be a suitable substituent to block the axial site of the nickel [57], and it can

potentially coordinate to the metal via the oxygen atoms. Bis(1,5-cyclooctadiene)nickel, PPh3, and methyl 2-bromobenzoate were combined to form an air-stable Ni2+ complex prior to ligand

exchange with sepp (Scheme 3).

Ligand exchange of sepp with PPh3 to prepare an externally initiated Ni2+ complex

The 31P{1H} NMR signals for (sepp)Ni(o-C6H4CO2Me)Br were nearly identical to those observed in the spectrum obtained from the polymerization reactions. For (sepp)Ni(o-C6H4CO2Me)Br in THF-d8,

signals were observed at 18.0 ppm (2JPP = 70.3 Hz) and −5.7 ppm (2JPP = 70.2 Hz). Two minor doublets that exchange with the major species are also observed at 4.8 and 1.9 ppm. The identity

of this species is still under investigation. A 2D 1H−31P HMQC experiment optimized for long-range coupling (J = 8 Hz) confirmed that the downfield signal near 19 ppm corresponds to the PPh2

group of sepp. The correlations of the P atom to the broad protons of the phenyl rings are indicated in Fig. 3.

Selected region of an 1H−31P HMQC experiment (500 MHz) recorded for (sepp)Ni(o-C6H4CO2Me)Br in THF-d8 at 22 °C

The signal near −5 ppm corresponds to the PEt2 group, as evidenced by the correlations to the methylene and methyl groups (marked by the blue box in Fig. 3). Notably, the phosphorus atom of

the PEt2 group also correlates strongly with nearly all the signals of the propyl backbone of the 6-membered metallacycle and the trans-2-methylbenzoate group. The assignments for

(sepp)Ni(o-C6H4CO2Me)Br are consistent with those of the spectra acquired during the polymerizations (Fig. 2, bottom) and suggest an energetic preference for the reactive arene ligand being

trans to the PEt2 group.

The formation of a single stereoisomer was also confirmed for (sepp)Ni(o-C6H4CO2Me)Br using single crystal X-ray diffraction (Fig. 4). There are two independent molecules in the unit cell

(similar to Ni(sepp)Cl2). In the solid-state molecular structure, the nickel–phosphine bond lengths are significantly different, and the PPh2−Ni bond trans to the Br is nearly 0.1 Å shorter

than the PEt2−Ni bond (2.1384(4) versus 2.2277(4) Å, respectively). This was expected considering the strong σ-donating properties of the reactive arene ligand opposite the PEt2 group. The

average bite angle of the ligands was nearly 98° for this complex. The oxygen atom (O12) and nickel are within the sum of their Van der Waals radii but there is not a strong bonding

interaction (Ni1–O12 distance is 2.545 Å).

Solid-state molecular structure of (sepp)Ni(o-C6H4CO2Me)Br (thermal ellipsoids at 50% probability) with H atoms omitted. Only one of the two independent molecules is shown

The formation of the major isomer during the polymerization (Fig. 2), and the observation of the same preference when synthesizing a complex by ligand exchange (Figs. 3 and 4), suggests a

significant energy difference with the relative orientation of the reactive arene and the bromide. The total energies of the two possible stereoisomers were calculated (3-methylthiophene was

used to mimic the polymer chain), and as expected, the trans relationship of the PEt2 group and the reactive ligand is favored by 3.7 kcal/mol (Fig. 5—computed structures A and B). We

suspect the energy difference is due to steric constraints as more distortion from square planar geometry around the nickel is observed in A. A mean plane was constructed from four atoms for

both the optimized geometries: the nickel, the two phosphines from the ligand, and the carbon atom of the 2-position for the thiophene ring. In A (the higher energy structure), the bromine

atom is distorted by 0.55 Å from the plane, while this value is only 0.25 Å in B (lower energy structure). A picture illustrating these distortions from the mean planes is provided in Fig. 

S26.

DFT-optimized structures of the two possible stereoisomers of (sepp)Ni(3-methylthiophene)Br (H atoms omitted). Optimization and total energy calculations were conducted with a mixed basis

set using B3LYP 6–31 G(d) for all ligand atoms and SDD for nickel

Once the energy preference for the Ni2+ species generated by oxidative addition was established, the two catalysts, Ni(sepp)Cl2 and (sepp)Ni(o-C6H4CO2Me)Br, were used to polymerize monomer

1. The MALDI-TOF mass spectra for the P3HT samples generated using these two catalysts (4 mol %) revealed some interesting features. For Ni(sepp)Cl2, a tail was observed in the

lower-molecular-weight region (along with a minor second distribution) and for (sepp)Ni(o-C6H4CO2Me)Br, a second mass distribution was observed at lower molecular weights (Fig. 6a, b,

respectively). The appearance of these lower-molecular-weight species suggested unproductive side-reactions (chain-transfer or chain-chain coupling) during the polymerization. In the GPC

traces, we observed higher-molecular-weight shoulders in some cases, which is indicative of chain–chain coupling. This can occur when two growing chains ((sepp)Ni(polymer)Br) exchange

reactive ligands producing (sepp)Ni(polymer)2 and (sepp)NiBr2. Reductive elimination from (sepp)Ni(polymer)2 can then produce higher-molecular-weight materials, and the resultant metal

species can begin growing new chains, affording lower-molecular-weight species.

MALDI-TOF mass spectra for P3HT samples. a Ni(sepp)Cl2 and b (sepp)Ni(o-C6H4CO2Me)Br. The spectra in c and d correspond to the same catalysts with an additional 1.0 equiv of sepp added to

the reaction mixture (relative to [Ni]). The reaction conditions were as described in Table 1, entries 1, 5, 3, and 7 for a, b, c, and d, respectively

In our previous report on Suzuki–Miyaura CTPs using Ni(dppp)Cl2, catalyst hydrolysis was critical to controlling the polymerization process, as it released free ligand into the reaction

mixture [25]. The free ligand does not have a significant impact on the rate of the polymerization, but it was necessary to obtain high-molecular-weight P3HT with good control over the end

groups. Since Ni(sepp)Cl2 and (sepp)Ni(o-C6H4CO2Me)Br are more resistant to hydrolysis, minimal free ligand is present in the reaction during the polymerization. To evaluate whether

additional free ligand could improve the polymerization, sepp was added to Ni(sepp)Cl2- and (sepp)Ni(o-C6H4CO2Me)Br-initiated polymerizations (Fig. 6c, d).

The change in the MALDI-TOF mass spectra for the as-obtained P3HT samples prepared using these two different catalyst systems was remarkable. When the polymerization was conducted in the

presence of additional sepp ligand, the distribution of chain lengths was in line with a living polymerization (Poisson distribution) (Fig. 6c, d). Very minor secondary distributions were

observed in both spectra, indicating that although the polymerization is not perfect, additional ligand greatly improves the control over the end groups (see the integrations of the P3HT

samples in the ESI) and the molecular weight distribution. For reference, the molecular weight data from GPC and NMR are included in Table 1.

A molecular weight versus conversion plot and a semilogarithmic plot were generated for (sepp)Ni(o-C6H4CO2Me)Br in the presence of additional sepp ligand. This experiment was completed in an

identical manner to our previous report though different time intervals were used [25]. The collected data were then compared to those of (dppp)Ni(o-tolyl)Br with additional dppp (Fig. 7)

[25]. As expected, the profile is very similar to that of the dppp system, but the newly synthesized catalyst produces the polymer at a slower rate. In both semilog plots, deviations from

linearity were observed, similar to the 3-hexylthiophene polymerization using a Kumada coupling [58]. As such, no rate constants were extracted; however, qualitatively, it is clear that

(dppp)Ni is faster. As expected, the Mn values increase for both catalysts with conversion, although they both begin to level off at higher conversions. This is indicative of chain-transfer

towards the end of the polymerization, which could be a result of repeated sampling because the dispersity of the final polymer samples was higher (~1.15) than in the isolated

polymerizations presented in Table 1 (entries 7 and 8).

Left: semilogarithmic plot of concentration versus time. Right: number-average molecular weight versus conversion. Black circles correspond to 2 mol% (dppp)Ni(o-C6H4Me)Br with dppp, and blue

squares correspond to 2 mol% (sepp)Ni(o-C6H4CO2Me)Br with sepp. For both polymerizations, [1] was 25 mM in THF with 0.5 mM catalyst and ligand. The (dppp)Ni experimental data was taken

directly from ref. [25]

Ultimately, the additional ligand clearly has a positive impact on the polymerization. Miyaura proposed in prior work that free ligand stabilizes Ni(0) [50,51,52, 59], and Van Der Boom has

illustrated that additional phosphine ligand can suppress intermolecular pathways in aryl halide bond activations with platinum species [60]. These reports highlight the importance of free

ligands for improving catalyst lifetime and chain fidelity, respectively. Future work will explore this effect in more detail to better elucidate how free ligand exerts this positive

effect.

In conclusion, we have prepared a new, unsymmetrical nickel diphosphine catalyst for CTP. The diphosphine comprises two electronically and sterically distinct moieties, one PPh2 group and

one stronger σ-donating PEt2 group. The nickel catalyst retains one ligand site that is identical to the highly successful Ni(dppp) system, while offering another donor in the scaffold as a

tunable site. Ni(sepp)Cl2 proved to be an excellent catalyst for the controlled polymerization of an organoboron precursor if excess free ligand is present in the reaction mixture. The P3HT

obtained using this catalyst system is essentially identical to that obtained using Ni(dppp)Cl2.

This work, as well as our prior efforts, are consistent with the reports from Yokozawa et al. [61], McNeil and co-workers [62], and Choi and co-workers [14] who have noted the benefits of

free ligand in CTP reactions. Lee and Luscombe [63] have also noted the importance of additional pyridine in recent studies on chain-growth polymerizations via direct arylation. In the

future, we will explore other classes of unsymmetrical diphosphines for CTP. Additionally, the Ni(sepp)Cl2 system and the externally initiated variant will be evaluated as catalysts for the

polymerization of other monomers using Suzuki–Miyaura CTPs.

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K.J.T.N. is grateful to the NSF for a CAREER Award (CHE-1455136). The NMR instrumentation at Carnegie Mellon University is partially supported by the NSF (CHE-0130903, CHE-1039870, and

CHE-1726525). The authors would also like to thank both Prof. Tsutomu Yokozawa and Prof. Anne McNeil for helpful discussions.

Author informationAuthors and Affiliations Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, PA, 15213-2617, USA

Matthew A. Baker, Josué Ayuso-Carrillo, Martin R. M. Koos, Anthony J. Varni, Roberto R. Gil & Kevin J. T. Noonan

Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell University, Ithaca, NY, 14853-1301, USA

Samantha N. MacMillan

AuthorsMatthew A. BakerView author publications You can also search for this author inPubMed Google Scholar

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About this articleCite this article Baker, M.A., Ayuso-Carrillo, J., Koos, M.R.M. et al. A robust nickel catalyst with an unsymmetrical propyl-bridged diphosphine ligand for

catalyst-transfer polymerization. Polym J 52, 83–92 (2020). https://doi.org/10.1038/s41428-019-0259-3

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Received: 07 June 2019

Revised: 09 July 2019

Accepted: 30 July 2019

Published: 03 September 2019

Issue Date: January 2020

DOI: https://doi.org/10.1038/s41428-019-0259-3

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