Redox Instability of Copper(II) Complexes of a Triazine‐Based PNP Pincer

• Published in: European journal of inorganic chemistry 2021-03, Vol.2021 (12), p.1140-1151
• Main Authors: Mohammadnezhad, Gholamhossein, Amirian, Ali Mohammad, Görls, Helmar, Plass, Winfried, Sandleben, Aaron, Schäfer, Sascha, Klein, Axel
• Format: Article
• Language: English
• Subjects: Alkynes; Azide click reaction; Catalytic activity; Coordination compounds; Coordination polymers; Copper; Copper chloride; Copper compounds; Cycloaddition; Electrochemistry; Inorganic chemistry; Ligands; Melting points; NMR; Nuclear magnetic resonance; PNP pincer ligands; Redox chemistry; Redox reactions; Spectroelectrochemistry; Spectrum analysis; Triazoles
• ISSN: 1434-1948; 1099-0682
• DOI: 10.1002/ejic.202001129

The new Cu(I) complex [Cu(PNPNTPh‐Ph)Cl] (1) containing the tridentate PNP pincer ligand N,N′‐bis(diphenylphosphino)‐2,6‐diamino‐4‐phenyl‐1,3,5‐triazine was obtained from the reaction of [Cu(SMe2)Cl]n with the ligand as ether solvate 1.0.5Et2O. 1 was independently obtained from a reaction mixture containing the ligand and the Cu(II) precursor CuCl2.2H2O in 50 % yield alongside with the Cu(II) coordination polymer [Cu(O2PPh2)2]n (2). From the reaction of Cu(NO3)2 ⋅ 3H2O with PNPNTPh‐Ph in the presence of pyridine the complexes [Cu(O2PPh2)2(Py)2(H2O)] (3), [Cu(O2PPh2)(Py)2(NO3)]2 (4), and [Cu(Py)4(NO3)2].Py (5), were obtained, 2, 3, and 4 contain diphenyl‐phosphinate ligands. The underlying redox reaction of the ligand and Cu(II) yielding the oxidised ligands observed in the by‐products and the Cu(I) product complex was further studied using electrochemistry and UV‐vis spectroelectrochemistry. Attempts to synthesise the Cu(II) complex [Cu(PNPNTPh‐Ph)(NO3)2] (6) in a mechanochemical experiment gave evidence for this unprecedented species from ESI‐MS(+) and EPR spectroscopy but also revealed its very high sensitivity to air and moisture. The catalytic activity of 1 was investigated in the azide‐alkyne cycloaddition yielding various 1‐benzyl‐4‐phenyl‐1H‐1,2,3‐triazoles. The environmentally benign (“green”) and cheap EtOH/H2O solvent mixture turned out to be very suitable. Melting points, FT‐IR, and NMR spectra of the triazole products were analysed. From the tridentate PNP pincer ligand N,N′‐bis(diphenylphosphino)‐2,6‐diamino‐4‐phenyl‐1,3,5‐triazine a Cu(I) chlorido complex was synthesised, while the corresponding Cu(II) complex undergoes electron transfer resulting in Cu(I) and oxidised ligand. The stable Cu(I) complex can be used for azide‐alkyne 1,3‐cycloaddition (AAC) catalysis.