Pincer complexes play an important role in organometallic chemistry; in particular, their use as homogeneous catalysts for organic transformations has increased dramatically in recent years. The high catalytic activity of such bis-cyclometallic complexes is associated with the easy tunability of their properties. Moreover, the phosphorus-based unsymmetrical pincers showed higher catalytic activity than the corresponding symmetrical analogues in several catalytic reactions. However, in modern literature, an increasing interest in the development of catalysts based on non-precious metals is observed. For example, nickel, which is an affordable and sustainable analogue of platinum and palladium, known for its low toxicity, has attracted increasing attention in the catalytic chemistry of transition metals in recent years. Thus, this mini-review is devoted to the recent advances in the chemistry of unsymmetrical phosphorus-based pincer nickel complexes, including the ligand design, the synthesis of nickel complexes and their catalytic applications.

A Pt(II) complex bearing chelating tridentate bis-aryloxide tetrahydropyrimidinium-based N-heterocyclic carbene (NHC) was synthesized and characterized by using different techniques. Both cyclic voltammetry and differential pulse voltammetry were used to study the electrochemical properties of the complex, revealing two reversible one-electron oxidation processes. The chemical generation and isolation of one-electron-oxidized species were performed oxidizing the initial complex by means of AgBF₄. A combination of spectroscopic (UV-Vis/NIR- and EPR-) and theoretical (density functional theory (DFT)) studies suggests the formation of a Pt(II)-phenoxyl radical complex. The latter open-shell derivative was structurally characterized by means of X-ray diffraction analysis. Finally, the neutral platinum complex was tested as a mediator in the process of electrocatalytic oxidation of 2-(methylamino)ethanol (MEA).

The low-stable organonickel sigma-complex (2,2'-bipyridyl)bromo(2,4,6-tricyclopentylphenyl)nickel(II) [NiBr(Tcpp)(bpy)] was obtained by a modified procedure of preparative electrochemical synthesis and isolation. The crystal structure of the resulting organonickel complex was determined by the X-ray diffraction method. Its structure was compared with that of its more stable analogue [NiBr(Tchp)(bpy)], where Tchp is 2,4,6-tricyclohexylphenyl. It was found that a more efficient network of intermolecular interactions is formed in the [NiBr(Tchp)(bpy)] crystal, which is responsible for its higher stability, whereas [NiBr(Tcpp)(bpy)] is more likely to enter into an equilibrium Schlenk reaction, leading to its decomposition.

A Pt(II) complex featuring chelating tridentate bis-aryloxide tetrahydropyrimidinium based N-heterocyclic carbene (NHC) was synthesized and characterized by using different techniques. The electrochemical properties of the complex were investigated by cyclic voltammetry as well as differential pulse voltammetry, which revealed two reversible one-electron oxidation processes. The chemical generation and isolation of one-electron-oxidized species were performed oxidizing the initial complex by means of AgBF4. The combined UV-Vis/NIR- and EPR-spectroscopy studies supported by density functional theory (DFT) calculations suggest the formation of Pt(II)-phenoxyl radical complex. The latter open-shell derivative was structurally characterized by means of X-ray diffraction analysis. Finally, the neutral platinum complex was tested as a mediator in the process of electrocatalytic oxidation of 2-(methylamino)ethanol (MEA).

The reaction between cobalt tetrafluoroborate hexahydrate with 2,2′-bipyridine (Вpy) and diphenylphosphinic acid leads to the formation of a new binuclear cobalt(II) complex [Co(µ-OPPh)(Вpy)](BF) featuring sterically demanding bridging ligands. The synthesized complex was characterized by various physicochemical methods of analysis, the crystal structure was determined by X-ray diffraction. The analysis of the X-ray structure of the complex showed that the cobalt ions have a distorted octahedral coordination and are linked by bridges through two {µ-OPPh} fragments. The results of EPR spectroscopy and magnetochemical studies indicate the presence of a weak antiferromagnetic interaction between the cobalt ions in the molecule.