Unveiling the Catalytic Potential of [1,3-Bis(diphenylphosphino)propane]nickel(II) chloride

[1,3-Bis(diphenylphosphino)propane]nickel(II) chloride can act as a catalyst, mainly used in organic synthesis processes such as C - C, C - N, and C - O coupling reactions, as well as in the research and development processes of chemical and pharmaceutical industries.

[1,3-Bis(diphenylphosphino)propane]nickel(II) chloride-Catalyzed Cross-Coupling of Aryl Halides

Scientists present a general approach to CP bond formation through the cross-coupling of aryl halides with a dialkyl phosphite, diphenylphosphine oxide, and diphenylphosphane by using [1,3-Bis(diphenylphosphino)propane]nickel(II) chloride as catalyst (dppp=1,3-bis(diphenylphosphino)propane). This catalyst system displays a broad applicability that is capable of catalyzing the cross-coupling of aryl bromides, particularly a range of unreactive aryl chlorides, with various types of phosphorus substrates, such as a dialkyl phosphite, diphenylphosphine oxide, and diphenylphosphane. Consequently, the synthesis of valuable phosphonates, phosphine oxides, and phosphanes can be achieved with one catalyst system. Moreover, the reaction proceeds not only at a much lower temperature (100–120 °C) relative to the classic Arbuzov reaction (ca. 160–220 °C), but also without the need of external reductants and supporting ligands. In addition, owing to the relatively mild reaction conditions, a range of labile groups, such as ether, ester, ketone, and cyano groups, are tolerated. Finally, a brief mechanistic study revealed that by using [1,3-Bis(diphenylphosphino)propane]nickel(II) chloride as a catalyst, the NiII center could be readily reduced in situ to Ni0 by the phosphorus substrates due to the influence of the dppp ligand, thereby facilitating the oxidative addition of aryl halides to a Ni0 center. This step is the key to bringing the reaction into the catalytic cycle.[1]

Despite the great advance in transition-metal-catalyzed Csp2-X (X=C, N, O, S, and so forth) bond formation over the past decades, relatively limited protocols for CP coupling are available. This fact belies the importance of phosphorus compounds, such as phosphonates, phosphine oxides, and phosphanes, in materials science,1 biochemistry,2 and catalytic chemistry. Initially, the catalytic efficiency of free [NiCl2 and various nickel complexes was evaluated. The results showed that among several nickel complexes [1,3-Bis(diphenylphosphino)propane]nickel(II) chloride is a potential catalyst, thus affording the desired product  in 75 % yield at a considerably lower temperature of 100 °C. Finally, to confirm whether the general and mild CP cross-coupling is really promoted by the ligand-based strategy (see above), three control experiments were carried out. When a mixture of 1-bromonaphthalene, K3PO4, and [1,3-Bis(diphenylphosphino)propane]nickel(II) chloride was heated in dioxane at 100 °C for one hour, no observable change resulted because the [1,3-Bis(diphenylphosphino)propane]nickel(II) chloride particles remained insoluble. In contrast, when the aryl halide was replaced by dimethyl phosphite or diphenylphosphine oxide, a slight- or deep-yellow homogeneous solution, respectively, was formed rapidly within several minutes. In contrast, the nickel 2p3/2 binding energy for the divalent [1,3-Bis(diphenylphosphino)propane]nickel(II) chloride complex was 855.1 eV. Thus, the combination of the control experiments and XPS analysis clearly demonstrated that by appropriate design of the ligands the reduction of the metal center from NiII to Ni0 by various phosphorus substrates in situ and the subsequent Ni0-catalyzed CP bond formation proceeded at a significantly lower temperature without the need of external reductants and supporting ligands.

Mechanism of Ni-catalyzed chain-growth polymerization

Scientists studied the mechanism of the chain-growth polymerization of 2-bromo-5-chloromagnesio-3-hexylthiophene (A) with [1,3-Bis(diphenylphosphino)propane]nickel(II) chloride, in which head-to-tail poly(3-hexylthiophene) (HT-P3HT) with a low polydispersity is obtained and the Mn is controlled by the feed ratio of the monomer to the Ni catalyst. Matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectra showed that the HT-P3HT uniformly had a hydrogen atom at one end of each molecule and a bromine atom at the other. The reaction of the polymer with aryl Grignard reagent gave HT-P3HT with aryl groups at both ends, which indicates that the H-end was derived from the propagating Ni complex. The degree of polymerization and the absolute molecular weight of the polymer could be evaluated from the 1H NMR spectra of the Ar/Ar-ended HT-P3HT, and it was found that one Ni catalyst molecule forms one polymer chain. Furthermore, by reaction of A with 50 mol % [1,3-Bis(diphenylphosphino)propane]nickel(II) chloride, the chain initiator was found to be a bithiophene−Ni complex, formed by a coupling reaction of A followed by insertion of the Ni(0) catalyst into the C−Br bond of the dimer.[2]

Evidence for Ligand-Dependent Mechanistic Changes in Nickel-Catalyzed Chain-Growth Polymerizations

The mechanisms for [1,3-Bis(diphenylphosphino)propane]nickel(II) chloride-catalyzed chain-growth polymerization of 4-bromo-2,5-bis(hexyloxy)phenylmagnesium chloride and 5-bromo-4-hexylthiophen-2-ylmagnesium chloride were investigated. A combination of rate and spectroscopic studies revealed that transmetalation is the rate-determining step of the catalytic cycle for both monomers. 31P NMR spectroscopic studies revealed that a Ni(II)−aryl halide and a Ni(II)−thienyl halide are the catalyst resting states. In addition, LiCl was found to alter the arene polymerization rates. These results are different than those previously obtained with an alternative catalyst (Ni(dppe)Cl2) and suggest that the ligand has a strong mechanistic influence on the polymerization.[3]

Monomers were generated in situ via Grignard metathesis (GRIM) with i-PrMgCl. Although monomer is an approximately 80:20 mixture of regioisomers, the minor regioisomer is not significantly consumed during polymerization. To generate a soluble catalyst species, the [1,3-Bis(diphenylphosphino)propane]nickel(II) chloride was preinitiated with 5−7 equiv of monomer prior to starting the rate studies. Initial rates of polymerization were measured by in situ IR spectroscopy or GC analysis of aliquots at varying concentrations of monomer and catalyst. For the [1,3-Bis(diphenylphosphino)propane]nickel(II) chloride-catalyzed polymerization of both monomers, an approximate first-order dependence of the initial rate on both [catalyst] and [monomer] was observed. These monomer reaction orders are different than those obtained with Ni(dppe)Cl2, suggesting a ligand-dependent change in rate-determining step. Indeed, the rate data obtained with [1,3-Bis(diphenylphosphino)propane]nickel(II) chloride are consistent with either rate-determining transmetalation or intermolecular oxidative addition because both steps involve the monomer and catalyst. To distinguish between these two scenarios, in situ NMR spectroscopic studies were used to characterize the catalyst resting state during polymerization.

References

[1]Zhao YL, Wu GJ, Li Y, Gao LX, Han FS. [NiCl2(dppp)]-catalyzed cross-coupling of aryl halides with dialkyl phosphite, diphenylphosphine oxide, and diphenylphosphine. Chemistry. 2012 Jul 27;18(31):9622-7.

[2]Miyakoshi R, Yokoyama A, Yokozawa T. Catalyst-transfer polycondensation. mechanism of Ni-catalyzed chain-growth polymerization leading to well-defined poly(3-hexylthiophene). J Am Chem Soc. 2005 Dec 14;127(49):17542-7.

[3]Lanni, E. L., & McNeil, A. J. (2010, September 1). Evidence for Ligand - Dependent Mechanistic Changes in Nickel - Catalyzed Chain - Growth Polymerizations. Macromolecules, 43(19), 8039 - 8044.

You may like